TY - JOUR
AU - McAllister, T. A.
AB - Abstract Many feedlot finishing diets include wheat when the relative wheat prices are low. This study was conducted to examine the responses in ruminal pH and fermentation as well as site and extent of digestion from substituting soft or hard wheat for barley grain and to determine whether an elevated monensin concentration might decrease indicators of ruminal acidosis in feedlot heifers. Five ruminally cannulated beef heifers were used in a 5 × 5 Latin square with 2 × 2 + 1 factorial arrangement. Treatments included barley (10% barley silage, 86% barley, 4% supplement, with 28 mg monensin/kg DM) and diets where barley was substituted by either soft or hard wheat with either 28 or 44 mg monensin/kg diet DM. Intake of DM was not affected by grain source, whereas increasing monensin with wheat diets reduced (P < 0.02) DMI. Mean ruminal pH was lower (P < 0.04) and durations of pH < 5.8 and pH < 5.5 greater (P < 0.03) for wheat than for barley diets. However, ruminal pH was not affected by wheat type or monensin level. Total VFA concentrations were greater (P < 0.03) for wheat than barley diets with no effect of wheat type. The molar proportion of propionate was greater (P < 0.04), whereas butyrate (P < 0.01) and ratio of acetate to propionate tended to be lower (P < 0.09), with the high as compared to low level of monensin. Replacing barley with wheat in finishing diets did not affect the duodenal flow or the digestibility of OM, likely as a result of greater (P < 0.01) NDF digestion from barley offsetting the increased (P < 0.03) supply of digested starch from wheat. Feeding soft vs. hard wheat delivered a greater (P < 0.03) duodenal supply of OM and nonammonia N with no differences in total tract nutrient digestion. The increased monensin concentration decreased the flow of OM (P < 0.01), total N (P < 0.05), and microbial protein (P < 0.05) to the small intestine due to decreased DMI. These results indicated that hard and soft wheat exhibited digestive characteristics similar to barley, but ruminal pH measurements indicate that compared with barley, wheat increased the risk of ruminal acidosis. Although an increased level of monensin had limited impact on ruminal indicators of acidosis, an increase in propionate would be expected to improve efficiency of feed use by heifers fed wheat-based finishing diets. INTRODUCTION Although many beef cattle feedlot operators often include some wheat in finishing diets, levels seldom exceed 50% of diet ingredients. Little attention has been paid to whether the wheat is soft or hard, though wheat type can alter the site or extent of starch digestion by feedlot cattle. Grain hardness has been described as the resistance of the kernel to fracture (Anjum and Walker, 1991), a trait used for decades by the wheat industry to differentiate quality and market classes. In corn, differences in hardness of floury (i.e., dent) vs. vitreous (i.e., flint) endosperm arise as a result of differences in affinity of starch and protein; higher affinity decreases both the rate and extent of starch digestion in the rumen (McAllister et al., 1990). The endosperm within different wheat types also differs in hardness, but all wheat types are digested more rapidly than corn in the rumen. Consequently, the extent to which hardness influences the site and extent of starch digestion in wheat is known less well. We hypothesized that as with flint corn, hard wheat would degrade more slowly in the rumen as a result of the protein matrix restricting access of starch to rumen microorganisms. Ionophores, particularly monensin, commonly are fed to finishing cattle in North American feedlots to modulate ruminal fermentation and improve feed efficiency. Of the cereal grains, wheat is fermented most rapidly in the rumen. As a result, the use of monensin to modulate ruminal fermentation may be applicable, particularly for cattle fed wheat-based finishing diets. However, diets with greater energy density (e.g., diets that contain highly processed grain) appear less responsive to monensin addition (DiLorenzo and Galyean, 2010). We hypothesize that increasing the level of monensin in the diet should reduce the severity of ruminal acidosis in cattle fed wheat-based diets and that this response should be greater for soft vs. hard wheat due to more rapid and extensive ruminal starch digestion of soft wheat. The objectives of this study were to investigate the effects of grain source (barley vs. wheat), wheat type (soft vs. hard), and monensin level on ruminal pH, fermentation, microbial protein synthesis, and the site and extent of nutrient digestion by finishing beef heifers. MATERIALS AND METHODS This study was approved by the Animal Care Committee of the Agriculture and Agri-Food Canada, Lethbridge Research Centre, Lethbridge, AB, and was conducted according to the guidelines of the Canadian Council on Animal Care (2009). Animals, Experimental Design, and Diets Five Angus beef heifers (initial BW = 605 ± 32 kg) fitted with rumen cannulae (10-cm center diameter, Bar Diamond, Parma, ID) and T-type duodenal cannulae with a 2.5-cm i.d. opening (Bar Diamond, Inc.) were used in the present study. Five heifers were assigned randomly to 1 of 5 treatments in a 5 × 5 Latin square with 2 × 2 + 1 factorial arrangement. The 5 treatments included barley (10% barley silage and 90% of a barley grain-based concentrate supplemented with 28 mg monensin/kg DM; BML) and diets substituting soft (SW) or hard (HW) wheat for barley grain with monensin at either 28 (ML) or 44 mg/kg dietary DM (MH). Single lots of barley, SW, and HW were obtained and used in the study. The grain was dry-rolled to be adjusted for a constant processing index for minimizing the impact of responses to grain processing; processing index is defined as the volume weight (g/L) of the grain (as is) after processing, expressed as a percentage of the volume weight before processing. Particle size distribution of whole and processed barley grain was measured using a series of sieves at 3.35, 2.36, 1.18, and 0.85 mm and a pan in a Ro-Tap machine (RX-29, W. S. Tyler, Mentor, OH; Table 1). Geometric mean particle size of the rolled grain kernels was determined according to the procedure of the American Society of Agricultural Engineers (ASAE, 1992). Diets were prepared (Table 2) daily using a feed mixer (Data Ranger, American Calan Inc., Northwood, NH). Heifers were adapted to their experimental diets by gradually increasing the proportion of grain in the diet over a period of 4 wk before starting the experiment. Each experimental period consisted of 14 d for adaptation followed by 7 d for experimental measurement. Table 1. Chemical composition and particle size distribution of feed ingredients (n = 5) Item  Barley silage  Barley grain  Soft wheat  Hard wheat  Chemical composition, % of DM  DM, %  33.9  91.4  90.8  92.5  OM  92.6  97.8  97.8  98.0  CP  11.9  12.9  13.7  15.4  NDF  49.5  25.4  13.5  17.1  ADF  32.1  5.4  2.6  3.3  Starch  19.5  58.3  65.5  64.8  Particle size on sieve, %      3.35 mm    21.2  2.9  2.3      2.36 mm    55.7  47.6  48.5      1.18 mm    21.3  44.4  46.5      0.85 mm    1.0  1.7  1.1      <0.85 mm    0.9  3.4  1.6  Geometric mean, mm    4.10  2.98  3.05  Test weight, g/L              Unprocessed    612  787  788      Processed    490  634  638      Processing index1, %    80  81  81  Item  Barley silage  Barley grain  Soft wheat  Hard wheat  Chemical composition, % of DM  DM, %  33.9  91.4  90.8  92.5  OM  92.6  97.8  97.8  98.0  CP  11.9  12.9  13.7  15.4  NDF  49.5  25.4  13.5  17.1  ADF  32.1  5.4  2.6  3.3  Starch  19.5  58.3  65.5  64.8  Particle size on sieve, %      3.35 mm    21.2  2.9  2.3      2.36 mm    55.7  47.6  