TY - JOUR
AU - Beauchemin, K. A.
AB - ABSTRACT The objectives of the study were to characterize the effects of CP concentration and ruminal degradability of barley-based backgrounding diets on route and chemical form of N excretion, ruminal fermentation, microbial protein synthesis, and nutrient digestion in beef cattle. Four Angus heifers (479 ± 14.6 kg average BW) with ruminal and duodenal cannulas were used in an experiment designed as a 4 × 4 Latin square. The basal diet consisted of 54% barley silage and 46% barley grain-based concentrate (DM basis). Dietary treatments included the basal diet with no added protein (12% CP) or diets formulated to contain 14% CP by supplementation with urea (UREA), urea and canola meal (UREA+CM), or urea, corn gluten meal, and xylose-treated soybean meal (UREA+CGM+xSBM). The amount of feed offered was restricted to 95% of ad libitum intake. There was no effect of the diets on DMI (P = 0.38), and therefore, N intake was less (P < 0.05) in heifers fed the 12% CP diets than the 14% CP diets. Fecal N output was not affected by the diet (P = 0.15), but urine N (P < 0.10) and urea N output were greater (P < 0.05) in heifers fed the 14% CP than the 12% CP diets. There was no effect of CP degradability (P > 0.10) on the amount of urine N output. Urine N output was 38.9 and 45.1 ± 5.50% of N intake in heifers fed the 12% CP and 14% CP diets (P < 0.05), respectively. Urea N, the form of N most susceptible to NH3–N volatilization and loss, was the major form of N in urine (75.5% in heifers fed the 12% CP diet and 81.4 ± 1.7% in heifers fed the 14% CP diets; P < 0.05). Supplemental RDP (UREA+CM) and RUP combined with urea (UREA+CGM+xSBM) to provide 14% CP increased (P < 0.05) ruminal NH3–N but had no effect on ruminal peptide N (P = 0.62) and free AA N (P = 0.18) concentration, the flow of microbial (P = 0.34) and feed (P = 0.55) N, and ruminal (starch, P = 0.11; NDF, P = 0.78) and total tract nutrient digestibility (OM, P = 0.21; starch, P = 0.16). Supplementation of barley-based backgrounding diets containing 12% CP with NPN alone or combined with ruminally degradable and undegradable true protein to attain 14% CP had no effect on fecal N output, but urine N and urea N increased irrespective of protein source. In addition, the ruminal degradability of the protein sources did not influence the composition of protein flowing to the intestine and site and extent of nutrient digestibility. INTRODUCTION Low production efficiency has in part led to the criticism of intensive beef cattle production as unsustainable and potentially threatening to the environment. When efficiency of N utilization is defined as grams of N in meat or milk per gram of N intake, the efficiency of ruminant production is low compared with nonruminant systems (Kohn et al., 2005). Feeding low CP diets to feedlot cattle is an effective means of increasing the efficiency of N utilization and reducing unnecessary N losses, providing there is no reduction in growth performance and overall feed efficiency (Cole et al., 2005). In a companion performance study with growing and finishing beef cattle fed barley-based diets, efficiency of N utilization during the backgrounding phase averaged 19.8% in cattle fed a 12% CP diet (low) or a 14% CP diet supplemented with a mixture of urea and true protein but was only 16.6% in cattle fed a 14% CP diet with urea as the sole supplemental protein source (Koenig et al., 2013). The ADG and feed:gain ratio were, however, reduced during the backgrounding phase in cattle fed the low CP diet and the 14% CP diet with urea, compared with cattle receiving the greater CP diets with supplementary true protein. The objectives of this study were to characterizing the effects of CP concentration and rumen degradability in backgrounding diets on 1) the route and chemical form of N excretion, as this will determine the susceptibility of excreted N to NH3–N volatilization, and 2) ruminal N metabolism and nutrient digestion that underlie the response in performance and efficiency of N utilization in growing cattle. The hypothesis was that providing ruminal degradable and undegradable CP to meet the microbial requirements for NH3–N and peptide N and host requirements for metabolizable protein would improve the efficiency of N utilization in the rumen and whole animal and thereby minimize N excretion and mitigate the potential loss of N to the environment from beef cattle feedlot operations. MATERIALS AND METHODS Animals were cared for and managed according to the guidelines of the Canadian Council on Animal Care (1993). Experimental procedures involving animals were approved by the Institutional Animal Care and Use Committee at the Lethbridge Research Centre. Animals and Experimental Design Four Black Angus heifers (approximately 15 mo of age at the beginning of the experiment) with ruminal and duodenal cannulas were used in an experiment designed as a 4 × 4 Latin square with 4 dietary treatments and 4 periods of 28 d (9 d for dietary adaptation and 19 d for measurements and sample collection). The heifers were spayed and surgically fitted with flexible permanent ruminal and duodenal cannulas by a licensed veterinarian 8 wk before the start of the experiment. Ruminal cannulas, manufactured of flexible polyvinyl chloride, with a 7.5 cm i.d. opening (number 4C; Bar Diamond, Inc., Parma, ID) were used for the first 6 to 8 wk after surgery and then replaced with a cannula with a 10 cm i.d. opening (number 9C; Bar Diamond). The surgical technique for placement of ruminal cannulas was as described by Bar Diamond (2011). The duodenal cannulas were designed as a simple T-type with a 2.5 cm i.d. opening and open gutter-type flanges and were constructed of Plastisol (Bar Diamond, Inc.) from a stainless steel mold. The general surgical principles for placement of the duodenal cannula were as described by McGilliard (1982). The site of the laparotomy incision and cannula position were modified to permit the cannula to be exteriorized between the ribs and requiring no removal of a rib. The duodenum and pylorus were located through a 15-cm incision made parallel to the right costal arch and 5 cm ventral and central to the 10th intercostal arch, and the cannula was placed proximal to the bile and pancreatic ducts, approximately 10 cm distal to the pylorus, and was exteriorized through an incision within the 10th intercostal arch. The heifers were housed in individual tie stalls with mattresses bedded with wood shavings. The heifers were released to an outdoor pen for 1 to 2 h of exercise daily, as the measurement and sampling schedule permitted. Body weight was measured (without feed restriction) at 1300 h at the beginning (d 1) and end (d 28) of each period and the 2 values were averaged for each period. Dietary Treatments Heifers were fed a diet containing 70% barley silage and 30% barley grain concentrate (DM basis) for 10 wk before and during recovery from surgery. The experimental diets consisted of 54% barley silage and 46% barley grain-based concentrate and were prepared as a total mixed ration (TMR; DM basis; Table 1). The 4 dietary treatments included the basal diet with no added protein (12% CP) or diets formulated to contain 14% CP by supplementation with urea (UREA; rumen degradable NPN), urea and canola meal [UREA+CM; approximately 50% N from NPN and 50% N from a rumen degradable true protein (peptides and AA) source], or urea, corn gluten meal and xylose-treated soybean meal (UREA+CGM+xSBM; approximately 50% N from NPN and 25% N from each of the rumen undegradable true protein sources). Urea was added to each of the diets supplemented with true protein to ensure a basal level of ruminal NH3–N for microbial growth. The diets were formulated to meet mineral and vitamin requirements (NRC, 2000). Heifers were fed the low 12% CP backgrounding diet for 3 wk before the initiation of the experiment. The heifers were then assigned to their dietary treatments such that each treatment followed every other treatment at 1 time during the experiment to balance for any residual effects. Table 1. Ingredients and nutrient composition of the barley silage-based backgrounding diets   Supplemental protein source2    UREA  UREA + CM  UREA + CGM + xSBM  Item  CP2:  12  14  14  14  Ingredient, % of DM      Barley silage3    53.6 ± 1.07  53.6 ± 1.09  53.6 ± 1.07  53.6 ± 1.10      Barley grain, dry rolled4    39.9 ± 0.92  39.9 ± 0.91  39.9 ± 0.92  39.9 ± 0.90      Supplement    6.51 ± 0.15  6.47 ± 0.19  6.57 ± 0.15  6.54 ± 0.20          Barley, ground    3.59  2.87  0.36  1.14          Urea    –  0.70  0.37  0.36          Canola meal    –  –  2.92  –          Soybean meal (xylose treated)5    –  –  –  1.17          Corn gluten meal    –  –  –  0.95          Limestone    1.47  1.45  1.46  1.46          Dicalcium phosphate    0.73  0.73  0.73  0.73          Salt    0.26  0.26  0.26  0.26          Trace mineral and vitamin premix6    0.07  0.07  0.07  0.07          Canola oil    0.12  0.12  0.12  0.12          Molasses    0.26  0.26  0.26  0.26          Rumensin premix7    0.01  0.01  0.01  0.01  Nutrient composition, % of DM unless noted otherwise      DM, % as fed    53.8 ± 1.01  52.8 ± 1.30  53.4 ± 1.32  53.0 ± 0.41      OM    93.2 ± 0.39  93.1 ± 0.35  92.4 ± 0.69  92.8 ± 0.17      CP    12.0 ± 0.67  14.2 ± 0.88  14.2 ± 0.48  14.3 ± 0.37      Starch    36.4 ± 2.74  38.3 ± 6.21  34.9 ± 2.69  36.2 ± 1.67      NDF    32.7 ± 1.78  33.1 ± 2.42  33.5 ± 2.15  33.0 ± 2.34      ADF    18.2 ± 1.73  18.9 ± 1.82  19.3 ± 1.50  18.8 ± 1.64      Crude fat    2.25  2.91  2.55  2.25      RDP8    6.18  8.04  7.70  7.29      Rumen NH3–N, % requirement8    54  74  69  62      Rumen peptide N, % requirement8    87  87  95  91      NEg, Mcal/kg DM8    0.83  0.83  0.82  0.83    Supplemental protein source2    UREA  UREA + CM  UREA + CGM + xSBM  Item  CP2:  12  14  14  14  Ingredient, % of DM      Barley silage3    53.6 ± 1.07  53.6 ± 1.09  53.6 ± 1.07  53.6 ± 1.10      Barley grain, dry rolled4    39.9 ± 0.92  39.9 ± 0.91  39.9 ± 0.92  39.9 ± 0.90      Supplement    6.51 ± 0.15  6.47 ± 0.19  6.57 ± 0.15  6.54 ± 0.20          Barley, ground    3.59  2.87  0.36  1.14          Urea    –  0.70  0.37  0.36          Canola meal    –  –  2.92  –          Soybean meal (xylose treated)5    –  –  –  1.17          Corn gluten meal    –  –  –  0.95          Limestone    1.47  1.45  1.46  1.46          Dicalcium phosphate    0.73  0.73  0.73  0.73          Salt    0.26  0.26  0.26  0.26          Trace mineral and vitamin premix6    0.07  0.07  0.07  0.07          Canola oil    0.12  0.12  0.12  0.12          Molasses    0.26  0.26  0.26  0.26          Rumensin premix7    0.01  0.01  0.01  0.01  Nutrient composition, % of DM unless noted otherwise      DM, % as fed    53.8 ± 1.01  52.8 ± 1.30  53.4 ± 1.32  53.0 ± 0.41      OM    93.2 ± 0.39  93.1 ± 0.35  92.4 ± 0.69  92.8 ± 0.17      CP    12.0 ± 0.67  14.2 ± 0.88  14.2 ± 0.48  14.3 ± 0.37      Starch    36.4 ± 2.74  38.3 ± 6.21  34.9 ± 2.69  36.2 ± 1.67      NDF    32.7 ± 1.78  33.1 ± 2.42  33.5 ± 2.15  33.0 ± 2.34      ADF    18.2 ± 1.73  18.9 ± 1.82  19.3 ± 1.50  18.8 ± 1.64      Crude fat    2.25  2.91  2.55  2.25      RDP8    6.18  8.04  7.70  7.29      Rumen NH3–N, % requirement8    54  74  69  62      Rumen peptide N, % requirement8    87  87  95  91      NEg, Mcal/kg DM8    0.83  0.83  0.82  0.83  1CGM = corn gluten meal; CM = canola meal; UREA= urea; xSBM = xylose-treated soybean meal. 2Dietary CP concentration, % of DM Mean ± SD, n = 4 periods. 3Contained on a DM basis: 39.9 ± 1.89% DM, 91.4 ± 0.44% OM, 10.4 ± 0.82% CP, 22.8 ± 2.62% starch, 49.2 ± 3.5% NDF, 31.3 ± 3.24% ADF, and 2.4% crude fat. 4Contained on a DM basis: 90.4 ± 0.52% DM, 97.0 ± 0.18% OM, 13.6 ± 1.75% CP, 58.2 ± 2.32% starch, 18.4 ± 1.59% NDF, 5.26 ± 0.46% ADF, and 1.50% crude fat. The processing index was 80 to 82% and was determined as the volume weight (0.5 L) of the grain after processing divided by the volume weight of whole barley grain before processing × 100% (0.5 L cup; Seedburo, Chicago, IL). 