TY - JOUR
AU - Schwartzkopf-Genswein, K.
AB - ABSTRACT Eighty continental crossbred beef heifers (414.9 ± 37.9 kg of BW), 16 of which were ruminally cannulated, were used in a 52-d experiment with a generalized randomized block design, to assess if self-selection of dietary ingredients modulates ruminal pH and improves rumen function of feedlot finishing cattle. Treatments were total mixed ration [TMR; 85% barley grain (BG), 10% corn silage (CS), 5% supplement]; or free-choice (self-selection; FC) diets of barley grain and corn silage (BGCS), barley grain and wheat distillers grain (BGDG), or corn silage and wheat distillers grain (CSDG). Heifers were housed in groups of 10 in 8 pens equipped with the GrowSafe System (Airdrie, AB, Canada) enabling feed intake and feeding behavior to be continuously monitored. Two cannulated heifers were randomly assigned to each pen and equipped with indwelling pH probes for continuous measurement of ruminal pH during 4 periods (d 1 to 4, d 7 to 14, d 21 to 28, and d 42 to 49). Rumen fluid samples were collected from cannulated heifers on d 7 and 42 before feed delivery, and on d 4 and 49 at 2 h post-feed delivery for determination of VFA. Heifers fed the TMR had shorter (P = 0.01) and smaller (P = 0.03) meals than those fed the FC diets. Cattle fed BGCS and BGDG increased (P < 0.01) intake of BG over time by up to 80 and 70%, respectively. Increased consumption of BG arose from an increase (P < 0.01) in eating rate over the same (P > 0.10) feeding time, which was accompanied by an increase (P < 0.05) in eating rate but a decrease (P < 0.05) in feeding time of either CS or DG. Even with increased BG consumption, ruminal pH and VFA profiles were not different (P > 0.10) among FC diets or compared with the TMR. Cattle fed FC CSDG consumed DG at 60% of dietary DM over the trial, resulting in greater (P < 0.05) mean ruminal pH and acetate-to-propionate ratio and less (P < 0.05) area under the curve than those given the other FC diets or the TMR. Finishing feedlot cattle fed FC diets containing BG self-regulate intake of diets that have a similar composition, intake level, and ruminal fermentation profile to those fed a TMR. INTRODUCTION Feedlot cattle are fed total mixed rations (TMR) containing a high proportion of grain to maximize growth performance and profitability. Feedlot diets usually contain a base level of physically effective fiber to reduce the risk of subclinical ruminal acidosis and enhance rumen function. Whereas pH in clinical ruminal acidosis is below 5.0, subclinical acidosis is characterized by prolonged bouts of ruminal pH between 5.0 and 5.5 (Nagaraja and Lechtenberg, 2007), which arises from the rapid production of VFA as a result of microbial fermentation of feed (Owens et al., 1998). The amount of fiber required to reduce subclinical acidosis depends on diet fermentability, feeding behavior, and rate of ruminal absorption of VFA in the host (Bevans et al., 2005). Previous studies reported that ruminants given free access to dietary ingredients select a diet that attenuates subclinical ruminal acidosis, although ruminal pH was not measured in those studies (Cooper et al., 1996; Phy and Provenza, 1998a,b; James and Kyriazakis, 2002). However, research that measured ruminal pH in relation to selection of dietary ingredients has not substantiated this claim (Keunen et al., 2003; Paton et al., 2006). Distillers grains (DG) are widely used in the diets of feedlot cattle (Klopfenstein et al., 2008). Although research on corn DG has been conducted (Vander Pol et al., 2005; Corrigan et al., 2009), little information exists on the risk of acidosis for finishing cattle fed diets containing wheat DG. A recent study showed that when wheat DG were used as a partial forage substitute, dairy cows spent less time chewing and had a greater risk of acidosis (Penner et al., 2009). The current study was conducted to determine if, when offered a choice of ingredients, feedlot cattle will select a diet that modulates ruminal pH. The study focused on barley-grain (BG), corn silage (CS), and wheat DG, which are common ingredients used in the diets of feedlot cattle in western Canada. MATERIALS AND METHODS All animals were cared for in accordance to the Canadian Council of Animal