48.5      1.18 mm    21.3  44.4  46.5      0.85 mm    1.0  1.7  1.1      <0.85 mm    0.9  3.4  1.6  Geometric mean, mm    4.10  2.98  3.05  Test weight, g/L              Unprocessed    612  787  788      Processed    490  634  638      Processing index1, %    80  81  81  1Processing index is the volume weight (g/L) of the grain after processing expressed as a percentage of the volume weight before processing. View Large Table 1. Chemical composition and particle size distribution of feed ingredients (n = 5) Item  Barley silage  Barley grain  Soft wheat  Hard wheat  Chemical composition, % of DM  DM, %  33.9  91.4  90.8  92.5  OM  92.6  97.8  97.8  98.0  CP  11.9  12.9  13.7  15.4  NDF  49.5  25.4  13.5  17.1  ADF  32.1  5.4  2.6  3.3  Starch  19.5  58.3  65.5  64.8  Particle size on sieve, %      3.35 mm    21.2  2.9  2.3      2.36 mm    55.7  47.6  48.5      1.18 mm    21.3  44.4  46.5      0.85 mm    1.0  1.7  1.1      <0.85 mm    0.9  3.4  1.6  Geometric mean, mm    4.10  2.98  3.05  Test weight, g/L              Unprocessed    612  787  788      Processed    490  634  638      Processing index1, %    80  81  81  Item  Barley silage  Barley grain  Soft wheat  Hard wheat  Chemical composition, % of DM  DM, %  33.9  91.4  90.8  92.5  OM  92.6  97.8  97.8  98.0  CP  11.9  12.9  13.7  15.4  NDF  49.5  25.4  13.5  17.1  ADF  32.1  5.4  2.6  3.3  Starch  19.5  58.3  65.5  64.8  Particle size on sieve, %      3.35 mm    21.2  2.9  2.3      2.36 mm    55.7  47.6  48.5      1.18 mm    21.3  44.4  46.5      0.85 mm    1.0  1.7  1.1      <0.85 mm    0.9  3.4  1.6  Geometric mean, mm    4.10  2.98  3.05  Test weight, g/L              Unprocessed    612  787  788      Processed    490  634  638      Processing index1, %    80  81  81  1Processing index is the volume weight (g/L) of the grain after processing expressed as a percentage of the volume weight before processing. View Large Table 2. Ingredient and chemical composition of the experimental diets   Diets  Item  Barley  Soft wheat  Hard wheat  Ingredient, % DM        Barley silage  10  10  10  Barley grain, dry-rolled  86  –  –  Soft wheat, dry-rolled  –  86  –  Hard wheat, dry-rolled  –  –  86  Supplement (pellet)        Barley grain, ground  1.8  1.8  1.8  Calcium carbonate  1.15  1.15  1.15  Salt  0.35  0.35  0.35  Mineral and vitamin premix1  0.08  0.08  0.08  Molasses  0.33  0.33  0.33  Canola oil  0.17  0.17  0.17  Urea  0.10  0.10  0.10  MGA 100 premix2  0.02  0.02  0.02  Chemical composition, % of DM  DM, %  76.7  76.3  77.5  CP  13.3  13.4  15.2  NDF  27.8  16.5  17.8  ADF  8.5  5.8  5.1  Starch  52.7  59.5  58.8  NEg,3 Mcal/kg  1.23  1.32  1.30    Diets  Item  Barley  Soft wheat  Hard wheat  Ingredient, % DM        Barley silage  10  10  10  Barley grain, dry-rolled  86  –  –  Soft wheat, dry-rolled  –  86  –  Hard wheat, dry-rolled  –  –  86  Supplement (pellet)        Barley grain, ground  1.8  1.8  1.8  Calcium carbonate  1.15  1.15  1.15  Salt  0.35  0.35  0.35  Mineral and vitamin premix1  0.08  0.08  0.08  Molasses  0.33  0.33  0.33  Canola oil  0.17  0.17  0.17  Urea  0.10  0.10  0.10  MGA 100 premix2  0.02  0.02  0.02  Chemical composition, % of DM  DM, %  76.7  76.3  77.5  CP  13.3  13.4  15.2  NDF  27.8  16.5  17.8  ADF  8.5  5.8  5.1  Starch  52.7  59.5  58.8  NEg,3 Mcal/kg  1.23  1.32  1.30  1Supplied per kilogram of dietary DM: 15 mg of Cu, 65 of mg Zn, 28 mg of Mn, 0.7 mg of I, 0.2 mg of Co, 0.3 mg of Se, 6000 IU of vitamin A, 600 IU of vitamin D, and 47 IU of vitamin E. 2MGA = Melengestrol acetate (220 mg/kg; Pfizer Canada Inc., Kirkland, Quebec, Canada). 3Estimated based on beef NRC (2000). View Large Table 2. Ingredient and chemical composition of the experimental diets   Diets  Item  Barley  Soft wheat  Hard wheat  Ingredient, % DM        Barley silage  10  10  10  Barley grain, dry-rolled  86  –  –  Soft wheat, dry-rolled  –  86  –  Hard wheat, dry-rolled  –  –  86  Supplement (pellet)        Barley grain, ground  1.8  1.8  1.8  Calcium carbonate  1.15  1.15  1.15  Salt  0.35  0.35  0.35  Mineral and vitamin premix1  0.08  0.08  0.08  Molasses  0.33  0.33  0.33  Canola oil  0.17  0.17  0.17  Urea  0.10  0.10  0.10  MGA 100 premix2  0.02  0.02  0.02  Chemical composition, % of DM  DM, %  76.7  76.3  77.5  CP  13.3  13.4  15.2  NDF  27.8  16.5  17.8  ADF  8.5  5.8  5.1  Starch  52.7  59.5  58.8  NEg,3 Mcal/kg  1.23  1.32  1.30    Diets  Item  Barley  Soft wheat  Hard wheat  Ingredient, % DM        Barley silage  10  10  10  Barley grain, dry-rolled  86  –  –  Soft wheat, dry-rolled  –  86  –  Hard wheat, dry-rolled  –  –  86  Supplement (pellet)        Barley grain, ground  1.8  1.8  1.8  Calcium carbonate  1.15  1.15  1.15  Salt  0.35  0.35  0.35  Mineral and vitamin premix1  0.08  0.08  0.08  Molasses  0.33  0.33  0.33  Canola oil  0.17  0.17  0.17  Urea  0.10  0.10  0.10  MGA 100 premix2  0.02  0.02  0.02  Chemical composition, % of DM  DM, %  76.7  76.3  77.5  CP  13.3  13.4  15.2  NDF  27.8  16.5  17.8  ADF  8.5  5.8  5.1  Starch  52.7  59.5  58.8  NEg,3 Mcal/kg  1.23  1.32  1.30  1Supplied per kilogram of dietary DM: 15 mg of Cu, 65 of mg Zn, 28 mg of Mn, 0.7 mg of I, 0.2 mg of Co, 0.3 mg of Se, 6000 IU of vitamin A, 600 IU of vitamin D, and 47 IU of vitamin E. 2MGA = Melengestrol acetate (220 mg/kg; Pfizer Canada Inc., Kirkland, Quebec, Canada). 3Estimated based on beef NRC (2000). View Large Heifers were fed a total mixed ration (TMR) once daily (1100 h) at a level that ensured at least 5% orts. Feed offered and refused were recorded daily for each heifer for the entire experiment. The TMR and barley silage were sampled weekly to determine DM content, and the diets were adjusted to account for changes in the moisture content of silage. Samples of barley and wheat grain were collected once during each period. Refusals were collected during the last 7 d of each period. The feed and orts samples were oven-dried at 55°C for 48 h and ground through a 1-mm screen (standard model 4, Arthur Thomas Co., Philadelphia, PA) for subsequent chemical analysis. Heifers were housed in individual tie-stalls on rubber mats and bedded with wood shavings. Water was available freely throughout the experiment. Heifers were exercised daily for 1 h in an outdoor pen, as the measurement and sampling schedule permitted. Heifers were weighed before feeding at the beginning of the first period and at the end of each period. Intake, Duodenal Flows, and Digestibility Feed intake for each heifer was calculated as the difference between feed offered and orts during the last 7 d of each period. Duodenal flows, nutrient digestion, and microbial N flow were determined using Yb (YbCl3·6H2O) as a marker for digesta flow and ammonia 15N ([15NH4]2SO4) as a microbial marker. Markers (5.5 g of YbCl3·6H2O, 1.2 g of Yb and 3 g of [15NH4]2SO4) were mixed with rolled barley and top dressed for each animal daily starting 7 d before duodenal and fecal sampling. Duodenal samples were collected from the duodenal cannulae over a 4-d period with 3 samplings per day. The samples were immediately subdivided into 3 fractions for determining DM content (oven dry at 55°C), NH3–N and nutrient analysis. The flows of DM to the duodenum were calculated as ratio of daily Yb consumed to Yb concentration in duodenal contents. Feces were collected from the rectum over a 4-d period at the same time as duodenal contents were sampled. Samples were combined by heifer within period, dried in an oven at 55°C for 48 h, and ground through 1-mm screen (standard model 4) for chemical analysis. Ruminal pH and Fermentation Ruminal pH was monitored continuously for 4 d using the Lethbridge Research Centre Ruminal pH Measurement System (Dascor, Escondido, CA; Penner et al., 2006) from d 14 through 18 of each period. Ruminal