5SoyPass; LignoTech USA, Inc., Rothschild, WI. 6Provided per kilogram of diet DM: 70 mg/kg of Zn, 18 mg/kg of Cu, 23 mg/kg of Mn, 0.8 mg/kg of I, 0.35 mg/kg of Se, 0.24 mg/kg of Co, 12,000 IU/kg of vitamin A, 601 IU/kg of vitamin D, and 16.5 IU/kg of vitamin E. 7Provided monensin at 22 mg/kg diet. Premix contained monensin at 200 g/kg; Elanco Animal Health, Guelph, ON. 8Estimated using Cornell Net Carbohydrate and Protein System (CNCPS v6.1) with the measured nutrient composition of the barley silage and grain, in situ protein degradation characteristics for the barley silage, barley grain, and protein sources determined in 3 heifers fed the UREA+CM backgrounding diet (data not shown), and animal, management, and environmental inputs. View Large Table 1. Ingredients and nutrient composition of the barley silage-based backgrounding diets   Supplemental protein source2    UREA  UREA + CM  UREA + CGM + xSBM  Item  CP2:  12  14  14  14  Ingredient, % of DM      Barley silage3    53.6 ± 1.07  53.6 ± 1.09  53.6 ± 1.07  53.6 ± 1.10      Barley grain, dry rolled4    39.9 ± 0.92  39.9 ± 0.91  39.9 ± 0.92  39.9 ± 0.90      Supplement    6.51 ± 0.15  6.47 ± 0.19  6.57 ± 0.15  6.54 ± 0.20          Barley, ground    3.59  2.87  0.36  1.14          Urea    –  0.70  0.37  0.36          Canola meal    –  –  2.92  –          Soybean meal (xylose treated)5    –  –  –  1.17          Corn gluten meal    –  –  –  0.95          Limestone    1.47  1.45  1.46  1.46          Dicalcium phosphate    0.73  0.73  0.73  0.73          Salt    0.26  0.26  0.26  0.26          Trace mineral and vitamin premix6    0.07  0.07  0.07  0.07          Canola oil    0.12  0.12  0.12  0.12          Molasses    0.26  0.26  0.26  0.26          Rumensin premix7    0.01  0.01  0.01  0.01  Nutrient composition, % of DM unless noted otherwise      DM, % as fed    53.8 ± 1.01  52.8 ± 1.30  53.4 ± 1.32  53.0 ± 0.41      OM    93.2 ± 0.39  93.1 ± 0.35  92.4 ± 0.69  92.8 ± 0.17      CP    12.0 ± 0.67  14.2 ± 0.88  14.2 ± 0.48  14.3 ± 0.37      Starch    36.4 ± 2.74  38.3 ± 6.21  34.9 ± 2.69  36.2 ± 1.67      NDF    32.7 ± 1.78  33.1 ± 2.42  33.5 ± 2.15  33.0 ± 2.34      ADF    18.2 ± 1.73  18.9 ± 1.82  19.3 ± 1.50  18.8 ± 1.64      Crude fat    2.25  2.91  2.55  2.25      RDP8    6.18  8.04  7.70  7.29      Rumen NH3–N, % requirement8    54  74  69  62      Rumen peptide N, % requirement8    87  87  95  91      NEg, Mcal/kg DM8    0.83  0.83  0.82  0.83    Supplemental protein source2    UREA  UREA + CM  UREA + CGM + xSBM  Item  CP2:  12  14  14  14  Ingredient, % of DM      Barley silage3    53.6 ± 1.07  53.6 ± 1.09  53.6 ± 1.07  53.6 ± 1.10      Barley grain, dry rolled4    39.9 ± 0.92  39.9 ± 0.91  39.9 ± 0.92  39.9 ± 0.90      Supplement    6.51 ± 0.15  6.47 ± 0.19  6.57 ± 0.15  6.54 ± 0.20          Barley, ground    3.59  2.87  0.36  1.14          Urea    –  0.70  0.37  0.36          Canola meal    –  –  2.92  –          Soybean meal (xylose treated)5    –  –  –  1.17          Corn gluten meal    –  –  –  0.95          Limestone    1.47  1.45  1.46  1.46          Dicalcium phosphate    0.73  0.73  0.73  0.73          Salt    0.26  0.26  0.26  0.26          Trace mineral and vitamin premix6    0.07  0.07  0.07  0.07          Canola oil    0.12  0.12  0.12  0.12          Molasses    0.26  0.26  0.26  0.26          Rumensin premix7    0.01  0.01  0.01  0.01  Nutrient composition, % of DM unless noted otherwise      DM, % as fed    53.8 ± 1.01  52.8 ± 1.30  53.4 ± 1.32  53.0 ± 0.41      OM    93.2 ± 0.39  93.1 ± 0.35  92.4 ± 0.69  92.8 ± 0.17      CP    12.0 ± 0.67  14.2 ± 0.88  14.2 ± 0.48  14.3 ± 0.37      Starch    36.4 ± 2.74  38.3 ± 6.21  34.9 ± 2.69  36.2 ± 1.67      NDF    32.7 ± 1.78  33.1 ± 2.42  33.5 ± 2.15  33.0 ± 2.34      ADF    18.2 ± 1.73  18.9 ± 1.82  19.3 ± 1.50  18.8 ± 1.64      Crude fat    2.25  2.91  2.55  2.25      RDP8    6.18  8.04  7.70  7.29      Rumen NH3–N, % requirement8    54  74  69  62      Rumen peptide N, % requirement8    87  87  95  91      NEg, Mcal/kg DM8    0.83  0.83  0.82  0.83  1CGM = corn gluten meal; CM = canola meal; UREA= urea; xSBM = xylose-treated soybean meal. 2Dietary CP concentration, % of DM Mean ± SD, n = 4 periods. 3Contained on a DM basis: 39.9 ± 1.89% DM, 91.4 ± 0.44% OM, 10.4 ± 0.82% CP, 22.8 ± 2.62% starch, 49.2 ± 3.5% NDF, 31.3 ± 3.24% ADF, and 2.4% crude fat. 4Contained on a DM basis: 90.4 ± 0.52% DM, 97.0 ± 0.18% OM, 13.6 ± 1.75% CP, 58.2 ± 2.32% starch, 18.4 ± 1.59% NDF, 5.26 ± 0.46% ADF, and 1.50% crude fat. The processing index was 80 to 82% and was determined as the volume weight (0.5 L) of the grain after processing divided by the volume weight of whole barley grain before processing × 100% (0.5 L cup; Seedburo, Chicago, IL). 5SoyPass; LignoTech USA, Inc., Rothschild, WI. 6Provided per kilogram of diet DM: 70 mg/kg of Zn, 18 mg/kg of Cu, 23 mg/kg of Mn, 0.8 mg/kg of I, 0.35 mg/kg of Se, 0.24 mg/kg of Co, 12,000 IU/kg of vitamin A, 601 IU/kg of vitamin D, and 16.5 IU/kg of vitamin E. 7Provided monensin at 22 mg/kg diet. Premix contained monensin at 200 g/kg; Elanco Animal Health, Guelph, ON. 8Estimated using Cornell Net Carbohydrate and Protein System (CNCPS v6.1) with the measured nutrient composition of the barley silage and grain, in situ protein degradation characteristics for the barley silage, barley grain, and protein sources determined in 3 heifers fed the UREA+CM backgrounding diet (data not shown), and animal, management, and environmental inputs. View Large Feed Intake (Day 1 to 28) Barley silage and grain for the 4 heifers were mixed daily in a Data Ranger (American Calan, Northwood, NH). For each animal, the daily allotment of the silage and grain mixture was then weighed into a feed cart with a scale system (Model 1015; Avery Weigh-Tronix, Fairmont, MN), a preweighed amount of the supplement and digesta flow marker (d 1 to 15) were added, and the diet was thoroughly mixed by hand. The diets were fed twice daily in 2 equal portions at 0830 and 1800 h. Water was freely available from individual automatic bowls throughout the experiment. Feed offered and refused were recorded daily (0800 h) before feeding in the morning. During the first 5 d of the period, feed was offered in an amount to permit ad libitum consumption (minimum of 10% feed refusal). Samples of the feed and orts were collected daily for the first 3 d of the period and combined to determine DM percentage. Mean daily DMI for each heifer was determined for the first 5 d of the period. For the duration of the experiment (d 6 to 28), all heifers were offered 95% of the mean daily DMI of the heifer with the least intake. The DMI was restricted to equalize intake among heifers fed the dietary treatments. Samples of the TMR and orts (if present) were collected daily and combined for marked (d 5 to 15) and unmarked (d 16 to 28) samples for each period. Barley silage and grain were collected 1 d each week and combined for each period. Samples were reduced in size as required using a riffle splitter. The DM content of the feed (∼200 g) and orts samples was determined by drying in a forced-air oven at 55°C for 48 h. Samples were ground using a cutter mill through a 4 mm diam. screen and then through a 1 mm diam. screen (model 4 Wiley mill; Thomas Scientific, Swedesboro, NJ). Dried and ground feed and ort samples were stored at room temperature until analyzed for analytical DM, OM, N, NDF, ADF, starch, crude fat, and Yb (marked feed and orts). Microbial Protein Synthesis, Digesta Flow, and Ruminal and Total Tract Nutrient Digestibility (Day 11 to 15) Ruminal microbial protein synthesis was determined using purine bases as an internal microbial marker. Digesta flow and nutrient digestion in the rumen and total tract were determined by reference to Yb as an external digesta marker. Ytterbium is not absorbed from the gastrointestinal tract, has minimal effect on rate and extent of digestion in the amount applied to the feed, and produces similar estimates of DM flow at the duodenum and feces as other digesta flow markers when continuously infused and representative samples are collected (Siddons et al., 1985). A solution containing the marker was prepared daily by diluting 11 g YbCl3 3 N solution (1.73 g of Yb; Rhodia Rare Earth, Inc., Shelton, CT) in 500 mL of deionized water. Feed offered from d 1 to 15 was marked by spraying the solution on the daily allotment of feed for each heifer. On d 1, a prime dose equal to one-half of the daily amount of the marker (5.5 g YbCl3 solution in 1 L of deionized water) was administered to the rumen through the cannula of each heifer at the time that feed was offered in the morning. Collection of duodenal contents and feces began on d 11 of the digesta flow marker administration. Samples were collected from the duodenal cannula (300 mL duodenal contents) and rectum (100 g feces wet weight) every 6 h, moving ahead 1.5 h each day, to enable over a 5-d period (d 11 to 15) the collection of samples representative of the digesta flowing over a 24-h feeding cycle (collected every 1.5 h for a total of 16 samples). Samples were collected at 0800, 1400, and 2000 h on d 11; 0200, 0930, 1530, and 2130 h on d 12; 0330, 1100, 1700, and 2300 h on d 13; 0500, 1230, and 1830 h on d 14; and 0030 and 0630 h on d 15. The pH of the duodenal contents collected was measured using a pH meter to ensure that the pH was <3.0, indicating correct positioning of the cannula and the absence of any contaminating backflow of digesta. If the pH of the duodenal sample was >3.0, the sample was discarded and another sample was collected. Duodenal samples for each time point (300 mL) were composited by heifer and period as collected and immediately frozen (–20°C). Fecal samples for each time point (100 g) were composited by heifer and period as collected and dried at 55°C. The composited duodenal samples were later thawed and homogenized in a blender and a 50-mL subsample was collected. The subsample was centrifuged at 20,000 × g and 4°C for 15 min and the supernatant was stored frozen (–20°C) until analysis of NH3–N. The remainder of the homogenized duodenal sample was freeze-dried. The dried duodenal contents and feces were ground and stored as described for feed samples and analyzed for DM, OM, N, NDF, ADF, starch, purine bases, Yb, and AA (duodenal contents). For determination of microbial N flow to the duodenum, mixed ruminal bacteria (fluid and particle associated) were isolated from ruminal contents collected at 8 of the time points previously listed for duodenal and fecal sampling from d 11 to 15 to provide samples representative of every 3 h during the feeding cycle (0930, 1530, and 2130 h on d 12, 0330 h on d 13, 1230 and 1830 h on d 14, and 0030 and 0630 h on d 15). Ruminal contents (∼1 L) were collected from 4 sites within the rumen (reticulum, dorsal sac, ventral sac, and caudal sacs) and squeezed through polyester monofilament fabric (355-μm mesh opening PECAP; Sefar Canada, Ville St. Laurent, QC) to obtain the filtrate and particles. The particles (400 g wet weight) were combined with 600 mL chilled 0.9% saline (4°C), homogenized using a Waring blender (Waring Products Division, New Hartford, CT) on high speed for two 30-s periods to dislodge particulate-associated bacteria, and then squeezed through the polyester fabric. Recovery of particulate-associated bacteria ranges from 33 to 54% when particles are homogenized with chilled saline (Merry and McAllan, 1983; Olubobokun et al., 1988). The filtrates containing the fluid-associated microorganisms from the whole ruminal contents and the particulate-associated microorganisms from the homogenized particulates were combined for differential centrifugation and isolation of mixed ruminal bacteria. The combined filtrate (2 × 500 mL) was centrifuged (800 × g for 15 min at 4 °C) to remove feed particles and protozoa and then the supernatant was centrifuged (20,000 × g for 45 min at 4°C) to obtain the mixed ruminal bacterial pellet. Bacterial pellets from each time point were frozen (–20°C), freeze-dried, and ground using a mortar and pestle. The bacterial samples from each time point were then combined by equal dry weight by heifer and period, further ground using a ball mill, and stored at room temperature until analysis of analytical DM, OM, starch, N, and purine bases. The DMI of each heifer was calculated as the difference between the amount of diet DM offered and refused (if orts were present) from d 10 to 15 of each period. Nutrient intakes were calculated by multiplying the DM offered and refused by the nutrient composition of the TMR and orts samples collected during marker administration (d 5 to 15). Flow of DM to the proximal duodenum and DM output in feces were calculated by dividing the intake of Yb (determined as the difference between the amount of Yb in the feed offered and orts) by the Yb concentration in duodenal digesta and feces, respectively. Nutrient flow to the duodenum and output in feces were then calculated by multiplying the DM flow at each site by the nutrient concentration measured in the composited duodenal and fecal samples, respectively. Microbial N flow was calculated as the duodenal flow of purine base N multiplied by the ratio of total N to purine base N of ruminal bacteria. The duodenal flow of feed N (including endogenous N) was calculated by subtracting NH3–N and microbial N flow from the total N entering the intestine. True ruminal N digestion was then calculated as N intake minus duodenal feed N flow. Microbial OM and starch flow was calculated by multiplying microbial N flow by the ratio of the OM and starch, respectively, to N concentration in ruminal bacteria. True ruminal digestion of OM and starch were calculated by subtracting the respective microbial flow of the nutrient from the total duodenal flow of the nutrient and then subtracting the corrected duodenal flow from the nutrient intake. Apparent ruminal digestion of NDF was determined as the difference between NDF intake and duodenal flow. Apparent postruminal digestion of nutrients was calculated as the difference between the amount of each nutrient entering the intestine and appearing in the feces. Continuous Ruminal pH (Day 11 to 15) Ruminal pH of each heifer was monitored continuously for 4 consecutive days beginning on d 11 and continuing until d 15 using a continuous indwelling ruminal pH measurement system (Maekawa et al., 2002; Bevans et al., 2005). The pH system consisted of an industrial pH electrode (S650CDHF; Sensorex, Garden Grove, CA) connected to an external pH controller (Model PHCN-37; Omega Engineering, Inc., Stamford, CT) with a 9 m cable (PHEH-65-30-ATC; Omega Engineering, Inc.). The pH probes were removed for 30 min each day between 0800 and 0900 h for calibration of the electrodes using pH 4.0 and 7.0 standards. Ruminal pH was measured continuously every 5 s and averaged for 5-min intervals by a data controller (CR10X; Campbell Scientific Inc., Logan, UT). Data for ruminal pH were summarized for each heifer in each period as daily mean, minimum and maximum pH, time that pH was below 5.8 and 5.5, and area under the curve of pH values of 5.8 and 5.5. A ruminal pH of <5.8 served as an indicator of impaired fiber digestion (Russell and Wilson, 1996) and a pH of <5.5 of subclinical ruminal acidosis (Nagaraja and Titgemeyer, 2007). Nitrogen Digestibility, Route, and Chemical Form of Excretion (Day 18 to 23) Urine and feces were quantitatively collected for 5 consecutive 24-h periods at 0800 h (just before the 0830 h feeding) from d 18 to 23. No bedding was used during total collections. Urine was collected from the bladder using indwelling catheters (Bardex Lubricath 75 cc Foley catheter; C. R. Bard, Inc., Covington, GA) and directed through tubing into 20 L plastic buckets. Urine collected during the first 24 h was collected into buckets submersed in an ice slurry and the pH of the urine was measured. Urine collected for the remaining four 24-h periods was collected into buckets containing a sufficient quantity of acid (500 mL of 2 M H2S04) to reduce the pH of the urine to less than 2.5 to prevent microbial activity and volatilization of NH3. The daily volume of urine was recorded. A sample (100 mL) of the chilled urine was acidified with 5 mL of 2 M H2SO4. Samples of the acidified urine for each day were diluted (20 mL acidified urine diluted to 100 mL with deionized water) and frozen (–20°C) until analysis of total N, urea N, NH3–N, and purine derivatives to characterize the chemical forms of N excreted. Feces were collected in pans positioned behind the animals. The daily output of feces was weighed and mixed, and a sample (∼1 kg wet weight) was dried at 55°C in an oven for 48 to 72 h to determine DM. The dry fecal samples were ground and stored as described for the feed samples. Daily fecal samples were analyzed for DM and N. Dry matter intake was calculated for each heifer during total collection as the difference between the amount of diet DM offered and refused from d 17 to 22 of each period (beginning 1 d before the total collection of feces and urine). Nitrogen intake was calculated by multiplying the DM offered and refused by the N content of the unmarked TMR and orts samples collected from d 16 to 28. Apparent total tract N digestibility was calculated as the difference between the N intake and the amount of N appearing in the feces. Ruminal Fermentation Characteristics and Plasma Urea N (Day 26) Ruminal fermentation characteristics and plasma urea N (PUN) were measured at multiple time points (–1.5, –0.5, 1, 1.5, 2.5, 3.5, 5, 6.5, 8, and 9.5 h) relative to the time of the morning feeding on d 26. A temporary indwelling catheter was inserted into the jugular vein of each heifer on d 25. Blood (10 mL) was collected into sterile vacuum tubes containing an anticoagulant (lithium heparin, Vacutainer Brand; Becton Dickenson, Franklin Lakes, NJ). Tubes were centrifuged at 3,000 × g and 4°C for 20 min and the plasma was stored at –20°C until analysis. Ruminal contents (∼1 L) were collected from 4 sites within the rumen. A subsample (200 mL) of the contents was squeezed through 355 μm polyester fabric to obtain the filtrate. Ruminal contents remaining after removal of the subsample were returned to the rumen. Ten milliliters of the ruminal fluid filtrate were combined with 2 mL of 0.2 M H2SO4 and 10 mL of the filtrate were combined with 2 mL of 25% (wt/vol) metaphosphoric acid and stored at –20°C until analysis of NH3–N and VFA concentrations, respectively. Another 10 mL of the ruminal filtrate was combined with 2.5 mL of 50% (wt/vol) trichloroacetic acid and let stand for 30 min at 4°C to inhibit enzymatic activity and precipitate protein. The sample was then centrifuged at 27,000 × g and 4°C for 30 min and the supernatant stored at –20°C until analysis of free AA and peptides. Chemical Analysis The analysis of samples for DM, ash, NDF, ADF, starch, N, and crude fat were as described by Koenig et al. (2013). Ytterbium in feed and orts was determined by inductively coupled plasma emission spectroscopy (SpectoCirosCCD; Spectro Analytical Instruments, GmbH and Co., Kleve, KG, Germany) after dry ashing at 550°C in an muffle furnace for 6 h and extraction of the Yb into 0.32 M HNO3. Nucleic acid N in bacteria and duodenal digesta were determined according to the method of Zinn and Owens (1986). Amino acids were quantified in duodenal contents after liquid-phase acid hydrolysis and solid phase extraction and derivatization (EZ:faast; Phenomenex, Inc., Torrance, CA) by GLC (HP 5890; Hewlett Packard, Palo Alto, CA) with flame ionization detection (HP 7673A; Hewlett Packard). Ammonia N concentration in ruminal fluid and urine was determined using the salicylate-nitroferricyanide-hypochlorite procedure (USEPA, 1983; method 351.2) and a continuous flow colorimetric analyzer (Astoria 2 Analyzer; Astoria-Pacific, Inc., Clackamas, OR). Total free AA N in ruminal fluid was determined by the ninhydrin reaction (ASBC, 2004) using leucine for the calibration standards and adapted for a continuous flow analyzer. Correction for the NH3–N contribution to the ninhydrin reaction was determined from the ninhydrin reaction of the leucine and NH3–N calibration curves as described by Broderick and Kang (1980). Peptide N was determined as the increase in AA N after acid hydrolysis. Ruminal fluid was mixed with an equal volume of 12 M HCl and hydrolyzed under N2 at 110°C for 24 h. Hydrolysates were dried (SpeedVac Concentrator SVC-100H; Savant Instruments, Inc., Farmingdale, NY) and then dissolved in deionized water. Volatile fatty acid concentrations in ruminal fluid were quantified using crotonic acid as an internal standard by GLC (Model 5890; Hewlett Packard, Little Falls, DE) with a capillary column (30 m by 0.25 mm internal diam., 1 mm phase thickness, bonded polyethylene glycol, Supelco Nukol; Sigma-Aldrich Canada, Oakville, ON, Canada) and flame ionization detection. Urea N was determined in urine and plasma by reaction with diacetyl monoxime and colorimetric detection using a flow analyzer (method A332, Astoria 2 Analyzer). Uric acid in urine was determined using a colorimetric procedure (Uric Acid; Pointe Scientific, Inc., Canton, MI) with volumes adjusted accordingly for a microplate reader (Appliskan; Thermo Electron Corporation, Waltham, MA). Allantoin was measured in urine according to the procedure of Chen et al. (1990) using a flow analyzer. Statistical Analysis The data for route of N excretion, nutrient digestibility, and ruminal pH were analyzed as a 4 × 4 Latin square design using a mixed linear model (SAS Inst. Inc., Cary, NC) with dietary treatment as a fixed effect and heifer and period as random effects. The REML method was used for estimating the variance components, and the Kenward-Roger's option was used to adjust the degrees of freedom. Differences among the dietary treatments were compared using Fisher's protected LSD test (i.e., when P < 0.05 for the main effect of diet). A similar model was used for data for rumen fermentation characteristics and PUN with time included as a repeated measure. The covariance structure used for the model with repeated measures was compound symmetry or heterogeneous compound symmetry. The most appropriate covariance structure was selected based on the lowest Akaike's information criterion. When the treatment × time interaction was significant, tests of the simple effects were determined using the SLICE option in the LSMEANS statement. When no interaction was present between treatment and time, differences among the time points were determined using the protected LSD test. An experiment (Koenig and Beauchemin, 2013) designed as a 4 × 4 Latin square with 4 ruminally and duodenally cannulated beef heifers fed barley-based finishing diets with similar supplemental protein concentrations and sources as in the present experiment was conducted concurrently (the start of the experiment was offset by 2 wk from the present experiment). To determine if the amount of forage in the diet affects the route and chemical form of N excretion, select data were compared in beef heifers fed the backgrounding diets of the present study and finishing diets by analyzing the data as a double 4 × 4 Latin square design using a mixed linear model with phase, diet, and the diet × phase interaction as fixed effects and animal(phase) and period(phase) as random effects. Differences were declared significant at P ≤ 0.05 and trends were discussed at 0.05 < P < 0.10 for all models. RESULTS Nitrogen Digestibility and Excretion Heifers averaged 479 ± 15 kg BW during the experiment (P = 0.81; data not shown). Dry matter intake did not differ (P = 0.38; Table 2) among the beef heifers fed the backgrounding diets and averaged 7.98 ± 0.43 kg/d. Therefore, as intended, N intake was less (P < 0.05) for heifers fed the 12% CP diet compared with the 14% CP diets, with no differences (P > 0.05) in N intake among heifers fed the 14% CP diets. Table 2. Nitrogen intake, flow, and ruminal and total tract digestibility in beef heifers fed barley silage-based backgrounding diets varying in protein concentration and rumen degradability1   Supplemental protein source2:    UREA  UREA + CM  UREA + CGM + xSBM      Item  CP3:  12  14  14  14  SEM  P-value  DMI, kg/d    7.90  7.86  8.09  8.06  0.43  0.38  Nitrogen      Intake, g/d    152b  178a  184a  184a  9.9  0.002      Duodenal flow, g/d    142  143  142  150  9.0  0.34          NH3–N flow, g/d    5.04  5.52  5.44  5.65  0.45  0.75          Nonammonia N flow, g/d    136  138  137  145  8.9  0.35          Microbial N flow, g/d    105  106  101  112  5.4  0.34              Microbial N flow, % duodenal flow    74.2  74.1  71.4  75.0  2.70  0.52              Microbial efficiency, g/kg OMTFR4    22.1  21.9  21.3  21.1  1.47  0.48              Microbial N efficiency, % ruminally degraded N    81.3a  68.8b  63.9b  69.5b  4.02  0.002          Feed N flow5, g/d    31.7  31.8  35.6  32.7  5.64  0.55      True ruminal digestion6, g/d    115b  141a  143a  146a  7.8  0.005      True ruminal digestion6, % intake    75.8  78.9  77.9  79.5  2.65  0.35      Postruminal digestion, % intestine    68.5  72.0  71.1  71.3  1.36  0.37      Apparent total tract digestion, % intake    70.5b  77.3a  77.8a  76.5a  1.59  0.01    Supplemental protein source2:    UREA  UREA + CM  UREA + CGM + xSBM      Item  CP3:  12  14  14  14  SEM  P-value  DMI, kg/d    7.90  7.86  8.09  8.06  0.43  0.38  Nitrogen      Intake, g/d    152b  178a  184a  184a  9.9  0.002      Duodenal flow, g/d    142  143  142  150  9.0  0.34          NH3–N flow, g/d    5.04  5.52  5.44  5.65  0.45  0.75          Nonammonia N flow, g/d    136  138  137  145  8.9  0.35          Microbial N flow, g/d    105  106  101  112  5.4  0.34              Microbial N flow, % duodenal flow    74.2  74.1  71.4  75.0  2.70  0.52              Microbial efficiency, g/kg OMTFR4    22.1  21.9  21.3  21.1  1.47  0.48              Microbial N efficiency, % ruminally degraded N    81.3a  68.8b  63.9b  69.5b  4.02  0.002          Feed N flow5, g/d    31.7  31.8  35.6  32.7  5.64  0.55      True ruminal digestion6, g/d    115b  141a  143a  146a  7.8  0.005      True ruminal digestion6, % intake    75.8  78.9  77.9  79.5  2.65  0.35      Postruminal digestion, % intestine    68.5  72.0  71.1  71.3  1.36  0.37      Apparent total tract digestion, % intake    70.5b  77.3a  77.8a  76.5a  1.59  0.01  a,bWithin a row, means without a common superscript differ (P < 0.05, n = 16). c,dWithin a row, means without a common superscript differ (P < 0.10, n = 16). 1Determined using microbial and external markers for determining site and extent of digestion from d 11 to 15. 2CGM = corn gluten meal; CM = canola meal; UREA= urea; xSBM = xylose-treated soybean meal. 3Dietary CP concentration, % of DM. 4OMTFR = OM truly fermented in the rumen. 5Includes endogenous N flow. 6Corrected for microbial N flow. View Large Table 2. Nitrogen intake, flow, and ruminal and total tract digestibility in beef heifers fed barley silage-based backgrounding diets varying in protein concentration and rumen degradability1   Supplemental protein source2:    UREA  UREA + CM  UREA + CGM + xSBM      Item  CP3:  12  14  14  14  SEM  P-value  DMI, kg/d    7.90  7.86  8.09  8.06  0.43  0.38  Nitrogen      Intake, g/d    152b  178a  184a  184a  9.9  0.002      Duodenal flow, g/d    142  143  142  150  9.0  0.34          NH3–N flow, g/d    5.04  5.52  5.44  5.65  0.45  0.75          Nonammonia N flow, g/d    136  138  137  145  8.9  0.35          Microbial N flow, g/d    105  106  101  112  5.4  0.34              Microbial N flow, % duodenal flow    74.2  74.1  71.4  75.0  2.70  0.52              Microbial efficiency, g/kg OMTFR4    22.1  21.9  21.3  21.1  1.47  0.48              Microbial N efficiency, % ruminally degraded N    81.3a  68.8b  63.9b  69.5b  4.02  0.002          Feed N flow5, g/d    31.7  31.8  35.6  32.7  5.64  0.55      True ruminal digestion6, g/d    115b  141a  143a  146a  7.8  0.005      True ruminal digestion6, % intake    75.8  78.9  77.9  79.5  2.65  0.35      Postruminal digestion, % intestine    68.5  72.0  71.1  71.3  1.36  0.37      Apparent total tract digestion, % intake    70.5b  77.3a  77.8a  76.5a  1.59  0.01    Supplemental protein source2:    UREA  UREA + CM  UREA + CGM + xSBM      Item  CP3:  12  14  14  14  SEM  P-value  DMI, kg/d    7.90  7.86  8.09  8.06  0.43  0.38  Nitrogen      Intake, g/d    152b  178a  184a  184a  9.9  0.002      Duodenal flow, g/d    142  143  142  150  9.0  0.34          NH3–N flow, g/d    5.04  5.52  5.44  5.65  0.45  0.75          Nonammonia N flow, g/d    136  138  137  145  8.9  0.35          Microbial N flow, g/d    105  106  101  112  5.4  0.34              Microbial N flow, % duodenal flow    74.2  74.1  71.4  75.0  2.70  0.52              Microbial efficiency, g/kg OMTFR4    22.1  21.9  21.3  21.1  1.47  0.48              Microbial N efficiency, % ruminally degraded N    81.3a  68.8b  63.9b  69.5b  4.02  0.002          Feed N flow5, g/d    31.7  31.8  35.6  32.7  5.64  0.55      True ruminal digestion6, g/d    115b  141a  143a  146a  7.8  0.005      True ruminal digestion6, % intake    75.8  78.9  77.9  79.5  2.65  0.35      Postruminal digestion, % intestine    68.5  72.0  71.1  71.3  1.36  0.37      Apparent total tract digestion, % intake    70.5b  77.3a  77.8a  76.5a  1.59  0.01  a,bWithin a row, means without a common superscript differ (P < 0.05, n = 16). c,dWithin a row, means without a common superscript differ (P < 0.10, n = 16). 1Determined using microbial and external markers for determining site and extent of digestion from d 11 to 15. 2CGM = corn gluten meal; CM = canola meal; UREA= urea; xSBM = xylose-treated soybean meal. 3Dietary CP concentration, % of DM. 4OMTFR = OM truly fermented in the rumen. 5Includes endogenous N flow. 6Corrected for microbial N flow. View Large Total amount of N flowing to the duodenum and the flow of its constituents including NH3–N (P = 0.75) and non-NH3–N (P = 0.35), which was further subdivided into microbial N (P = 0.34) and feed N (P = 0.55), were similar among heifers fed the various dietary treatments (Table 2). Microbial N flow averaged 73.6 ± 2.70% of duodenal N flow (P = 0.52). Efficiency of use of rumen available energy for microbial protein synthesis did not differ (P = 0.48) among heifers fed the dietary treatments and averaged 21.6 ± 1.47 g/kg of OM fermented in the rumen. Efficiency of use of rumen available N for microbial protein synthesis was 81.3 ± 4.02% in heifers fed the 12% CP diet and was greater (P < 0.05) than in heifers fed the 14% CP diets, which averaged 67.4% with no differences in efficiency among heifers fed the supplemented diets. Supplementing the diet with an additional 1 percentage unit of CP from rumen undegradable CP sources (UREA+CGM+xSBM) did not increase (P = 0.55) the flow of feed N from the rumen to the duodenum. The duodenal flow of total (718 ± 51.6 g/d; P = 0.86) and essential (42.0 ± 1.38% of total AA; P = 0.45) AA were also not affected by the dietary treatments (data not shown). Flow of individual essential AA averaged (P ≥ 0.17; data not shown) His 6.6 ± 0.20%, Ile 13.1 ± 0.24%, Leu 19.8 ± 0.27%, Lys 17.3 ± 0.55%, Met 3.9 ± 0.12%, Phe 14.3 ± 0.67%, Thr 9.5 ± 0.54%, and Val 15.5 ± 0.21% of essential AA flow. The amount of N digested in the rumen was greater (P < 0.05; Table 2) in heifers fed the 14% CP diets than the 12% CP diet although when expressed as a percentage of N intake, N digestibility did not differ (P > 0.35) among the diets and averaged 78.0 ± 2.65%. Postruminal N digestion as a percentage of the N entering the intestine also did not differ (P = 0.37) among the diets and averaged 70.7 ± 1.36%. However, total tract digestion of N was greater (P < 0.05) for heifers fed the 14% CP (77.2 ± 1.59%) diets compared with the 12% CP (70.5%) diet. Dry matter intake during total fecal and urine collection did not differ (P = 0.16; Table 3) among heifers fed the backgrounding diets and averaged 7.81 ± 0.26 kg/d, which was in agreement with average DMI during the measurement of nutrient flow and site of digestion. There was also no effect of the dietary treatments on daily output of fecal DM (P = 0.69), urine volume (P = 0.35), and urine pH (P = 0.85). Nitrogen intake was greater (P < 0.05) for heifers fed the 14% CP diets compared with the 12% CP diet. There was no effect of diet on microbial N flow (P = 0.60) when estimated from the excretion of purine derivatives in urine and was in agreement with the estimate determined using purine bases as a microbial marker. Total tract N digestion was greater (P < 0.05) in heifers fed the 14% CP diets with UREA+CM and UREA+CGM+xSBM than the 12% CP diet and was in agreement with total tract N digestion determined using digesta flow markers. Total tract digestion was intermediate for the 14% CP UREA diet. The dietary treatments had no effect (P = 0.22) on N balance, which averaged 28.5 ± 6.04% of N intake. Table 3. Route and chemical form of nitrogen excretion in beef heifers fed barley-silage based backgrounding diets varying in protein concentration and rumen degradability1   Supplemental protein source2:    UREA  UREA + CM  UREA + CGM + xSBM      Item  CP3:  12  14  14  14  SEM  P-value  DMI, kg/d    8.08  7.78  7.88  7.51  0.26  0.16  Fecal DM, kg/d    2.20  2.22  2.22  2.11  0.10  0.69  Urine volume, L/d    6.71  7.84  8.20  6.29  1.09  0.35  Urine pH    7.35  7.78  7.13  7.46  0.52  0.85  Nitrogen      Intake, g/d    154b  168a  174a  168a  6.93  0.03      Microbial N flow, g/d    92.0  99.7  89.5  99.3  10.0  0.60      Apparent total tract digestion, % intake    69.4b  71.9ab  73.0a  74.1a  1.08  0.02      Balance, % intake    30.5  25.9  28.8  28.7  6.04  0.22      Total N output, g/d    107  125  122  118  10.6  0.13          Fecal N, g/d    47.1  47.1  46.9  43.1  2.11  0.15          Fecal N, % intake    30.6a  28.1ab  27.0b  25.9b  1.08  0.02          Fecal N, % total N output    44.2a  38.6bc  40.4b  37.1c  2.88  0.007              Nucleic acid N, % fecal N    6.88  7.43  9.83  12.4  3.57  0.68          Urine N, g/d    60.4f  77.9e  74.4e  74.8e  9.20  0.06          Urine N, % intake    38.9b  46.2a  43.5ab  45.5a  5.50  0.03          Urine N, % total N output    55.8c  61.4ab  59.6b  62.9a  2.88  0.007              Urea N, g/d    45.3b  61.5a  61.2a  60.2a  6.55  0.01              Urea N, % urine N    75.5d  79.6c  81.2b  83.4a  1.74  <0.001              Urea N, % total N output    42.0c  48.6b  49.1b  51.7a  1.58  <0.001              Ammonia N, % urine N    1.42  1.33  1.03  0.95  0.55  0.48              Allantoin N, % urine N    12.4b  10.2a  9.42a  9.64a  1.28  0.03              Uric acid N, % urine N    0.73a  0.60b  0.63b  0.49c  0.10  0.003    Supplemental protein source2:    UREA  UREA + CM  UREA + CGM + xSBM      Item  CP3:  12  14  14  14  SEM  P-value  DMI, kg/d    8.08  7.78  7.88  7.51  0.26  0.16  Fecal DM, kg/d    2.20  2.22  2.22  2.11  0.10  0.69  Urine volume, L/d    6.71  7.84  8.20  6.29  1.09  0.35  Urine pH    7.35  7.78  7.13  7.46  0.52  0.85  Nitrogen      Intake, g/d    154b  168a  174a  168a  6.93  0.03      Microbial N flow, g/d    92.0  99.7  89.5  99.3  10.0  0.60      Apparent total tract digestion, % intake    69.4b  71.9ab  73.0a  74.1a  1.08  0.02      Balance, % intake    30.5  25.9  28.8  28.7  6.04  0.22      Total N output, g/d    107  125  122  118  10.6  0.13          Fecal N, g/d    47.1  47.1  46.9  43.1  2.11  0.15          Fecal N, % intake    30.6a  28.1ab  27.0b  25.9b  1.08  0.02          Fecal N, % total N output    44.2a  38.6bc  40.4b  37.1c  2.88  0.007              Nucleic acid N, % fecal N    6.88  7.43  9.83  12.4  3.57  0.68          Urine N, g/d    60.4f  77.9e  74.4e  74.8e  9.20  0.06          Urine N, % intake    38.9b  46.2a  43.5ab  45.5a  5.50  0.03          Urine N, % total N output    55.8c  61.4ab  59.6b  62.9a  2.88  0.007              Urea N, g/d    45.3b  61.5a  61.2a  60.2a  6.55  0.01              Urea N, % urine N    75.5d  79.6c  81.2b  83.4a  1.74  <0.001              Urea N, % total N output    42.0c  48.6b  49.1b  51.7a  1.58  <0.001              Ammonia N, % urine N    1.42  1.33  1.03  0.95  0.55  0.48              Allantoin N, % urine N    12.4b  10.2a  9.42a  9.64a  1.28  0.03              Uric acid N, % urine N    0.73a  0.60b  0.63b  0.49c  0.10  0.003  a−dWithin a row, means without a common superscript differ (P < 0.05, n = 16). e,fWithin a row, means without a common superscript differ (P < 0.10, n = 16). 