pH readings were taken every 30 s and stored by the data logger (model M1b-pH-1KRTD, Dascor, Escondido, CA). The daily ruminal pH data were averaged for each min and summarized as minimum pH, mean pH, maximum pH, and duration below and area under the curve at pH 5.8, pH 5.5, and pH 5.2. The area under the curve was calculated as the sum of the absolute value of pH deviations below the curve multiplied by the duration below the curve and reported as pH × h/d. Ruminal fermentation parameters were measured at 1, 3, 5, and 7 h post feeding on 2 consecutive days (d 16 and 17). Ruminal contents were collected from 4 different sites within the rumen and combined before being squeezed through a nylon mesh (pore size 355µm; PECAP, B & SH Thompson, Ville Mont-Royal, QC, Canada). Subsamples (5 mL) of filtrate were preserved with 1 mL of 25% (wt/vol) HPO3 and 1 mL of 1% H2SO4 for VFA and NH3–N analysis, respectively. Microbial Protein Synthesis Flow of microbial protein to the duodenum was determined by the ratio of 15N flow at the duodenum to 15N concentration of isolated mixed ruminal bacteria (Yang et al., 2010). Ruminal samples (750 g/sample) were collected from 4 different locations in the rumen and immediately squeezed through 4 layers of cheesecloth. Particles that passed through the cheesecloth were blended with an equal amount of 0.9% NaCl in a waring blender for 1 min and then squeezed through 4 layers of cheesecloth. Filtrates from squeezed and strained homogenates were mixed and centrifuged (800 × g for 15 min at 4°C) to remove protozoa and feed particles. The supernatant fluid was centrifuged (20,000 × g for 40 min at 4°C) to obtain a pellet of mixed ruminal bacteria. Bacterial pellets were combined by period for each heifer, freeze-dried, and ground using a mortar and pestle. Ground pellets were analyzed for OM, 15N, and total N with the results used to calculate ruminal microbial protein synthesis as described by Yang et al. (2010). Chemical Analyses Chemical analyses were performed on each sample in duplicate; when the CV for the replicate analysis exceeded 5%, analysis was repeated. Analytical DM was determined by oven drying at 135°C for 2 h (AOAC, 1995; method 930.15). Ash was determined by combustion at 550°C for 5 h and OM was calculated (AOAC, 1995; method 942.05). The NDF was determined as described by Van Soest et al. (1991) using heat-stable α-amylase (Termamyl 120 L, Novo Nordisk Biochem, Franklinton, NC) without sodium sulfite. The ADF was determined according to AOAC (1995; method 973.18). Both NDF and ADF were expressed inclusive of residual ash. For starch and CP (N × 6.25), samples were ground to a fine powder using a ball mill (Mixer Mill MM 2000; Retsch, Haan, Germany). Starch was determined by enzymatic hydrolysis of α-linked glucose polymers as described by Rode et al. (1999). Total N was determined by flash combustion and thermal conductivity detection (Model 1500; Carlo Erba Instruments, Milan, Italy). Ether extract was determined using an E-816 extraction unit, (Büchi Labortechnik AG, Flawil, Switzerland) according to AOAC (1995; method 920.39). Ruminal VFA were separated and quantified using a gas chromatograph (model 5890, Hewlett-Packard Lab, Palo Alto, CA) equipped with a capillary column (30 m × 0.32 mm i.d., 1-μm phase thickness, Zebron ZB-FAAP, Phenomenex, Torrance, CA) and flame ionization detector. Crotonic acid (trans-2-butenoic acid) was used as an internal standard. Concentration of NH3–N in ruminal contents was determined as described by Rhine et al. (1998). Statistical Analyses Data were analyzed using the MIXED procedure (SAS Inst. Inc., Cary, NC) for a 5 × 5 Latin square with 2 × 2 + 1 factorial arrangement. The mixed model included the fixed effects of diet and the random effects of heifer and period. Day (or time within day) was considered a repeated measure for variables measured over time. For repeated measures, various covariance structures were tested with the final choice exhibiting the lowest value for Akaike's information criteria. Contrasts were used to compare barley vs. wheat diets at the same monensin level, SW vs. HW, ML vs. MH within wheat, and the interaction between wheat source and monensin levels. Differences were declared significant at P ≤ 0.05. Trends were discussed at 0.05 < P ≤ 0.10 unless otherwise stated. RESULTS Chemical Composition of Grains Although no statistical analysis could be performed as single lots of each grain were obtained, wheat grain obviously had greater CP and starch contents and less NDF and ADF contents than barley grain (Table 1). Hard wheat also had greater CP and NDF contents but starch content was similar compared to SW. In comparison with NRC (2000), the CP content of barley (13.2%) and wheat (14.2%) was, respectively, similar to that of present barley and SW, but the NDF content of barley (18.1%) and wheat (11.8%) from NRC (2000) was less than the NDF content of grains used in the present study. The sum of CP, starch, and NDF appeared lower in SW (92.7%) than in HW (97.3%). McAllister and Sultana (2011) reported that the sum of CP, starch, and NDF decreased from 97.5, 97.2, 95.1, to 90.3% with decreasing kernel hardness from 0, 17, 32, to 35.5, respectively (lower number indicate harder kernels). We speculate that the soluble and insoluble nonstarch polysaccharide content may be responsible for the difference between SW and HV (Lean et al., 2013). Intake, Duodenal Flow, and Digestibility Intakes of DM and OM were not different, but intake of NDF was less (P < 0.01) for heifers fed wheat-based diets than those fed the barley-based diet as a result of lower NDF content of wheat (Table 3). There was no interaction between wheat type and monensin levels on nutrient intakes, but the high level of monensin decreased intakes of DM (P < 0.02), NDF (P < 0.02), and starch (P < 0.04). Table 3. Effect of wheat source and monensin level on intake, flow to duodenum, and digestibility in finishing beef heifers   Diet1    P-value2  Item  BML  SWML  SWMH  HWML  HWMH  SEM  Grain  Wheat  Mon  Intake, kg/d                        DM  8.41  7.82  7.30  8.27  7.34  0.356  0.25  0.35  0.02      OM  8.19  7.65  7.15  8.07  7.17  0.346  0.28  0.38  0.02      NDF  2.37  1.29  1.21  1.46  1.28  0.083  0.01  0.08  0.07      Starch  4.44  4.68  4.44  4.89  4.39  0.207  0.11  0.60  0.04  Flow to duodenum, kg/d              OM  3.80  3.57  3.28  3.85  3.39  0.104  0.31  0.03  0.01      Microbial OM  0.88  0.88  0.76  0.96  0.91  0.061  0.55  0.06  0.16      NDF  1.18  0.87  0.75  0.86  0.74  0.037  0.01  0.85  0.01      Starch  0.96  1.08  0.92  1.24  1.06  0.117  0.06  0.08  0.05      RFOM3  5.27  4.96  4.63  5.18  4.69  0.311  0.51  0.55  0.11      Digestibility, % intake          Rumen                        OM4  64.3  64.5  64.7  63.9  65.3  1.64  0.95  0.99  0.60      NDF  49.6  32.1  37.7  40.4  41.4  3.66  0.01  0.09  0.33      Starch  78.7  76.5  79.2  74.4  75.9  2.53  0.23  0.22  0.33  Intestine (small + large)              OM  25.9  30.2  30.1  27.8  31.8  2.78  0.33  0.88  0.46      NDF  10.4  15.6  9.5  8.4  10.6  4.76  0.67  0.31  0.52      Starch  16.8  21.4  19.0  22.7  21.2  2.57  0.08  0.45  0.39  Total                        OM  79.5  83.1  84.1  79.7  84.3  1.90  0.40  0.40  0.15      NDF  60.6  47.7  47.3  48.8  52.0  3.44  0.01  0.26  0.59      Starch  95.5  97.9  98.2  97.2  97.1  0.71  0.03  0.19  0.88    Diet1    P-value2  Item  BML  SWML  SWMH  HWML  HWMH  SEM  Grain  Wheat  Mon  Intake, kg/d                        DM  8.41  7.82  7.30  8.27  7.34  0.356  0.25  0.35  0.02      OM  8.19  7.65  7.15  8.07  7.17  0.346  0.28  0.38  0.02      NDF  2.37  1.29  1.21  1.46  1.28  0.083  0.01  0.08  0.07      Starch  4.44  4.68  4.44  4.89  4.39  0.207  0.11  0.60  0.04  Flow to duodenum, kg/d              OM  3.80  3.57  3.28  3.85  3.39  0.104  0.31  0.03  0.01      Microbial OM  0.88  0.88  0.76  0.96  0.91  0.061  0.55  0.06  0.16      NDF  1.18  0.87  0.75  0.86  0.74  0.037  0.01  0.85  0.01      Starch  0.96  1.08  0.92  1.24  1.06  0.117  0.06  0.08  0.05      RFOM3  5.27  4.96  4.63  5.18  4.69  0.311  0.51  0.55  0.11      Digestibility, % intake          Rumen                        OM4  64.3  64.5  64.7  63.9  65.3  1.64  0.95  0.99  0.60      NDF  49.6  32.1  37.7  40.4  41.4  3.66  0.01  0.09  0.33      Starch  78.7  76.5  79.2  74.4  75.9  2.53  0.23  0.22  0.33  Intestine (small + large)              OM  25.9  30.2  30.1  27.8  31.8  2.78  0.33  0.88  0.46      NDF  10.4  15.6  9.5  8.4  10.6  4.76  0.67  0.31  0.52      Starch  16.8  21.4  19.0  22.7  21.2  2.57  0.08  0.45  0.39  Total                        OM  79.5  83.1  84.1  79.7  84.3  1.90  0.40  0.40  0.15      NDF  60.6  47.7  47.3  48.8  52.0  3.44  0.01  0.26  0.59      Starch  95.5  97.9  98.2  97.2  97.1  0.71  0.03  0.19  0.88  1Diets were barley (BML; 10% silage and 90% barley grain-based concentrate supplemented with 28 mg monensin/kg DM) and diets substituting soft (SW) or hard (HW) wheat for barley grain combined with either 28 (ML) or 44 mg (MH) monensin/kg DM (SWML, SWMH, HWML, and HWMH). 2Grain = BML vs. average of SWML and HWML diets; Wheat = average of the SWML and SWMH diets vs. average of the HWML and HWMH diets; Mon = average of the SWML and HWML diets vs. average of the SWMH and HWMH diets; the interaction between wheat type and monensin level was not significant (P > 0.15). 3RFOM = OM truly fermented in the rumen calculated by correcting for microbial OM. 4Corrected for microbial OM. View Large Table 3. Effect of wheat source and monensin level on intake, flow to duodenum, and digestibility in finishing beef heifers   Diet1    P-value2  Item  BML  SWML  SWMH  HWML  HWMH  SEM  Grain  Wheat  Mon  Intake, kg/d                        DM  8.41  7.82  7.30  8.27  7.34  0.356  0.25  0.35  0.02      OM  8.19  7.65  7.15  8.07  7.17  0.346  0.28  0.38  0.02      NDF  2.37  1.29  1.21  1.46  1.28  0.083  0.01  0.08  0.07      Starch  4.44  4.68  4.44  4.89  4.39  0.207  0.11  0.60  0.04  Flow to duodenum, kg/d              OM  3.80  3.57  3.28  3.85  3.39  0.104  0.31  0.03  0.01      Microbial OM  0.88  0.88  0.76  0.96  0.91  0.061  0.55  0.06  0.16      NDF  1.18  0.87  0.75  0.86  0.74  0.037  0.01  0.85  0.01      Starch  0.96  1.08  0.92  1.24  1.06  0.117  0.06  0.08  0.05      RFOM3  5.27  4.96  4.63  5.18  4.69  0.311  0.51  0.55  0.11      Digestibility, % intake          Rumen                        OM4  64.3  64.5  64.7  63.9  65.3  1.64  0.95  0.99  0.60      NDF  49.6  32.1  37.7  40.4  41.4  3.66  0.01  0.09  0.33      Starch  78.7  76.5  79.2  74.4  75.9  2.53  0.23  0.22  0.33  Intestine (small + large)              OM  25.9  30.2  30.1  27.8  31.8  2.78  0.33  0.88  0.46      NDF  10.4  15.6  9.5  8.4  10.6  4.76  0.67  0.31  0.52      Starch  16.8  21.4  19.0  22.7  21.2  2.57  0.08  0.45  0.39  Total                        OM  79.5  83.1  84.1  79.7  84.3  1.90  0.40  0.40  0.15      NDF  60.6  47.7  47.3  48.8  52.0  3.44  0.01  0.26  0.59      Starch  95.5  97.9  98.2  97.2  97.1  0.71  0.03  0.19  0.88    Diet1    P-value2  Item  BML  SWML  SWMH  HWML  HWMH  SEM  Grain  Wheat  Mon  Intake, kg/d                        DM  8.41  7.82  7.30  8.27  7.34  0.356  0.25  0.35  0.02      OM  8.19  7.65  7.15  8.07  7.17  0.346  0.28  0.38  0.02      NDF  2.37  1.29  1.21  1.46  1.28  0.083  0.01  0.08  0.07      Starch  4.44  4.68  4.44  4.89  4.39  0.207  0.11  0.60  0.04  Flow to duodenum, kg/d              OM  3.80  3.57  3.28  3.85  3.39  0.104  0.31  0.03  0.01      Microbial OM  0.88  0.88  0.76  0.96  0.91  0.061  0.55  0.06  0.16      NDF  1.18  0.87  0.75  0.86  0.74  0.037  0.01  0.85  0.01      Starch  0.96  1.08  0.92  1.24  1.06  0.117  0.06  0.08  0.05      RFOM3  5.27  4.96  4.63  5.18  4.69  0.311  0.51  0.55  0.11      Digestibility, % intake          Rumen                        OM4  64.3  64.5  64.7  63.9  65.3  1.64  0.95  0.99  0.60      NDF  49.6  32.1  37.7  40.4  41.4  3.66  0.01  0.09  0.33      Starch  78.7  76.5  79.2  74.4  75.9  2.53  0.23  0.22  0.33  Intestine (small + large)              OM  25.9  30.2  30.1  27.8  31.8  2.78  0.33  0.88  0.46      NDF  10.4  15.6  9.5  8.4  10.6  4.76  0.67  0.31  0.52      Starch  16.8  21.4  19.0  22.7  21.2  2.57  0.08  0.45  0.39  Total                        OM  79.5  83.1  84.1  79.7  84.3  1.90  0.40  0.40  0.15      NDF  60.6  47.7  47.3  48.8  52.0  3.44  0.01  0.26  0.59      Starch  95.5  97.9  98.2  97.2  97.1  0.71  0.03  0.19  0.88  1Diets were barley (BML; 10% silage and 90% barley grain-based concentrate supplemented with 28 mg monensin/kg DM) and diets substituting soft (SW) or hard (HW) wheat for barley grain combined with either 28 (ML) or 44 mg (MH) monensin/kg DM (SWML, SWMH, HWML, and HWMH). 2Grain = BML vs. average of SWML and HWML diets; Wheat = average of the SWML and SWMH diets vs. average of the HWML and HWMH diets; Mon = average of the SWML and HWML diets vs. average of the SWMH and HWMH diets; the interaction between wheat type and monensin level was not significant (P > 0.15). 3RFOM = OM truly fermented in the rumen calculated by correcting for microbial OM. 4Corrected for microbial OM. View Large Duodenal flows of OM and microbial OM did not differ between barley and wheat diets, but flow of NDF to the duodenum was less (P < 0.01) and flow of starch tended (P < 0.06) to be greater with the wheat diets than with the barley diet (Table 3). There was no interaction between wheat type and monensin level on nutrient flow to the duodenum. The flow of OM to the duodenum was less (P < 0.01) and that of microbial OM (P < 0.06) and starch (P < 0.08) tended to be less for heifers fed SW than HW. Increasing the dietary monensin level reduced flows of OM (P < 0.01), NDF (P < 0.01), and starch (P < 0.05) to the small intestine. However, there were no differences in the amount of OM truly fermented in the rumen among the treatments. Ruminal digestibility of OM and starch did not differ among heifers fed barley or wheat, whereas ruminal digestibility of NDF was less (P < 0.01) with wheat (Table 3). Digestibility of OM and NDF in the intestine did not differ among grain source, whereas the digestibility of NDF in the total digestive tract was less (P < 0.01) and that of starch was greater (P < 0.03) by heifers when fed wheat than when fed barley. Site and extent of feed digestion in heifers was not affected by either wheat type or monensin level in finishing diets. Nitrogen Metabolism and Ruminal Microbial Protein Synthesis Intake of N did not differ between heifers fed barley- or wheat-based diets (Table 4). Duodenal flows of total N, nonammonia N (NAN), and dietary N (i.e., nonammonia and nonmicrobial N) were also not affected by grain source. However, microbial protein synthesis and microbial efficiency were improved (P < 0.05) in heifers fed wheat vs. barley diets. Digestibility of N in the rumen and in the intestine was not affected by diet. Consequently the digestibility of N in the total digestive tract did not differ among diets. Table 4. Effect of wheat source and monensin level on N metabolism in finishing beef heifers   Diet1    P-value2  Item  BML  SWML  SWMH  HWML  HWMH  SEM  Grain  Wheat  Mon  N intake, g/d  178  168  157  201  178  10.1  0.64  0.02  0.11      Flow to duodenum                        Total N, g/d  177  174  163  191  173  6.8  0.54  0.07  0.05      Total N, % of intake  101.1  105.7  104.4  96.2  98.4  6.72  0.99  0.27  0.95      NAN,3 g/d  173  169  158  186  