1Determined during the total collection of urine and feces from d 18 to 23. 2CGM = corn gluten meal; CM = canola meal; UREA= urea; xSBM = xylose-treated soybean meal. 3Dietary CP concentration, % of DM. View Large Table 3. Route and chemical form of nitrogen excretion in beef heifers fed barley-silage based backgrounding diets varying in protein concentration and rumen degradability1   Supplemental protein source2:    UREA  UREA + CM  UREA + CGM + xSBM      Item  CP3:  12  14  14  14  SEM  P-value  DMI, kg/d    8.08  7.78  7.88  7.51  0.26  0.16  Fecal DM, kg/d    2.20  2.22  2.22  2.11  0.10  0.69  Urine volume, L/d    6.71  7.84  8.20  6.29  1.09  0.35  Urine pH    7.35  7.78  7.13  7.46  0.52  0.85  Nitrogen      Intake, g/d    154b  168a  174a  168a  6.93  0.03      Microbial N flow, g/d    92.0  99.7  89.5  99.3  10.0  0.60      Apparent total tract digestion, % intake    69.4b  71.9ab  73.0a  74.1a  1.08  0.02      Balance, % intake    30.5  25.9  28.8  28.7  6.04  0.22      Total N output, g/d    107  125  122  118  10.6  0.13          Fecal N, g/d    47.1  47.1  46.9  43.1  2.11  0.15          Fecal N, % intake    30.6a  28.1ab  27.0b  25.9b  1.08  0.02          Fecal N, % total N output    44.2a  38.6bc  40.4b  37.1c  2.88  0.007              Nucleic acid N, % fecal N    6.88  7.43  9.83  12.4  3.57  0.68          Urine N, g/d    60.4f  77.9e  74.4e  74.8e  9.20  0.06          Urine N, % intake    38.9b  46.2a  43.5ab  45.5a  5.50  0.03          Urine N, % total N output    55.8c  61.4ab  59.6b  62.9a  2.88  0.007              Urea N, g/d    45.3b  61.5a  61.2a  60.2a  6.55  0.01              Urea N, % urine N    75.5d  79.6c  81.2b  83.4a  1.74  <0.001              Urea N, % total N output    42.0c  48.6b  49.1b  51.7a  1.58  <0.001              Ammonia N, % urine N    1.42  1.33  1.03  0.95  0.55  0.48              Allantoin N, % urine N    12.4b  10.2a  9.42a  9.64a  1.28  0.03              Uric acid N, % urine N    0.73a  0.60b  0.63b  0.49c  0.10  0.003    Supplemental protein source2:    UREA  UREA + CM  UREA + CGM + xSBM      Item  CP3:  12  14  14  14  SEM  P-value  DMI, kg/d    8.08  7.78  7.88  7.51  0.26  0.16  Fecal DM, kg/d    2.20  2.22  2.22  2.11  0.10  0.69  Urine volume, L/d    6.71  7.84  8.20  6.29  1.09  0.35  Urine pH    7.35  7.78  7.13  7.46  0.52  0.85  Nitrogen      Intake, g/d    154b  168a  174a  168a  6.93  0.03      Microbial N flow, g/d    92.0  99.7  89.5  99.3  10.0  0.60      Apparent total tract digestion, % intake    69.4b  71.9ab  73.0a  74.1a  1.08  0.02      Balance, % intake    30.5  25.9  28.8  28.7  6.04  0.22      Total N output, g/d    107  125  122  118  10.6  0.13          Fecal N, g/d    47.1  47.1  46.9  43.1  2.11  0.15          Fecal N, % intake    30.6a  28.1ab  27.0b  25.9b  1.08  0.02          Fecal N, % total N output    44.2a  38.6bc  40.4b  37.1c  2.88  0.007              Nucleic acid N, % fecal N    6.88  7.43  9.83  12.4  3.57  0.68          Urine N, g/d    60.4f  77.9e  74.4e  74.8e  9.20  0.06          Urine N, % intake    38.9b  46.2a  43.5ab  45.5a  5.50  0.03          Urine N, % total N output    55.8c  61.4ab  59.6b  62.9a  2.88  0.007              Urea N, g/d    45.3b  61.5a  61.2a  60.2a  6.55  0.01              Urea N, % urine N    75.5d  79.6c  81.2b  83.4a  1.74  <0.001              Urea N, % total N output    42.0c  48.6b  49.1b  51.7a  1.58  <0.001              Ammonia N, % urine N    1.42  1.33  1.03  0.95  0.55  0.48              Allantoin N, % urine N    12.4b  10.2a  9.42a  9.64a  1.28  0.03              Uric acid N, % urine N    0.73a  0.60b  0.63b  0.49c  0.10  0.003  a−dWithin a row, means without a common superscript differ (P < 0.05, n = 16). e,fWithin a row, means without a common superscript differ (P < 0.10, n = 16). 1Determined during the total collection of urine and feces from d 18 to 23. 2CGM = corn gluten meal; CM = canola meal; UREA= urea; xSBM = xylose-treated soybean meal. 3Dietary CP concentration, % of DM. View Large The concentration and degradability of the supplemental protein sources in the backgrounding diets had no effect on total N (P = 0.13) and fecal N (P = 0.15) output of beef heifers although fecal N as a percentage of total N output was greater (P < 0.05) for heifers fed the 12% CP than the 14% CP diets (Table 3). There was no effect (P = 0.68) of the diets on nucleic acid N output in feces, which averaged 9.15 ± 3.57% of fecal N. Urine N output tended (P < 0.10) to be greater for heifers fed the 14% CP diets. In contrast to what was observed for the fecal N, urine N output expressed as a percentage of total N output was reduced (P < 0.05) for the 12% CP diet compared with the 14% CP diets. Greater urinary N output in heifers fed the 14% CP diets reflected greater (P < 0.05) urea N excretion in heifers fed the 14% CP diets compared with the 12% CP diet. As a percentage of the urine N output, urea N represented 75.5% of the N in urine of heifers fed the 12% CP diet and 79.6% to 83.4% in urine of heifers fed the 14% CP diets (SEM = 1.74%; P < 0.05). Urinary urea N accounted for 42% of the total N output in heifers fed the 12% CP diet and increased to 48.6 to 51.7% for the 14% CP diets (SEM = 1.58%; P < 0.05). Excretion of NH3–N accounted for only 1.2 ± 0.55% of the urinary N output and was not affected (P = 0.48) by the amount and degradability of the protein of the diet. Urinary N output was less for heifers fed the low CP diet; therefore, allantoin and uric acid N as a percentage of the total urine N output was greater (P < 0.05) for the 12% CP diet compared with the 14% CP diets. Nutrient Digestion Organic matter (P = 0.60) and starch (P = 0.59) intake were similar among heifers fed the 12 and 14% CP diets (Table 4). The NDF concentrations of the diets were similar (Table 1) although the intake of NDF tended (P < 0.10) to be greater for heifers fed the 14% CP diets supplemented with UREA+CM and UREA+CGM+xSBM compared with the 12% CP. Ruminal digestion of OM as a percentage of intake was greater (P < 0.05) for heifers fed the 14% CP diet with UREA+CGM+xSBM compared with heifers fed the 12% CP and 14% CP with UREA+CM and was intermediate for heifers fed the 14% CP diet with UREA. There were, however, no differences among the heifers fed the dietary treatments for ruminal digestion of starch (P = 0.50) and NDF (P = 0.69), which averaged 72.3 ± 3.24% and 46.1 ± 3.01% of intake, respectively. Total tract digestion of OM (P = 0.21) and starch (P = 0.16) averaged 77.7 ± 1.27% and 93.9 ± 1.01%, respectively. Total tract digestion of NDF was less (P < 0.05) in heifers fed the 12% CP diet, intermediate in heifers fed the UREA and UREA+CGM+xSBM and greater for heifers fed the UREA+CM diet. Table 4. Nutrient intake and ruminal and total tract digestibility in beef heifers fed barley silage-based backgrounding diets varying in protein concentration and rumen degradability1   Supplemental protein source2    UREA  UREA + CM  UREA + CGM + xSBM      Item  CP3:  12  14  14  14  SEM  P-value  Intake, kg/d      OM    7.36  7.31  7.46  7.49  0.42  0.60      Starch    2.94  3.09  2.85  2.93  0.30  0.59      NDF    2.53e  2.55de  2.69c  2.65cd  0.09  0.08  Ruminal digestion, % intake      OM4    65.0b  67.3ab  64.4b  70.9a  2.60  0.04      Starch4    72.7  70.8  67.5  78.0  3.24  0.11      NDF    44.8  48.2  45.3  45.9  3.01  0.78  Apparent total tract digestion, % intake      OM    76.0  77.8  78.8  78.1  1.27  0.21      Starch    92.3  93.9  94.6  94.7  1.01  0.16      NDF    56.8b  61.4ab  64.6a  61.4ab  2.19  0.04    Supplemental protein source2    UREA  UREA + CM  UREA + CGM + xSBM      Item  CP3:  12  14  14  14  SEM  P-value  Intake, kg/d      OM    7.36  7.31  7.46  7.49  0.42  0.60      Starch    2.94  3.09  2.85  2.93  0.30  0.59      NDF    2.53e  2.55de  2.69c  2.65cd  0.09  0.08  Ruminal digestion, % intake      OM4    65.0b  67.3ab  64.4b  70.9a  2.60  0.04      Starch4    72.7  70.8  67.5  78.0  3.24  0.11      NDF    44.8  48.2  45.3  45.9  3.01  0.78  Apparent total tract digestion, % intake      OM    76.0  77.8  78.8  78.1  1.27  0.21      Starch    92.3  93.9  94.6  94.7  1.01  0.16      NDF    56.8b  61.4ab  64.6a  61.4ab  2.19  0.04  a,bWithin a row, means without a common superscript differ (P < 0.05, n = 16). c−eWithin a row, means without a common superscript differ (P < 0.10, n = 16). 1Determined using microbial and external markers for determining site and extent of digestion from d 11 to 15. 2CGM = corn gluten meal; CM = canola meal; UREA= urea; xSBM = xylose-treated soybean meal. 3Dietary CP concentration, % of DM. 4Corrected for microbial flow. View Large Table 4. Nutrient intake and ruminal and total tract digestibility in beef heifers fed barley silage-based backgrounding diets varying in protein concentration and rumen degradability1   Supplemental protein source2    UREA  UREA + CM  UREA + CGM + xSBM      Item  CP3:  12  14  14  14  SEM  P-value  Intake, kg/d      OM    7.36  7.31  7.46  7.49  0.42  0.60      Starch    2.94  3.09  2.85  2.93  0.30  0.59      NDF    2.53e  2.55de  2.69c  2.65cd  0.09  0.08  Ruminal digestion, % intake      OM4    65.0b  67.3ab  64.4b  70.9a  2.60  0.04      Starch4    72.7  70.8  67.5  78.0  3.24  0.11      NDF    44.8  48.2  45.3  45.9  3.01  0.78  Apparent total tract digestion, % intake      OM    76.0  77.8  78.8  78.1  1.27  0.21      Starch    92.3  93.9  94.6  94.7  1.01  0.16      NDF    56.8b  61.4ab  64.6a  61.4ab  2.19  0.04    Supplemental protein source2    UREA  UREA + CM  UREA + CGM + xSBM      Item  CP3:  12  14  14  14  SEM  P-value  Intake, kg/d      OM    7.36  7.31  7.46  7.49  0.42  0.60      Starch    2.94  3.09  2.85  2.93  0.30  0.59      NDF    2.53e  2.55de  2.69c  2.65cd  0.09  0.08  Ruminal digestion, % intake      OM4    65.0b  67.3ab  64.4b  70.9a  2.60  0.04      Starch4    72.7  70.8  67.5  78.0  3.24  0.11      NDF    44.8  48.2  45.3  45.9  3.01  0.78  Apparent total tract digestion, % intake      OM    76.0  77.8  78.8  78.1  1.27  0.21      Starch    92.3  93.9  94.6  94.7  1.01  0.16      NDF    56.8b  61.4ab  64.6a  61.4ab  2.19  0.04  a,bWithin a row, means without a common superscript differ (P < 0.05, n = 16). c−eWithin a row, means without a common superscript differ (P < 0.10, n = 16). 1Determined using microbial and external markers for determining site and extent of digestion from d 11 to 15. 2CGM = corn gluten meal; CM = canola meal; UREA= urea; xSBM = xylose-treated soybean meal. 3Dietary CP concentration, % of DM. 4Corrected for microbial flow. View Large Ruminal pH and Fermentation Characteristics and Plasma Urea N Maximum ruminal pH was lowest (P < 0.05) in heifers fed the UREA diet, but otherwise, there were no effects of protein concentration and degradability in the background diets (data not shown) on ruminal pH, including mean pH (6.24 ± 0.10; P = 0.23), time below pH of 5.8 (217 ± 90 min/d; P = 0.53) and 5.5 (81 ± 49 min/d; P = 0.35), and area under the curve of pH 5.8 [3,431 ± 1,863 (min × pH)/d; P = 0.42] and 5.5 [883 ± 648 (min × pH)/d; P = 0.26]. Ruminal NH3–N concentration was greater (P < 0.05; Table 5) in heifers fed the backgrounding diets containing 14% CP with UREA (172 mg N/L) and UREA+CGM+xSBM (161 mg N/L), intermediate for those fed the UREA+CM (139 mg N/L), and least for heifers fed the low 12% CP diet (118 mg N/L; SEM = 23.3). Ruminal NH3–N peaked 1 h after feeding at concentrations >290 mg N/L and then declined to concentrations of <50 mg N/L for heifers fed the 12% CP diet and <125 mg N/L for the 14% CP diets for ≥4 h, between 5 to 9.5 h after the morning feeding (P < 0.05; Fig.1; Table 5). Ruminal peptide N concentration was not affected (P = 0.62) by concentration and degradability of the protein and averaged 164 ± 32.9 mg N/L. For all of the backgrounding diets, ruminal peptide N peaked at 1 to 1.5 h after feeding and then steadily declined to concentrations below that observed before the morning feeding (P < 0.05). Ruminal AA concentration gradually increased (P < 0.05) in concentration to 8.8 mg N/L at 6.5 h after feeding, but the concentration was relatively low and averaged 3.37 ± 2.93 mg N/L. Figure 1. View largeDownload slide Ruminal NH3–N, peptide N, free AA N (AA-N), and plasma urea N concentration in beef cattle fed barley-based backgrounding diets varying in protein concentration and rumen degradability. Supplemental protein sources: CGM = corn gluten meal, CM = canola meal, xSBM = xylose-treated soybean meal. (SEM: ruminal NH3–N = 23 mg N/L, peptide N = 33 mg N/L, free AA-N = 2.9 mg N/L, and plasma urea N = 9 mg N/L; n = 16). Figure 1. View largeDownload slide Ruminal NH3–N, peptide N, free AA N (AA-N), and plasma urea N concentration in beef cattle fed barley-based backgrounding diets varying in protein concentration and rumen degradability. Supplemental protein sources: CGM = corn gluten meal, CM = canola meal, xSBM = xylose-treated soybean meal. (SEM: ruminal NH3–N = 23 mg N/L, peptide N = 33 mg N/L, free AA-N = 2.9 mg N/L, and plasma urea N = 9 mg N/L; n = 16). Table 5. Ruminal pH, ruminal N, and volatile fatty acid concentration and plasma urea N in beef heifers fed barley silage-based backgrounding diets varying in protein concentration and rumen degradability   Supplemental protein source1    UREA  UREA + CM  UREA + CGM + xSBM    P-value  Item  CP2:  12  14  14  14  SEM  Diet  Time  Diet × time  Ruminal N, mg N/L      NH3–N    118b  172a  139ab  161a  23.3  0.04  < 0.001  0.29      Peptide N    154  165  169  171  32.9  0.62  < 0.001  0.39      AA N    4.88  1.96  2.51  4.15  2.93  0.18  0.02  0.82  Time below NH3–N threshold, min/12 h      125 mg N/L    429a  256b  343ab  327ab  56  0.04  –  –      50 mg N/L    235a  72b  82b  180ab  71  0.03  –  –  VFA      Total VFA, mM    136  133  137  127  6.0  0.43  0.003  0.71  Individual VFA, mol/100 mol      Acetic acid    60.8a  61.1a  56.9b  60.7a  2.36  0.02  0.79  0.94      Propionic acid    24.1b  22.6b  29.1a  21.0b  2.43  0.02  0.02  0.79      Butyric acid    10.2b  11.3ab  9.62b  13.2a  0.95  0.05  0.32  0.005      Branched chain acids    3.28  3.20  2.86  3.36  0.27  0.21  0.008  0.10      Acetate:propionate ratio    2.75a  2.80a  2.04b  2.95a  0.32  0.046  0.07  0.99  Plasma urea N, mg N/L    96.7b  128a  117ab  123a  9.1  0.04  <0.001  0.90    Supplemental protein source1    UREA  UREA + CM  UREA + CGM + xSBM    P-value  Item  CP2:  12  14  14  14  SEM  Diet  Time  Diet × time  Ruminal N, mg N/L      NH3–N    118b  172a  139ab  161a  23.3  0.04  < 0.001  0.29      Peptide N    154  165  169  171  32.9  0.62  < 0.001  0.39      AA N    4.88  1.96  2.51  4.15  2.93  0.18  0.02  0.82  Time below NH3–N threshold, min/12 h      125 mg N/L    429a  256b  343ab  327ab  56  0.04  –  –      50 mg N/L    235a  72b  82b  180ab  71  0.03  –  –  VFA      Total VFA, mM    136  133  137  127  6.0  0.43  0.003  0.71  Individual VFA, mol/100 mol      Acetic acid    60.8a  61.1a  56.9b  60.7a  2.36  0.02  0.79  0.94      Propionic acid    24.1b  22.6b  29.1a  21.0b  2.43  0.02  0.02  0.79      Butyric acid    10.2b  11.3ab  9.62b  13.2a  0.95  0.05  0.32  0.005      Branched chain acids    3.28  3.20  2.86  3.36  0.27  0.21  0.008  0.10      Acetate:propionate ratio    2.75a  2.80a  2.04b  2.95a  0.32  0.046  0.07  0.99  Plasma urea N, mg N/L    96.7b  128a  117ab  123a  9.1  0.04  <0.001  0.90  a,bWithin a row, means without a common superscript differ (P < 0.05, n = 16). 1CGM = corn gluten meal; CM = canola meal; UREA= urea; xSBM = xylose-treated soybean meal. 2Dietary CP concentration, % of DM. View Large Table 5. Ruminal pH, ruminal N, and volatile fatty acid concentration and plasma urea N in beef heifers fed barley silage-based backgrounding diets varying in protein concentration and rumen degradability   Supplemental protein source1    UREA  UREA + CM  UREA + CGM + xSBM    P-value  Item  CP2:  12  14  14  14  SEM  Diet  Time  Diet × time  Ruminal N, mg N/L      NH3–N    118b  172a  139ab  161a  23.3  0.04  < 0.001  0.29      Peptide N    154  165  169  171  32.9  0.62  < 0.001  0.39      AA N    4.88  1.96  2.51  4.15  2.93  0.18  0.02  0.82  Time below NH3–N threshold, min/12 h      125 mg N/L    429a  256b  343ab  327ab  56  0.04  –  –      50 mg N/L    235a  72b  82b  180ab  71  0.03  –  –  VFA      Total VFA, mM    136  133  137  127  6.0  0.43  0.003  0.71  Individual VFA, mol/100 mol      Acetic acid    60.8a  61.1a  56.9b  60.7a  2.36  0.02  0.79  0.94      Propionic acid    24.1b  22.6b  29.1a  21.0b  2.43  0.02  0.02  0.79      Butyric acid    10.2b  11.3ab  9.62b  13.2a  0.95  0.05  0.32  0.005      Branched chain acids    3.28  3.20  2.86  3.36  0.27  0.21  0.008  0.10      Acetate:propionate ratio    2.75a  2.80a  2.04b  2.95a  0.32  0.046  0.07  0.99  Plasma urea N, mg N/L    96.7b  128a  117ab  123a  9.1  0.04  <0.001  0.90    Supplemental protein source1    UREA  UREA + CM  UREA + CGM + xSBM    P-value  Item  CP2:  12  14  14  14  SEM  Diet  Time  Diet × time  Ruminal N, mg N/L      NH3–N    118b  172a  139ab  161a  23.3  0.04  < 0.001  0.29      Peptide N    154  165  169  171  32.9  0.62  < 0.001  0.39      AA N    4.88  1.96  2.51  4.15  2.93  0.18  0.02  0.82  Time below NH3–N threshold, min/12 h      125 mg N/L    429a  256b  343ab  327ab  56  0.04  –  –      50 mg N/L    235a  72b  82b  180ab  71  0.03  –  –  VFA      Total VFA, mM    136  133  137  127  6.0  0.43  0.003  0.71  Individual VFA, mol/100 mol      Acetic acid    60.8a  61.1a  56.9b  60.7a  2.36  0.02  0.79  0.94      Propionic acid    24.1b  22.6b  29.1a  21.0b  2.43  0.02  0.02  0.79      Butyric acid    10.2b  11.3ab  9.62b  13.2a  0.95  0.05  0.32  0.005      Branched chain acids    3.28  3.20  2.86  3.36  0.27  0.21  0.008  0.10      Acetate:propionate ratio    2.75a  2.80a  2.04b  2.95a  0.32  0.046  0.07  0.99  Plasma urea N, mg N/L    96.7b  128a  117ab  123a  9.1  0.04  <0.001  0.90  a,bWithin a row, means without a common superscript differ (P < 0.05, n = 16). 1CGM = corn gluten meal; CM = canola meal; UREA= urea; xSBM = xylose-treated soybean meal. 2Dietary CP concentration, % of DM. View Large There was no effect (Table 5; P = 0.43) of the dietary treatments on ruminal total VFA concentration. In the first hour after feeding, ruminal VFA concentrations remained relatively stable and then increased (P < 0.05) up to 5 to 7 h after feeding before returning to the concentration observed before the morning feeding. There was, however, an effect of diet on the molar proportion of individual VFA. For cattle fed UREA+CM, the molar proportion of acetate was lower and the molar proportion of propionate was greater, and therefore, the ratio of acetate to propionate was less than in cattle fed the other dietary treatments (P < 0.05). There were minor changes (P < 0.05) in ruminal butyrate concentration over time in cattle fed the protein supplemented diets, but the concentration in cattle fed the control diet was stable. Before feeding and from 5 to 10 h after feeding the molar proportion of butyrate was greater for UREA+CGM+xSBM, intermediate for UREA, and lower for UREA+CM and 12% CP diets. There was no effect (P = 0.21) of the diets on the molar proportion of branched chain acids (isobutyric and isovaleric acid). The PUN concentration was decreased (P < 0.05; Fig.1) in heifers fed 12% CP (96.7 mg N/L) compared with those fed 14% CP with UREA (128 mg N/L) and UREA+CGM+xSBM (123 mg N/L) and was intermediate for UREA+CM (117 mg N/L; SEM = 9.1). The PUN concentration increased gradually reaching its greatest concentration at 1.5 to 3.5 h after feeding. DISCUSSION Route and Chemical Form of N Excretion and Potential for Volatilization as Ammonia-N Drying fecal samples can result in the loss of volatile N and consequently an underestimation of total fecal N excreted. The reduction in Kjeldahl N (organic N and ammonia N) concentration of oven and freeze dried fecal samples can be as high as 15% compared with wet samples (Spanghero and Kowaski, 1997). The potential N loss upon drying primarily reflects the volatilization of NH3–N from fresh feces. In dairy cattle fed diets ranging in CP concentration from 9 to 21% and providing a wide range of N intakes, NH3–N concentration ranged from 3 to 14.4% of total fecal N (Marini and Van Amburgh, 2005; Misselbrook et al., 2005). Based on the results of these studies, fecal N from the beef heifers fed the 12 to 14% CP diets was likely underestimated due to a loss of volatile NH3–N upon drying before N analysis. The potential loss of NH3–N from manure depends on the amount and chemical form of N in feces and urine. Increasing dietary CP concentration and N intake increases the total amount of N in manure by primarily increasing the amount and proportion of urea N excreted in urine (Huntington et al., 2001; Marini and Van Amburgh, 2005; Vasconcelos et al., 2009). Increasing the CP concentration of the backgrounding diets over the relatively narrow range of 12 to 14% increased N intake, but total N output was not affected. Fecal N excretion in heifers was also unaffected by dietary CP concentration and N intake. Fecal N is primarily of microbial origin with lesser amounts of undegraded feed protein and endogenous secretions (NRC, 1985). True digestibility of feed protein in the total tract is typically high (85 to 95%; NRC, 1985) unless the protein is heat damaged or bound in other unavailable forms (e.g., lignin, tannin-protein complexes). Fecal N excretion can also be influenced by the site and extent of carbohydrate fermentation (Bierman et al., 1999; Adams et al., 2004). A greater proportion of dietary N intake (27.9 vs. 19.9%) and total N output (40.1 vs. 31.2%) were excreted in feces by heifers fed the forage-based backgrounding diets than the grain-based finishing diets (Koenig and Beauchemin, 2013), respectively. Decreased ruminal OM degradability, a trend towards greater postruminal flow of NDF, and greater fecal output of OM, NDF, and nucleic acid N indicated greater microbial fermentation and microbial N contribution to fecal N in the lower tract of heifers fed the forage-based diets (33.1% NDF) compared with the grain-based diets (20.4% NDF). When the dietary fiber content of a corn grain-based (9.9% NDF) diet was increased by substitution with 7.5% roughage (13.6% NDF) or 42.5% wet corn gluten meal (28.5% NDF), the route of N excretion was shifted from urine to feces, and fecal N excretion increased from 33% to an average of 53% of N output in feedlot cattle (Bierman et al., 1999). Urine was the major route of N excretion in the beef heifers. Feeding 12% CP compared with 14% CP diets reduced the amount and proportion of N excreted in urine from 45.1 to 38.9% of N intake and from 61.3 to 55.8% of total N output. Except at very low dietary N intakes, urea N is the major form of N in urine and ranges between 60 and 95% of total urine N in cattle (Bristow et al., 1992; Huntington et al., 2001; Cole et al., 2005; Vasconcelos et al., 2009). Urinary urea N excretion in heifers was reduced by feeding 12% CP compared with 14% CP backgrounding diets (75.4 vs. 81.4% of urine N, respectively) and was consistent with the reduced plasma urea N concentration in cattle fed the low CP diets (97 vs. 123 mg N/L, respectively). Route of N excretion and PUN concentration were determined from d 18 to 23 and d 26, respectively, of each period, but the length of time