169  6.6  0.56  0.05  0.05      NAN, % of intake  98.4  102.5  101.3  93.6  96.3  6.48  0.97  0.30  0.91      Ammonia N, g/d  4.7  5.2  4.7  5.2  3.7  0.42  0.25  0.20  0.02      NANMN,4 g/d  89  69  77  89  77  7.9  0.32  0.23  0.82      NANMN, % intake  50.7  41.9  49.4  44.3  44.1  4.61  0.16  0.73  0.39      Microbial N, g/d  84  100  81  97  92  5.8  0.05  0.47  0.05      Microbial N, %  47.7  60.6  51.9  49.3  52.2  4.81  0.21  0.23  0.53      Microbial efficiency5  15.9  20.7  17.6  19.4  19.5  1.67  0.04  0.83  0.32  Digestibility, % intake                        Ruminal (truly)  49.3  58.1  50.6  55.7  55.9  4.61  0.16  0.73  0.40      Postruminal  75.4  82.4  81.9  74.8  80.8  7.03  0.70  0.53  0.69      Total tract  74.3  76.7  77.5  78.6  82.4  1.84  0.14  0.07  0.20    Diet1    P-value2  Item  BML  SWML  SWMH  HWML  HWMH  SEM  Grain  Wheat  Mon  N intake, g/d  178  168  157  201  178  10.1  0.64  0.02  0.11      Flow to duodenum                        Total N, g/d  177  174  163  191  173  6.8  0.54  0.07  0.05      Total N, % of intake  101.1  105.7  104.4  96.2  98.4  6.72  0.99  0.27  0.95      NAN,3 g/d  173  169  158  186  169  6.6  0.56  0.05  0.05      NAN, % of intake  98.4  102.5  101.3  93.6  96.3  6.48  0.97  0.30  0.91      Ammonia N, g/d  4.7  5.2  4.7  5.2  3.7  0.42  0.25  0.20  0.02      NANMN,4 g/d  89  69  77  89  77  7.9  0.32  0.23  0.82      NANMN, % intake  50.7  41.9  49.4  44.3  44.1  4.61  0.16  0.73  0.39      Microbial N, g/d  84  100  81  97  92  5.8  0.05  0.47  0.05      Microbial N, %  47.7  60.6  51.9  49.3  52.2  4.81  0.21  0.23  0.53      Microbial efficiency5  15.9  20.7  17.6  19.4  19.5  1.67  0.04  0.83  0.32  Digestibility, % intake                        Ruminal (truly)  49.3  58.1  50.6  55.7  55.9  4.61  0.16  0.73  0.40      Postruminal  75.4  82.4  81.9  74.8  80.8  7.03  0.70  0.53  0.69      Total tract  74.3  76.7  77.5  78.6  82.4  1.84  0.14  0.07  0.20  1Diets were barley (BML; 10% silage and 90% barley grain-based concentrate supplemented with 28 mg monensin/kg DM) and diets substituting soft (SW) or hard (HW) wheat for barley grain combining with 28 (ML) or 44 mg (MH) monensin/kg DM (SWML, SWMH, HWML and HWMH). 2Grain = BML vs. average of SWML and HWML diets; Wheat = average of the SWML and SWMH diets vs. average of the HWML and HWMH diets; Mon = average of the SWML and HWML diets vs. average of the SWMH and HWMH diets; the interaction between wheat type and monensin level was not significant (P > 0.15). 3NAN = nonammonia N. 4NANMN = nonammonia nonmicrobial N. 5Grams of microbial N/kg of OM truly fermented in the rumen. View Large Table 4. Effect of wheat source and monensin level on N metabolism in finishing beef heifers   Diet1    P-value2  Item  BML  SWML  SWMH  HWML  HWMH  SEM  Grain  Wheat  Mon  N intake, g/d  178  168  157  201  178  10.1  0.64  0.02  0.11      Flow to duodenum                        Total N, g/d  177  174  163  191  173  6.8  0.54  0.07  0.05      Total N, % of intake  101.1  105.7  104.4  96.2  98.4  6.72  0.99  0.27  0.95      NAN,3 g/d  173  169  158  186  169  6.6  0.56  0.05  0.05      NAN, % of intake  98.4  102.5  101.3  93.6  96.3  6.48  0.97  0.30  0.91      Ammonia N, g/d  4.7  5.2  4.7  5.2  3.7  0.42  0.25  0.20  0.02      NANMN,4 g/d  89  69  77  89  77  7.9  0.32  0.23  0.82      NANMN, % intake  50.7  41.9  49.4  44.3  44.1  4.61  0.16  0.73  0.39      Microbial N, g/d  84  100  81  97  92  5.8  0.05  0.47  0.05      Microbial N, %  47.7  60.6  51.9  49.3  52.2  4.81  0.21  0.23  0.53      Microbial efficiency5  15.9  20.7  17.6  19.4  19.5  1.67  0.04  0.83  0.32  Digestibility, % intake                        Ruminal (truly)  49.3  58.1  50.6  55.7  55.9  4.61  0.16  0.73  0.40      Postruminal  75.4  82.4  81.9  74.8  80.8  7.03  0.70  0.53  0.69      Total tract  74.3  76.7  77.5  78.6  82.4  1.84  0.14  0.07  0.20    Diet1    P-value2  Item  BML  SWML  SWMH  HWML  HWMH  SEM  Grain  Wheat  Mon  N intake, g/d  178  168  157  201  178  10.1  0.64  0.02  0.11      Flow to duodenum                        Total N, g/d  177  174  163  191  173  6.8  0.54  0.07  0.05      Total N, % of intake  101.1  105.7  104.4  96.2  98.4  6.72  0.99  0.27  0.95      NAN,3 g/d  173  169  158  186  169  6.6  0.56  0.05  0.05      NAN, % of intake  98.4  102.5  101.3  93.6  96.3  6.48  0.97  0.30  0.91      Ammonia N, g/d  4.7  5.2  4.7  5.2  3.7  0.42  0.25  0.20  0.02      NANMN,4 g/d  89  69  77  89  77  7.9  0.32  0.23  0.82      NANMN, % intake  50.7  41.9  49.4  44.3  44.1  4.61  0.16  0.73  0.39      Microbial N, g/d  84  100  81  97  92  5.8  0.05  0.47  0.05      Microbial N, %  47.7  60.6  51.9  49.3  52.2  4.81  0.21  0.23  0.53      Microbial efficiency5  15.9  20.7  17.6  19.4  19.5  1.67  0.04  0.83  0.32  Digestibility, % intake                        Ruminal (truly)  49.3  58.1  50.6  55.7  55.9  4.61  0.16  0.73  0.40      Postruminal  75.4  82.4  81.9  74.8  80.8  7.03  0.70  0.53  0.69      Total tract  74.3  76.7  77.5  78.6  82.4  1.84  0.14  0.07  0.20  1Diets were barley (BML; 10% silage and 90% barley grain-based concentrate supplemented with 28 mg monensin/kg DM) and diets substituting soft (SW) or hard (HW) wheat for barley grain combining with 28 (ML) or 44 mg (MH) monensin/kg DM (SWML, SWMH, HWML and HWMH). 2Grain = BML vs. average of SWML and HWML diets; Wheat = average of the SWML and SWMH diets vs. average of the HWML and HWMH diets; Mon = average of the SWML and HWML diets vs. average of the SWMH and HWMH diets; the interaction between wheat type and monensin level was not significant (P > 0.15). 3NAN = nonammonia N. 4NANMN = nonammonia nonmicrobial N. 5Grams of microbial N/kg of OM truly fermented in the rumen. View Large Interactions between wheat source and monensin level for intake, duodenal flows, or the digestibility of N (Table 4) were not significant (P > 0.15). Heifers fed SW had less (P < 0.01) N intake as compared to those fed HW (163 vs. 190 g/d). As a result, the heifers fed SW diet had less flow of total N (P < 0.07) and NAN (P < 0.05) to the duodenum compared with heifers fed HW but no differences in the flows of microbial or dietary N to the duodenum were observed. Total tract digestibility of N tended (P < 0.07) to be less for heifers fed SW (77.1%) vs. those fed HW (80.5%). Elevated monensin numerically reduced (P < 0.11) N intake and decreased the flows of total N (P < 0.05), NAN (P < 0.05), ammonia N (P < 0.02), and microbial N (P < 0.05) to the duodenum; however, the site and extent of N digestion did not differ with monensin level. Ruminal pH and Fermentation Compared with heifers fed barley, heifers fed wheat exhibited a lower mean (5.56 vs. 5.91; P < 0.04) and minimum pH (4.96 vs. 5.24; P < 0.02), increases in the duration of a reduced pH < 5.8 (P < 0.02), < 5.5 (P < 0.03), and in the area below the curve at pH 5.8 (P < 0.05; Table 5). However, ruminal pH was affected neither by wheat type nor by monensin level, and no wheat type × monensin level interactions were observed for parameters related to pH. Table 5. Effect of wheat source and monensin level on ruminal pH and fermentation characteristics in finishing beef cattle   Diet1    P-value2  Item  BML  SWML  SWMH  HWML  HWMH  SEM  Grain  Wheat  Mon  pH                        Mean  5.91  5.51  5.69  5.60  5.57  0.120  0.04  0.92  0.56      Minimum  5.24  4.92  5.12  4.99  5.06  0.070  0.01  0.88  0.08      Maximum  6.63  6.53  6.48  6.49  6.50  0.146  0.48  0.95  0.85      pH < 5.8, h/d  9.9  17.5  15.2  17.0  18.1  2.34  0.02  0.64  0.80      pH < 5.5, h/d  4.3  12.8  10.2  11.3  12.1  2.64  0.03  0.94  0.73      pH < 5.2, h/d  0.8  7.8  4.4  4.1  3.5  2.16  0.07  0.30  0.37      Area pH 5.8, pH × h/d  2.8  9.4  6.6  7.1  7.3  1.89  0.04  