for adaptation from a high to a low CP diet (and vice versa) and the mechanisms controlling the transfer of urea from plasma to the rumen for use by ruminal microorganisms instead of being eliminated in urine are not fully understood (Marini et al., 2004; Muscher et al., 2010). Allantoin and uric acid, purine derivatives formed from the digestion and metabolism of microbial nucleic acids and endogenous purine metabolism, accounted for 11% of urine N. Ammonia N was a minor component that accounted for <1.5% of urine N. Other nitrogenous constituents in the urine of cattle include hippuric acid (3.8 to 8.0% of urine N), creatinine (1.8 to 5.3%), creatine (1.8 to 4.1%), and AA (<3.7%; Bristow et al., 1992). Urea N (and NH3–N) in urine is the most susceptible form of manure N contributing to NH3–N emissions (Cole et al., 2005). In simulated short-term storage of dairy manure, urinary N contributed to 90% of the emitted NH3–N (Lee et al., 2011). Hydrolysis and formation of NH3–N from urinary urea N under similar conditions was reportedly complete (Whitehead and Raistrick, 1993) with additional contributions to emitted NH3–N from the decomposition of allantoin, uric acid, and creatinine (Whitehead et al., 1989). Fecal N is less readily mineralized and emitted as NH3–N, with only 10% of fecal N converted to NH3–N when stored for 10 d at 25°C (Lee et al., 2011) or 30 d at 20°C (Whitehead and Raistrick, 1993). In addition to manure N composition, the effective loss of NH3–N from the feedlot will be determined by environmental factors including temperature, precipitation, and pH. In the companion beef feedlot study (Koenig et al., 2013), NH3–N emissions were reduced in cattle fed the 12% CP diet compared with the 14% CP diets reflecting the reduced amount of urinary urea N excretion although NH3–N emission averaged only 7.7 and 10.4% for cattle fed the backgrounding diets. Decreased NH3–N emissions were attributed to the low mean air temperatures of <5°C during the backgrounding phase. Protein Feeding Strategies to Improve N Efficiency Improving the efficiency of N utilization in the rumen will have the greatest influence on increasing the efficiency of N use in the whole animal (Tamminga, 1992). Inefficient assimilation of NH3–N by ruminal microorganisms and loss of N from the rumen results from an imbalance in the amount and lack of synchronization in rate of ruminal carbohydrate fermentation and protein availability, microbial degradation of dietary protein, intraruminal microbial N recycling, and limiting nutrients for microbial growth. Ammonia is the major end product of protein degradation in the rumen and forms the primary N source for the majority of ruminal bacteria (Leng and Nolan, 1984; Koenig et al., 2000). In addition to NH3–N, peptides and AA are formed as intermediary products of protein degradation. In vitro studies have demonstrated that peptides and AA are stimulatory to ruminal bacteria for microbial growth and ruminal digestion (Argyle and Baldwin, 1989; Griswold et al., 1996; Carro and Miller, 1999), particularly with rapidly fermentable carbohydrate sources (Russell et al., 1992; Chikunya et al., 1996). A ruminal NH3–N concentration of 50 mg N/L, derived from in vitro continuous culture experiments, is often accepted as the concentration required to meet N requirements and support maximal growth of ruminal microorganisms (Satter and Slyter, 1974). When sheep were fed semipurified diets (containing 5 to 6% CP) with mixed forages and concentrates (corn and barley grain) at ratios of 12:88, 26:74, and 50:50 and continuously infused with incremental amounts of urea, microbial protein synthesis was maximized at ruminal NH3–N concentrations of 39, 82 and 22 mg N/L, respectively, which were considered consistent with the 50 mg N/L in vitro estimate (Pisulewski et al., 1981). In contrast, the ruminal NH3–N concentration required to maximize the in situ degradation rate of barley grain was 125 mg N/L and was greater than the concentration of 61 mg N/L for corn grain (Odle and Schaefer, 1987). The optimal concentration of ruminal NH3–N was influenced by the chemical and structural characteristics of the substrate and appeared to be greater for barley grain-based diets with a more rapid rate of ruminal fermentation. The optimal ruminal NH3–N concentration may be even greater to ensure adequate N in microenvironments created by the physical association of bacteria cells with fermentable substrates (Odle and Schaefer, 1987) and for animals fed once or twice per day, which can create variable ruminal NH3–N concentrations throughout the feeding cycle (Pisulewski et al., 1981). Ruminal NH3–N concentrations measured over 12 h relative to feed offered at 0900 h fell below 50 and 125 mg N/L for approximately 4 and 7 h, respectively, in heifers fed the 12% CP diets. Assuming that the ruminal NH3–N concentration followed a similar profile after feed was offered at 1800 h (Coppock et al., 1976; Dixon, 1999), NH3–N concentration could be expected to be below 50 and 125 mg N/L for approximately 8 and 14 h/d, respectively. Increasing the CP concentration of the diets to 14% (all diets included a portion of the supplementary CP as RDP) reduced the time that ruminal NH3–N concentration was below 50 mg N/d to <3.5 h/d although ruminal NH3–N concentration was still below 125 mg N/L for an average of 10 h/d. Maintaining the ruminal NH3–N concentration above 50 mg N/L by feeding the 14% CP backgrounding diets did not improve microbial protein synthesis and efficiency or ruminal fermentation of OM, starch, and fiber indicating that ruminal NH3–N was not limiting for ruminal microorganisms in heifers fed the low CP diets. In the Cornell net carbohydrate and protein system (CNCPS), bacteria that degrade structural carbohydrates preferentially use NH3–N as their primary N source (Russell et al., 1992) and in the CNCPS evaluation of the beef backgrounding diets, a shortfall for ruminal NH3–N was predicted for all diets. Ruminal NH3–N was 54% of requirement for cattle fed the diets with 12% CP, 74 and 69% for cattle fed the diets with RDP (UREA and UREA+CM, respectively), and intermediate at 62% for cattle fed the RUP (UREA+CGM+xSBM). Unlike ruminal NH3–N, ruminal peptide and AA concentrations are not routinely measured in ruminant digestion and metabolism studies and adequate concentrations required to meet microbial requirements are not defined. Providing RDP (UREA+CM) to provide an additional source of peptide N to ruminal microorganisms did not affect ruminal peptide and free AA concentrations, microbial protein synthesis, or nutrient digestibility. Ruminal peptide concentration averaged 165 mg N/L in cattle fed the backgrounding diets. Ruminal peptide concentrations in the cattle fed barley-based diets were greater than that observed in lactating dairy cows fed grass silage (60 to 109 mg N/L; Choi et al., 2002), corn silage and hay crop silage (84 to 154 mg N/L; Chen et al., 1987), and alfalfa hay and timothy silage (99 to 114 mg N/L; Robinson and McQueen, 1994) combined with concentrates and various protein sources. In lactating dairy cows fed diets supplemented with protein of varying rumen degradabilities free AA concentration averaged 60 to 82 mg N/L in ruminal fluid (Reynal et al., 2006). Ruminal AA N was less than ruminal peptide N concentration and averaged <5 mg N/L. In lactating dairy cows fed diets supplemented with protein of varying rumen degradabilities free AA concentration averaged 60 to 82 mg N/L in ruminal fluid (Reynal et al., 2006). Recent revisions to the CNCPS (version 6.1) to expand the number of soluble carbohydrate pools, repartition soluble N between the protein A and B1 pools, and reduce degradation rates and increase passage rates of soluble pools have resulted in reduced microbial yield and, therefore, reduced microbial N requirements including peptides, which are now considered in excess of microbial requirements in dairy diets (Van Amburgh et al., 2010). The CNCPS evaluation of the beef backgrounding diets predicted that ruminal peptide N was limiting at 87% of requirement in cattle fed the 12% CP diet and the 14% CP diet with UREA and 95 and 91% in cattle fed the 14% CP diets with true protein sources (UREA+CM and UREA+CGM+xSBM, respectively). In addition to providing a source of N to ruminal microorganisms, peptides that accumulate in the rumen may pass to the small intestine to provide AA to the host (Chen et al., 1987; Reynal et al., 2006). The provision of rumen degradable and undegradable true protein sources, however, did not affect the flow of feed N (13% of total N flow) from the rumen to the small intestine. The inability to detect differences among the dietary treatments may have been due in part to the relatively small amount of true protein supplemented to the UREA+CM and UREA+CGM+xSBM diets combined with the imprecision (Titgemeyer, 1997) of estimating the flow of feed N by difference between the duodenal flow of total N and the flow of microbial N and ammonia N. Feed N flow averaged 33 g N/d. When calculated from DMI and in situ estimates of RUP for the major ingredients of the diets (data not shown), feed N flow in heifers was greater than when estimated from the digestive flow markers and there were numerical differences among the diets. Feed N flow was 53, 58, and 69 g N/d, respectively, in heifers fed the 12% CP, UREA+CM, and UREA+CGM+xSBM backgrounding diets. There were, however, no differences detected in the duodenal flow of AA. Microbial N formed the majority of the total N flow to the intestine {74% of which approximately 80% of microbial N was AA N [(total microbial N – nucleic acid N)/total microbial N]}, and because the treatments were similar in ingredient composition, other than differing in the protein source, the AA profile of microbial protein would be expected to be unaffected by the diet (Hvelplund, 1986; Martin et al., 1996). The concentration and degradability of protein did not affect total ruminal VFA concentration, which was consistent with similar amounts of OM, starch, and fiber digestion in the rumen. In addition, there was no increase in branched chain VFA derived from the deamination and decarboxylation of branched chain AA when rumen degradable true protein was provided (UREA+CM). In cattle fed the 14% CP diet with UREA+CM there was a greater molar proportion of propionate, a glucogenic precursor, which could suggest an improvement in energy efficiency (Huntington et al., 2006). The explanation for the improvement in growth performance and N efficiency of growing cattle fed the barley-based backgrounding diets supplemented with a mixture of urea and true protein sources (UREA+CM and UREA+CGM+xSBM; Koenig et al., 2013) is inconclusive from the results of the metabolism experiment. Feed intake in the metabolism experiments was restricted to ensure feed and energy intake were similar among the heifers fed the dietary treatments as was observed for the growing cattle of the performance trial fed similar diets. Restricting feed intake, however, decreases rate of passage from the rumen, increases ruminal digestion of nutrients, and decreases the flow and efficiency of microbial protein synthesis, which may have diminished relative differences in digestive function among cattle fed the diets and may not reflect practical feedlot feeding conditions. Footnotes 1 Contribution number 387-12022. LITERATURE CITED Adams J. R. Farran T. B. Erickson G. E. Klopfenstein T. J. Macken C. N. Wilson C. B. 2004. Effect of organic matter addition to the pen surface and pen cleaning frequency on nitrogen mass balance in open feedlots. J. Anim. Sci.  82: 2153– 2163. Google Scholar CrossRef Search ADS PubMed  American Society of Brewing Chemists (ASBC) 2004. Wort-12. Free amino nitrogen. In: Methods of analysis of the American society of brewing chemists,  9th ed. ASBC, St. Paul, MN. Argyle J. L. Baldwin R. L. 1989. Effects of amino acids and peptides on rumen microbial growth yields. J. Dairy Sci.  72: 2017– 2027. Google Scholar CrossRef Search ADS PubMed  Diamond Bar. 2011. Bovine surgery for fistulation of the rumen and cannula placement.  