0.66  0.51      Area pH 5.5, pH × h/d  0.7  4.8  2.8  2.7  2.6  1.22  0.08  0.36  0.40      Area pH 5.2, pH × h/d  0.1  1.7  0.6  0.5  0.4  0.53  0.14  0.18  0.27  VFA                        Total, mM  160.9  180.0  176.8  179.0  172.8  9.27  0.03  0.69  0.45      Acetate (A), %  48.9  46.5  45.1  46.4  45.8  1.14  0.10  0.76  0.40      Propionate (P), %  35.2  36.5  41.8  35.2  41.2  2.70  0.85  0.70  0.04      Butyrate, %  10.2  11.1  7.9  11.8  7.7  1.29  0.36  0.83  0.01      BCFA3, %  3.2  2.7  1.9  3.0  2.3  0.43  0.55  0.38  0.10      A:P  1.45  1.34  1.09  1.43  1.13  0.15  0.76  0.68  0.09      NH3–N, mM  10.2  11.5  9.9  12.2  10.8  1.45  0.34  0.56  0.29    Diet1    P-value2  Item  BML  SWML  SWMH  HWML  HWMH  SEM  Grain  Wheat  Mon  pH                        Mean  5.91  5.51  5.69  5.60  5.57  0.120  0.04  0.92  0.56      Minimum  5.24  4.92  5.12  4.99  5.06  0.070  0.01  0.88  0.08      Maximum  6.63  6.53  6.48  6.49  6.50  0.146  0.48  0.95  0.85      pH < 5.8, h/d  9.9  17.5  15.2  17.0  18.1  2.34  0.02  0.64  0.80      pH < 5.5, h/d  4.3  12.8  10.2  11.3  12.1  2.64  0.03  0.94  0.73      pH < 5.2, h/d  0.8  7.8  4.4  4.1  3.5  2.16  0.07  0.30  0.37      Area pH 5.8, pH × h/d  2.8  9.4  6.6  7.1  7.3  1.89  0.04  0.66  0.51      Area pH 5.5, pH × h/d  0.7  4.8  2.8  2.7  2.6  1.22  0.08  0.36  0.40      Area pH 5.2, pH × h/d  0.1  1.7  0.6  0.5  0.4  0.53  0.14  0.18  0.27  VFA                        Total, mM  160.9  180.0  176.8  179.0  172.8  9.27  0.03  0.69  0.45      Acetate (A), %  48.9  46.5  45.1  46.4  45.8  1.14  0.10  0.76  0.40      Propionate (P), %  35.2  36.5  41.8  35.2  41.2  2.70  0.85  0.70  0.04      Butyrate, %  10.2  11.1  7.9  11.8  7.7  1.29  0.36  0.83  0.01      BCFA3, %  3.2  2.7  1.9  3.0  2.3  0.43  0.55  0.38  0.10      A:P  1.45  1.34  1.09  1.43  1.13  0.15  0.76  0.68  0.09      NH3–N, mM  10.2  11.5  9.9  12.2  10.8  1.45  0.34  0.56  0.29  1Diets were barley (BML; 10% silage and 90% barley grain-based concentrate supplemented with 28 mg monensin/kg DM) and diets substituting soft (SW) or hard (HW) wheat for barley grain combining with 28 (ML) or 44 mg (MH) monensin/kg DM (SWML, SWMH, HWML and HWMH). 2Grain = BML vs. average of SWML and HWML diets; Wheat = average of the SWML and SWMH diets vs. average of the HWML and HWMH diets; Mon = average of the SWML and HWML diets vs. average of the SWMH and HWMH diets; the interaction between wheat type and monensin level was not significant (P > 0.15). 3BCFA = Branched-chain VFA (isobutyrate + isovalerate). View Large Table 5. Effect of wheat source and monensin level on ruminal pH and fermentation characteristics in finishing beef cattle   Diet1    P-value2  Item  BML  SWML  SWMH  HWML  HWMH  SEM  Grain  Wheat  Mon  pH                        Mean  5.91  5.51  5.69  5.60  5.57  0.120  0.04  0.92  0.56      Minimum  5.24  4.92  5.12  4.99  5.06  0.070  0.01  0.88  0.08      Maximum  6.63  6.53  6.48  6.49  6.50  0.146  0.48  0.95  0.85      pH < 5.8, h/d  9.9  17.5  15.2  17.0  18.1  2.34  0.02  0.64  0.80      pH < 5.5, h/d  4.3  12.8  10.2  11.3  12.1  2.64  0.03  0.94  0.73      pH < 5.2, h/d  0.8  7.8  4.4  4.1  3.5  2.16  0.07  0.30  0.37      Area pH 5.8, pH × h/d  2.8  9.4  6.6  7.1  7.3  1.89  0.04  0.66  0.51      Area pH 5.5, pH × h/d  0.7  4.8  2.8  2.7  2.6  1.22  0.08  0.36  0.40      Area pH 5.2, pH × h/d  0.1  1.7  0.6  0.5  0.4  0.53  0.14  0.18  0.27  VFA                        Total, mM  160.9  180.0  176.8  179.0  172.8  9.27  0.03  0.69  0.45      Acetate (A), %  48.9  46.5  45.1  46.4  45.8  1.14  0.10  0.76  0.40      Propionate (P), %  35.2  36.5  41.8  35.2  41.2  2.70  0.85  0.70  0.04      Butyrate, %  10.2  11.1  7.9  11.8  7.7  1.29  0.36  0.83  0.01      BCFA3, %  3.2  2.7  1.9  3.0  2.3  0.43  0.55  0.38  0.10      A:P  1.45  1.34  1.09  1.43  1.13  0.15  0.76  0.68  0.09      NH3–N, mM  10.2  11.5  9.9  12.2  10.8  1.45  0.34  0.56  0.29    Diet1    P-value2  Item  BML  SWML  SWMH  HWML  HWMH  SEM  Grain  Wheat  Mon  pH                        Mean  5.91  5.51  5.69  5.60  5.57  0.120  0.04  0.92  0.56      Minimum  5.24  4.92  5.12  4.99  5.06  0.070  0.01  0.88  0.08      Maximum  6.63  6.53  6.48  6.49  6.50  0.146  0.48  0.95  0.85      pH < 5.8, h/d  9.9  17.5  15.2  17.0  18.1  2.34  0.02  0.64  0.80      pH < 5.5, h/d  4.3  12.8  10.2  11.3  12.1  2.64  0.03  0.94  0.73      pH < 5.2, h/d  0.8  7.8  4.4  4.1  3.5  2.16  0.07  0.30  0.37      Area pH 5.8, pH × h/d  2.8  9.4  6.6  7.1  7.3  1.89  0.04  0.66  0.51      Area pH 5.5, pH × h/d  0.7  4.8  2.8  2.7  2.6  1.22  0.08  0.36  0.40      Area pH 5.2, pH × h/d  0.1  1.7  0.6  0.5  0.4  0.53  0.14  0.18  0.27  VFA                        Total, mM  160.9  180.0  176.8  179.0  172.8  9.27  0.03  0.69  0.45      Acetate (A), %  48.9  46.5  45.1  46.4  45.8  1.14  0.10  0.76  0.40      Propionate (P), %  35.2  36.5  41.8  35.2  41.2  2.70  0.85  0.70  0.04      Butyrate, %  10.2  11.1  7.9  11.8  7.7  1.29  0.36  0.83  0.01      BCFA3, %  3.2  2.7  1.9  3.0  2.3  0.43  0.55  0.38  0.10      A:P  1.45  1.34  1.09  1.43  1.13  0.15  0.76  0.68  0.09      NH3–N, mM  10.2  11.5  9.9  12.2  10.8  1.45  0.34  0.56  0.29  1Diets were barley (BML; 10% silage and 90% barley grain-based concentrate supplemented with 28 mg monensin/kg DM) and diets substituting soft (SW) or hard (HW) wheat for barley grain combining with 28 (ML) or 44 mg (MH) monensin/kg DM (SWML, SWMH, HWML and HWMH). 2Grain = BML vs. average of SWML and HWML diets; Wheat = average of the SWML and SWMH diets vs. average of the HWML and HWMH diets; Mon = average of the SWML and HWML diets vs. average of the SWMH and HWMH diets; the interaction between wheat type and monensin level was not significant (P > 0.15). 3BCFA = Branched-chain VFA (isobutyrate + isovalerate). View Large Total ruminal VFA concentration was greater (179 vs. 161 mM; P < 0.03), whereas the molar proportions of acetate tended to be less (46 vs. 49%; P < 0.10) for heifers fed wheat vs. those fed barley (Table 5). No differences in the molar proportion of other individual VFA or in ruminal ammonia N concentration were observed between heifers fed wheat vs. barley. Wheat type did not alter the total VFA concentrations or the molar proportion of individual VFA in ruminal fluid. In contrast, elevated monensin levels increased (P < 0.04) the molar proportion of propionate (35.9 vs. 41.5%), decreased (P < 0.01) the proportion of butyrate (11.5 vs. 7.8%), and tended to reduce (P < 0.09) the acetate to propionate ratio (1.39 vs. 1.11). DISCUSSION Barley vs. Wheat Although wheat, corn, and barley are the major feed grains used in livestock production in North America (Owens et al., 1997), most of the wheat has been fed to poultry and swine. Although wheat often comprises a portion of the grain in feedlot diets in Canada, its level often is restricted to 50% of dietary DM or less due to fear that greater proportions will promote digestive disturbances such as ruminal acidosis (Zinn, 1994; Owens et al., 1998). Among cereal grains, wheat has the most rapid rate of starch digestion in the rumen, with a rate twice that of barley and almost 4 times that of corn for grains processed to a similar degree (Herrera-Saldana et al., 1990). Rapid starch digestion in the rumen increases the rate of VFA production, enhancing the likelihood of subclinical or clinical ruminal acidosis. The greater rate of ruminal starch digestion of wheat than barley was reflected in the present study by the decreased mean ruminal pH and increased the duration of pH < 5.8 or < 5.5, which was accompanied by greater VFA concentrations in the rumen of heifers fed wheat vs. barley grain. In a comparison among cereal grains, McAllister and Sultana (2011) reported the highest in situ DM, CP, and starch disappearances were for wheat, with a ranking of wheat > barley > corn. He et al. (2013a) observed a trend (P = 0.06) of linear decrease in ruminal pH along with linear increase in the duration of pH < 5.8 when wheat replaced 0, 33, 67, and 100% of the barley in a finishing beef diet. Although ruminal pH was less in heifers fed wheat as compared to those fed barley, DMI did not differ between heifers fed these 2 grain sources. This contradicts the contention that ruminal pH below a threshold value of 5.5 to 5.8 will reduce DMI and lead to cyclic responses in DMI (Beauchemin et al., 2006). Based on this concept, a low ruminal pH causes the cow to go “off-feed,” reducing the ruminal production of VFA and enabling pH to recover. After recovery, the cow resumes her high feed intake that once again leads to excessive VFA production and a decline in DMI as the cycle is repeated. Similarly, it was reported that feeding a highly fermented carbohydrate such as extensively processed barley (Nocek et al., 2002) or wheat (Zinn, 1994) results in low ruminal pH and predisposes ruminants to an acidotic state, which in turn will reduce DMI. However, our lack of a difference in DMI between barley and wheat is consistent with an earlier study (Owens et al., 1997) and recent reports from our laboratory (He et al., 2013a). The lack of a reduction in DMI with a reduced ruminal pH in the present study may reflect the extended period of time that was allowed for cattle to adapt to their high-grain diets. The heifers in the present study were used in the previous trial with diets rich in grain diet. Thus the animals were adapted to high-grain diets. This suggestion is supported by the study of Xu, He, Liang, McAllister, and Yang (personal communication) from a long-term growing study (120 d). Schwaiger et al. (2013) reported that beef heifers that spent more time on a high-concentrate diet before the bout of imposed ruminal acidosis exhibited a decreased day-to-day variation in ruminal pH and no difference in DMI. Those authors suggest that a longer time on a high-grain feed stabilizes ruminal pH both before and after a bout of ruminal acidosis, although adaptation time does not affect the risk of ruminal acidosis. Replacing barley with wheat in a finishing diet did not alter the flow of OM to the duodenum, but wheat resulted in an increase in the amount of microbial protein (+90 g/d) and delivered more digestible starch (+0.43 kg/d) and less digestible NDF (−0.77 kg/d) for the total digestive tract. An increase in microbial protein production without an increase in ruminally fermented OM (5.07 vs. 5.27 kg/d; wheat vs. barley) resulted in an improvement in the efficiency of microbial protein synthesis (+26%). He et al. (2013b) detected no differences in ADG but did find a tendency for DMI to be reduced and feed efficiency to be improved in steers fed wheat as compared to those fed barley. In the present study, the slightly greater intake of digestible starch could have been offset by lower intake of digestible NDF. The reduced ruminal NDF digestibility along with the trend of less acetate in the rumen suggests that fiber digestion in the rumen may have been more adversely affected by heifers fed wheat than those fed barley. The reduced mean ruminal pH plus decreased pH status of heifers fed wheat diets might explain the reduction in NDF digestion in the rumen and total digestive tract. Others have found that a decreased ruminal pH reduces ruminal NDF digestion (Russell and Wilson, 1996). Whether the NDF of wheat is less digestible than the NDF of barley grain is not clear. He et al. (2013a) reported no significant difference in NDF digestion in the total digestive tract of steers fed finishing diets with various ratios of barley to wheat from 100:0, 67:33, 33:67, to 0:100. There were also no differences in NDF digestion in Holstein steers fed bermudagrass hay and supplemented with corn, sorghum, or wheat grains at a rate of 0.7% of BW. The lower intake of digestible NDF by heifers fed wheat grain suggests that the potential for a deficiency of effective fiber in finishing cattle is greater for those fed a wheat diet. It is interesting to notice that the feeding value of barley and wheat grain would vary as well if different processing methods were applied. Dry-rolling, temper-rolling, and steam-rolling are commonly used for processing grain in North American feedlots. Temper- or steam-rolling is an ideal processing method to control size or thickness of the products, whereas dry-rolling is a cheap and simple processing method but produces more fines, resulting in a greater surface area for fermentation. Owens et al. (1997) compiled results from 605 comparisons from published feeding trials in North America and indicated that feedlot cattle fed barley responded differently from those fed wheat to the processing method. Cattle fed steam-rolled wheat reduced DMI, improved feed efficiency, and increased metabolizable energy content of processed grain compared to those fed dry-rolled wheat, whereas no differences in DMI or feed efficiency were detected between cattle fed steam-rolled and dry-rolled barley (Owens et al., 1997). This emphasize another critical factor that impacts the nutritional value of grain through manipulating processing method. Soft vs. Hard Wheat Grain hardness reflects the degree of adhesion between starch granules and the protein matrix (Swan et al., 2006), though packing density of starch (kernel density) and pericarp rigidity can alter starch availability as well. Increased kernel hardness usually is associated with a decrease in the rate of starch digestion, likely as a result of more extensive protection of starch granules by protein from microbial attack (McAllister and Sultana, 2011). However, hard kernels shatter more than soft kernels when dry rolled; fine particles generated can increase the rate of ruminal starch digestion (Varner and Woods, 1975). Swan et al. (2006) reported that starch granules from SW appeared more resistant to microbial digestion than the starch granules from HW as there was greater damage to the surface of starch granules in HW after cracking using a mill. In the present study, failure to observe differences in the ruminal pH and VFA concentrations and site and extent of nutrient digestibility between SW and HW may be due to the fact that wheat grain for this study was deliberately processed coarsely (PI > 80%) to avoid digestive upsets. In fact, the particle size distributions of rolled SW and HW were overall similar except for a slightly greater proportion of fine particles (< .85 mm) of rolled wheat for SW than HW. The impact of the difference in fines on ruminal pH and OM digestion appeared to be minimal. However, the present results suggest that the particle size distribution of processed wheat could differ even though the processing index was identical between SW and HW. Using barley grain, several studies showed that the levels of test weight (g/L), degree of processing, and moisture of grain could significantly affect the particle size distribution of processed grain (Mathison et al., 1997; Yang et al., 2013). Although SW generally exhibits a faster rate of digestion than HW in the rumen (McAllister and Sultana, 2011), the rate of fermentation of wheat is also dependent on the degree of processing. The digestibility of wheat varied from 44% when fed whole to over 88% when properly processed (Nordin and Campling, 1976). It generally is recommended that wheat be rolled coarsely in a manner that breaks kernels into 2 or 3 pieces while avoiding the production of fines, which presumably promote acidosis. He et al. (2013a) reported no significant differences in ruminal