http://www.bardiamond.com/uploads/Hodges_Simpson_RevisedTM.pdf (Accessed 4 October 2012). Bevans D. W. Beauchemin K. A. Schwarzkopf-Genswein K. S. McKinnon J. J. McAllister T. A. 2005. Effect of rapid or gradual grain adaptation on subacute acidosis and feed intake by feedlot cattle. J. Anim. Sci.  83: 1116– 1132. Google Scholar CrossRef Search ADS PubMed  Bierman S. Erickson G. E. Klopfenstein T. J. Stock R. A. Shain D. H. 1999. Evaluation of nitrogen and organic matter balance in the feedlot as affected by level and source of dietary fiber. J. Anim. Sci.  77: 1645– 1653. Google Scholar CrossRef Search ADS PubMed  Bristow A. W. Whitehead D. C. Cockburn J. E. 1992. Nitrogenous constituents in the urine of cattle, sheep and goats. J. Sci. Food Agric.  59: 387– 394. Google Scholar CrossRef Search ADS   Broderick G. A. Kang J. H. 1980. Automated simultaneous determination of ammonia and total amino acids in ruminal fluid and in vitro media. J. Dairy Sci.  63: 64– 75. Google Scholar CrossRef Search ADS PubMed  Canadian Council on Animal Care 1993. Guide to the use of experimental animals, vol. 1. 2nd ed. Olfert E. D. Cross B. M. McWilliams A. A. editors. Canadian Council on Animal Care, Ottawa, ON. Carro M. D. Miller E. L. 1999. Effect of supplementing a fibre basal diet with different nitrogen forms on ruminal fermentation and microbial growth in an in vitro semi-continuous culture system (RUSITEC). Br. J. Nutr.  82: 149– 157. Google Scholar CrossRef Search ADS PubMed  Chen G. Sniffen C. J. Russell J. B. 1987. Concentration and estimated flow of peptides from the rumen of dairy cattle: Effects of protein quantity, protein solubility, and feeding frequency. J. Dairy Sci.  70: 983– 992. Google Scholar CrossRef Search ADS PubMed  Chen X. B. Mathieson J. DeB Hovell F. D. Reeds P. J. 1990. Measurement of purine derivatives in urine of ruminants using automated methods. J. Sci. Food Agric.  53: 23– 33. Google Scholar CrossRef Search ADS   Chikunya S. Newbold C. J. Rode L. Chen X. B. Wallace R. J. 1996. Influence of dietary rumen-degradable protein on bacterial growth in the rumen of sheep receiving different energy sources. Anim. Feed Sci. Technol.  63: 333– 340. Google Scholar CrossRef Search ADS   Choi C. W. Ahvenjärvi S. Vanhatalo A. Toivonen V. Huhtanen P. 2002. Quantitation of the flow of soluble non-ammonia nitrogen entering the omasal canal of dairy cows fed grass silage based diets. Anim. Feed Sci. Technol.  96: 203– 220. Google Scholar CrossRef Search ADS   Cole N. A. Clark R. N. Todd R. W. Richardson C. R. Gueye A. Greene L. W. McBride K. 2005. Influence of dietary crude protein concentration and source on potential ammonia emissions from beef cattle manure. J. Anim. Sci.  83: 722– 731. Google Scholar CrossRef Search ADS PubMed  Coppock C. E. Peplowski M. A. Lake G. B. 1976. Effect of urea form and method of feeding on rumen ammonia concentration. J. Dairy Sci.  59: 1152– 1156. Google Scholar CrossRef Search ADS PubMed  Dixon R. M. 1999. Effects of addition of urea to a low nitrogen diet on the rumen digestion of a range of roughages. Aust. J. Agric. Res.  50: 1091– 1097. Google Scholar CrossRef Search ADS   Griswold K. E. Hoover W. H. Miller T. K. Thayne W. V. 1996. Effect of form of nitrogen on growth of ruminal microbes in continuous culture. J. Anim. Sci.  74: 483– 491. Google Scholar CrossRef Search ADS PubMed  Huntington G. B. Harmon D. L. Richards C. J. 2006. Sites, rates, and limits of starch digestion and glucose metabolism in growing cattle. J. Anim. Sci.  84(E. Suppl.): E14– E24. Google Scholar CrossRef Search ADS PubMed  Huntington G. Poore M. Hopkins B. Spears J. 2001. Effect of ruminal protein degradability on growth and N metabolism in growing beef steers. J. Anim. Sci.  79: 533– 541. Google Scholar CrossRef Search ADS PubMed  Hvelplund T. 1986. The influence of diet on nitrogen and amino acid content of mixed rumen bacteria. Acta Agric. Scand.  36: 325– 331. Koenig K. M. Beauchemin K. A. 2013. Nitrogen metabolism and route of excretion in beef feedlot cattle fed barley-based finishing diets varying in protein concentration and rumen degradability. J. Anim. Sci.  91: 2310– 2320. Google Scholar CrossRef Search ADS PubMed  Koenig K. M. McGinn S. M. Beauchemin K. A. 2013. Ammonia emissions and performance of backgrounding and finishing beef feedlot cattle fed barley-based diets varying in dietary crude protein concentration and rumen degradability. J. Anim. Sci.  91: 2278– 2294. Google Scholar CrossRef Search ADS PubMed  Koenig K. M. Newbold C. J. McIntosh F. M. Rode L. M. 2000. Effects of protozoa on bacterial nitrogen recycling in the rumen. J. Anim. Sci.  78: 2431– 2445. Google Scholar CrossRef Search ADS PubMed  Kohn R. A. Dinneen M. M. Russek-Cohen E. 2005. Using blood urea nitrogen to predict nitrogen excretion and efficiency of nitrogen utilization in cattle, sheep, goats, horses, pigs, and rats. J. Anim. Sci.  83: 879– 889. Google Scholar CrossRef Search ADS PubMed  Lee C. Hristov A. N. Cassidy T. Heyler K. 2011. Nitrogen isotope fractionation and origin of ammonia nitrogen volatilization from cattle manure in simulated storage. Atmosphere  2: 256– 270. Google Scholar CrossRef Search ADS   Leng R. A. Nolan J. V. 1984. Nitrogen metabolism in the rumen. J. Dairy Sci.  67: 1072– 1089. Google Scholar CrossRef Search ADS PubMed  Maekawa M. Beauchemin K. A. Christensen D. A. 2002. Effect of concentrate level and feeding management of chewing activities, saliva production, and ruminal pH of lactating dairy cows. J. Dairy Sci.  85: 1165– 1175. Google Scholar CrossRef Search ADS PubMed  Marini J. C. Klein J. D. Sands J. M. Van Amburgh M. E. 2004. Effect of nitrogen intake on nitrogen recycling and urea transporter abundance in lambs. J. Anim. Sci.  82: 1157– 1164. Google Scholar CrossRef Search ADS PubMed  Marini J. C. Van Amburgh M. E. 2005. Partition of nitrogen excretion in urine and the feces of Holstein replacement heifers. J. Dairy Sci.  88: 1778– 1784. Google Scholar CrossRef Search ADS PubMed  Martin C. Bernard L. Michalet-Doreau B. 1996. Influence of sampling time and diet on amino acid composition of protozoal and bacterial fractions from bovine ruminal contents. J. Anim. Sci.  74: 1157– 1163. Google Scholar CrossRef Search ADS PubMed  McGilliard A. D. 1982. Surgical techniques – Advances and cautions. In: Protein requirements for cattle: Symposium.  Oklahoma State Univ., Stillwater, OK. p. 31– 36. Merry R. J. McAllan A. B. 1983. A comparison of the chemical composition of mixed bacteria harvested from the liquid and solid fractions of rumen digesta. Brit. J. Nutr.  50: 701– 709. Google Scholar CrossRef Search ADS   Misselbrook T. H. Powell J. M. Broderick G. A. Grabber J. H. 2005. Dietary manipulation in dairy cattle: laboratory experiments to assess the influence on ammonia emissions. J. Dairy Sci.  88: 1765– 1777. Google Scholar CrossRef Search ADS PubMed  Muscher A. S. Schröder B. Breves G. Huber K. 2010. Dietary nitrogen reduction enhances urea transport across the goat epithelium. J. Anim. Sci.  88: 3390– 3398. Google Scholar CrossRef Search ADS PubMed  Nagaraja T. G. Titgemeyer E. C. 2007. Ruminal acidosis in beef cattle: the current microbiological and nutritional outlook. J. Dairy Sci.  90(E. Suppl.): E17– E38. Google Scholar CrossRef Search ADS PubMed  NRC 1985. Ruminant nitrogen usage. Natl. Acad. Press, Washington, DC. NRC 2000. Nutrient requirements of beef cattle. Update. 7th rev. ed. Natl. Acad. Press, Washington, DC. Odle J. Schaefer D. M. 1987. Influence of rumen ammonia concentration on the rumen degradation rates of barley and maize. Br. J. Nutr.  57: 127– 138. Google Scholar CrossRef Search ADS PubMed  Olubobokun J. A. Craig W. M. Nipper W. A. 1988. Characteristics of protozoal and bacterial fractions from microorganisms associated with ruminal fluid or particles. J. Anim. Sci.  66: 2701– 2710. Google Scholar CrossRef Search ADS   Pisulewski P. M. Okorie A. U. Buttery P. J. Haresign W. Lewis D. 1981. Ammonia concentration and protein synthesis in the rumen. J. Sci. Food Agric.  32: 759– 766. Google Scholar CrossRef Search ADS PubMed  Reynal S. M. Ipharraguerre I. R. Liñeiro M. Brito A. F. Broderick G. A. Clark J. H. 2006. Omasal flow of soluble proteins, peptides, and free amino acids in dairy cows fed diets supplemented with proteins of varying ruminal degradabilities. J. Dairy Sci.  90: 1887– 1903. Google Scholar CrossRef Search ADS   Robinson P. H. McQueen R. E. 1994. Influence of supplemental protein source and feeding frequency on rumen fermentation and performance in dairy cows. J. Dairy Sci.  77: 1340– 1353. Google Scholar CrossRef Search ADS PubMed  Russell J. B. O'Conner J. D. Fox D. G. Van Soest P. J. Sniffen C. J. 1992. A net carbohydrate and protein system for evaluating cattle diets: I. Ruminal fermentation. J. Anim. Sci.  70: 3551– 3561. Google Scholar CrossRef Search ADS PubMed  Russell J. B. Wilson D. B. 1996. Why are ruminal cellulolytic bacteria unable to digest cellulose at low pH. J. Dairy Sci.  79: 1503– 1509. Google Scholar CrossRef Search ADS PubMed  Satter L. D. Slyter L. L. 1974. Effect of ammonia concentration on rumen microbial protein production in vitro. Br. J. Nutr.  32: 199– 208. Google Scholar CrossRef Search ADS PubMed  Siddons R. C. Paradine J. Beever D. E. Cornell P. R. 1985. Ytterbium acetate as a particulate-phase digesta-flow marker. Br. J. Nutr.  54: 509– 519. Google Scholar CrossRef Search ADS PubMed  Spanghero M. Kowalski Z. M. 1997. Critical analysis of N balance experiments with lactating cows. Livest. Prod. Sci.  52: 113– 122. Google Scholar CrossRef Search ADS   Tamminga S. 1992. Nutrition management of dairy cows as a contribution to pollution control. J. Dairy Sci.  75: 345– 357. Google Scholar CrossRef Search ADS   Titgemeyer E. C. 1997. Design and interpretation of nutrient digestion studies. J. Anim. Sci.  75: 2235– 2247. Google Scholar CrossRef Search ADS PubMed  U.S. Environmental Protection Agency (USEPA) 1983. Methods for chemical analysis of water and wastes. EPA/600/4–79/020. Method 351.2 determination of total Kjeldahl nitrogen by semi-automated colorimetry. Rev. 2. USEPA Cincinnati, OH. Van.Amburgh M. E. Chase L. E. Overton T. R. Ross D. A. Recktenwald E. B. Higgs R. J. Tylutki T. P. 2010. Updates to the Cornell net carbohydrate and protein system v6.1 and implications for ration formulation. In: Proc. Cornell Nutr. Conf. Feed Manufacturers,  Syracruse, NY. p. 144– 159 Vasconcelos J. T. Cole N. A. McBride K. W. Gueye A. Galyean M. L. Richardson C. R. Greene L. W. 2009. Effects of dietary crude protein and supplemental urea levels on nitrogen and phosphorus utilization by feedlot cattle. J. Anim. Sci.  87: 1174– 1183. Google Scholar CrossRef Search ADS PubMed  Whitehead D. C. Lockyer D. R. Raistrick N. 1989. Volatilzation of ammonia from urea applied to soil: influence of hippuric acid and other constituents of livestock urine. Soil Biol. Biochem.  21: 803– 808. Google Scholar CrossRef Search ADS   Whitehead D. C. Raistrick N. 1993. Nitrogen in the excreta of dairy cattle: changes during short-term storage. J. Agric. Sci. Camb.  121: 73– 81. Google Scholar CrossRef Search ADS   Zinn R. A. Owens F. N. 1986. A rapid procedure for purine measurement and its use for estimating net ruminal protein synthesis. Can. J. Anim. Sci.  66: 157– 166. Google Scholar CrossRef Search ADS   American Society of Animal Science
TI - Nitrogen metabolism and route of excretion in beef feedlot cattle fed barley-based backgrounding diets varying in protein concentration and rumen degradability,
JF - Journal of Animal Science
DO - 10.2527/jas.2012-5652
DA - 2013-05-01
UR - https://www.deepdyve.com/lp/oxford-university-press/nitrogen-metabolism-and-route-of-excretion-in-beef-feedlot-cattle-fed-e1TIFIt0yr
SP - 2295
EP - 2309
VL - 91
IS - 5
DP - DeepDyve
ER -