pH or VFA concentrations or in nutrient digestibility when SW (PI = 80%) was substituted for barley grain and accounted for 30, 60, or 90% of the diet DM in a finishing diet fed to beef steers. We speculated that coarsely rolling wheat to a PI of 81% did not damage the starch granules in HW, while maintaining partial integrity of the protein matrix so as to modulate the access of ruminal microorganisms to starch granules. In a feedlot experiment, steers had a greater DMI (0.4 kg/d) when fed coarsely (PI = 85%) vs. more extensively (PI = 75%) processed rolled SW (He et al., 2013b). The results suggested that feeding more extensively processed rolled wheat may have resulted in a decline in DMI as a result of the increased ruminal starch digestion rate leading to a greater decline in ruminal pH. Alternatively, coarsely rolled wheat may have resulted in lower total tract DM digestibility, resulting in an increase in feed intake of steers to meet their energy requirements. Yet our results suggest that SW and HW have a similar feed value based on OM digestibility if they are moderately processed to a similar degree. McAllister and Sultana (2011) reported that protein concentration of the wheat varieties was correlated negatively (r = −0.77) with the in situ rate of DM disappearance, further implicating the role of protein in influencing digestion of wheat in the rumen. Two major proteins, puroindolines A and B associated with the fribilin protein complex on the surface of wheat starch granules, play a central role in kernel hardness (Morris, 2002) and in determining the in situ disappearance of wheat starch (Swan et al., 2006). Soft wheat results from both puroindolines A and B genes being present in their wild-type form, whereas HW has an absence or alteration in either puroindoline gene with a wide range of grain textures (Hogg et al., 2004). As HW had greater CP than SW, HW may exhibit increased resistance to starch damage during mechanical processing, but if this was true it did not result in any difference in the feeding value between HW and SW. Monensin Levels Monensin levels in beef cattle diets have tended to be elevated recently to concentrations of over 40 mg/kg diet DM (Duffield et al., 2012), likely because high energy density diets, such as diets containing highly processed grain are less responsive to monensin addition (DiLorenzo and Galyean, 2010). The reduced monensin response has been attributed to a reduced change in ruminal fermentation end products as a result of increased grain feeding (an increasing proportion of grain and more extensive grain processing) and addition of dietary fat (e.g., high fat distillers grain). Each of these feeding practices has decreased the ratio of acetate to propionate and methane production, hence reducing the potential for marginal improvement from addition of monensin. In a meta-analysis, Duffield et al. (2012) identified a 0.008-kg reduction in DMI/kg BW gain per 1 mg/kg increase in monensin in the diet. This translates into estimated feed efficiency improvements of −0.55, −0.64, and −0.73 kg DMI/kg gain for doses at 22, 33, and 44 mg/kg feed DM. The reduction in DMI with increasing monensin from 28 to 44 mg/kg DM in this study is in agreement with the conclusions drawn by that meta-analysis and is consistent with our studies that have examined the impact of elevated monensin on the DMI of feedlot cattle fed barley-based diets (Xu et al., 2013). The reduction in the flows of OM, NDF, and starch to the duodenum with elevated levels of monensin reflects the decline in DMI, whereas extent of digestion of OM, NDF, and starch in the rumen was similar between these 2 monensin levels. This lack of an effect of monensin level on ruminal digestibility contrasts with previous reports that ruminal digestibility of OM and NDF declined with elevated levels of monensin when barley grain was substituted for a portion of the wheat distillers grain in high-grain diets (Xu et al., 2013). These authors suggested that the increase in ruminal digestibility of OM may have resulted from an increase in ruminal retention time of feed, thereby enhancing the extent of feed digestion by ruminal microorganisms. Monensin is known to inhibit ruminal protein degradation and decrease the flow of microbial protein to the small intestine (Bergen and Bates, 1984; Ruiz et al., 2001). Although ruminal N digestibility was not affected (−6%; P < 0.40), the reduction in the amount of N truly digested in the rumen appeared substantial (−15%) with the higher level of monensin. The numerical decrease in microbial protein flow to the duodenum was consistent with the numerical reduction in the amount of ruminally fermented OM with elevated monensin. These results indicate that the high level of monensin supplementation of wheat-based finishing diet effectively lowered ruminal protein degradation. Increasing the monensin level decreased microbial protein flow to the duodenum an average of 12% regardless of whether heifers were being fed SW or HW, an outcome consistent with reduction in DMI (9%) and ruminal OM digestion (8%) as seen in previous reports (Ruiz et al., 2001; Mwenya et al., 2005). Monensin has been shown to reduce variation in feed intake level (Stock et al., 1995) and meal frequency to increase the ruminal pH of cattle fed high-grain diets (Burrin and Britton, 1986). Therefore, increasing the level of monensin in the diet may help reduce the risk of acidosis associated with wheat-based finishing diets. However, in the present study, elevated monensin levels had little impact on ruminal pH, though minimum pH tended (P < 0.08) to be increased. Other factors, such as sufficient time for dietary adaptation, bunk management, and increasing the level of silage in the diet, are effective management strategies (Erickson et al., 2003; Li et al., 2011) for reducing the risk of acidosis in cattle fed wheat diets. The greater proportion of propionate in VFA and trend for a reduction in the ratio of acetate to propionate with high monensin is consistent with the literature and its positive effect on feed efficiency (Richardson et al., 1976; Duffield et al., 2012). With finishing diets, the monensin-mediated increase in propionate may have less effect on rate of gain and more effect on reducing DMI, an observation confirmed by several recent studies with feedlot cattle fed high-grain diets. In conclusion, heifers fed wheat-based finishing diets had no differences in DMI, duodenal OM flow, or digestibility of OM in the rumen or in the intestine as compared to heifers fed barley grain. However, substituting wheat for barley in finishing diets increased intake of digested starch and microbial protein production but decreased intake of digested NDF. Feeding wheat also decreased ruminal pH, potentially increasing the risk of ruminal acidosis. There was no interaction of wheat type (HW vs. SW) with monensin level on the parameters measured. Feeding SW vs. HW resulted in overall no differences in the DMI, ruminal pH or fermentation, and nutrient digestion, with the exception that HW delivered slightly more OM and NAN to the duodenum. These results indicated that HW and SW exhibited similar to only slightly superior feed value relative to barley when processed to a similar degree, despite their greater starch and lower NDF content. 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TI - Impact of hard vs. soft wheat and monensin level on rumen acidosis in feedlot heifers
JF - Journal of Animal Science
DO - 10.2527/jas.2014-8092
DA - 2014-11-01
UR - https://www.deepdyve.com/lp/oxford-university-press/impact-of-hard-vs-soft-wheat-and-monensin-level-on-rumen-acidosis-in-MSrfCH0u2V
SP - 5088
EP - 5098
VL - 92
IS - 11
DP - DeepDyve
ER -