Care (1997) guidelines. Experimental Design Animals. Eighty continental crossbred beef heifers, with an initial BW of 414.9 ± 37.9 kg, were used in a 52-d experiment at the Agriculture and Agri-Food Canada Research Centre in Lethbridge (Alberta, Canada). The experiment was conducted as a generalized randomized block design with 4 dietary treatments and 20 replications (heifers). One month before the study, cattle were ear tagged, fitted with a radio frequency transponder (Allflex USA Inc., Dallas-Ft. Worth, TX) in the left ear, and given a growth promotant implant (Component E-H, Elanco Animal Health, Guelph, ON, Canada). In addition, 16 of the 80 heifers were ruminally cannulated (model 9C, Bar Diamond, Parma, ID) under local anesthesia using full aseptic precautions. Dietary Treatments. Treatments included (Table 1) 1) a TMR comprising mainly barley grain and corn silage as a control (TMR); 2) a free-choice (i.e., self-selection) diet (BGCS) providing cattle with separate access to dry-rolled BG and CS; 3) a free-choice diet (BGDG) of BG and pelleted wheat DG offered separately; or 4) a free-choice diet (CSDG) of CS and DG offered separately. The control TMR was typical of rations used in western Canadian feedlots before the availability of DG. To ensure adequate vitamin and mineral consumption (NRC, 2000), a pelleted supplement was added to the TMR and to all dietary components (i.e., BG, CS, and DG) at a rate of 5% (DM basis). Melengesterol acetate (Pfizer, Kirkland, Québec, Canada) was added to the supplement to suppress estrus. Furthermore, to ensure N intake was not a limiting factor, CS was blended with a small amount of wheat DG to increase its CP content to 12.9% (DM basis, Table 1). Table 1. Dietary components and chemical composition Item  Dietary component1  TMR  BG  DG  CS  Ingredient composition, % of diet DM               Tempered barley-grain  85.0  95.0  —2  —   Wheat dry distillers grain  —  —  95.0  12.0   Corn silage  10.0  —  —  83.0   Supplement  5.0  5.0  5.0  5.0  Chemical composition               DM, %  81.2  86.2  93.1  46.0   CP, % of DM  14.1  15.6  40.7  12.9   NDF, % of DM  20.3  17.5  27.8  42.2   ADF, % of DM  6.9  4.4  16.8  26.6   NEm, Mcal/kg  1.97  2.03  2.03  1.56   NEg, Mcal/kg  1.33  1.38  1.38  0.97  Item  Dietary component1  TMR  BG  DG  CS  Ingredient composition, % of diet DM               Tempered barley-grain  85.0  95.0  —2  —   Wheat dry distillers grain  —  —  95.0  12.0   Corn silage  10.0  —  —  83.0   Supplement  5.0  5.0  5.0  5.0  Chemical composition               DM, %  81.2  86.2  93.1  46.0   CP, % of DM  14.1  15.6  40.7  12.9   NDF, % of DM  20.3  17.5  27.8  42.2   ADF, % of DM  6.9  4.4  16.8  26.6   NEm, Mcal/kg  1.97  2.03  2.03  1.56   NEg, Mcal/kg  1.33  1.38  1.38  0.97  1TMR = total mixed ration; BG = dry-rolled barley grain component of the free-choice diets BGCS and BGDG; DG = wheat dry distillers grain component of the free-choice diets BGDG and DGCS; CS = corn silage component of the free-choice diets BGCS and DGCS. 2Ingredient not included in the diet. View Large Table 1. Dietary components and chemical composition Item  Dietary component1  TMR  BG  DG  CS  Ingredient composition, % of diet DM               Tempered barley-grain  85.0  95.0  —2  —   Wheat dry distillers grain  —  —  95.0  12.0   Corn silage  10.0  —  —  83.0   Supplement  5.0  5.0  5.0  5.0  Chemical composition               DM, %  81.2  86.2  93.1  46.0   CP, % of DM  14.1  15.6  40.7  12.9   NDF, % of DM  20.3  17.5  27.8  42.2   ADF, % of DM  6.9  4.4  16.8  26.6   NEm, Mcal/kg  1.97  2.03  2.03  1.56   NEg, Mcal/kg  1.33  1.38  1.38  0.97  Item  Dietary component1  TMR  BG  DG  CS  Ingredient composition, % of diet DM               Tempered barley-grain  85.0  95.0  —2  —   Wheat dry distillers grain  —  —  95.0  12.0   Corn silage  10.0  —  —  83.0   Supplement  5.0  5.0  5.0  5.0  Chemical composition               DM, %  81.2  86.2  93.1  46.0   CP, % of DM  14.1  15.6  40.7  12.9   NDF, % of DM  20.3  17.5  27.8  42.2   ADF, % of DM  6.9  4.4  16.8  26.6   NEm, Mcal/kg  1.97  2.03  2.03  1.56   NEg, Mcal/kg  1.33  1.38  1.38  0.97  1TMR = total mixed ration; BG = dry-rolled barley grain component of the free-choice diets BGCS and BGDG; DG = wheat dry distillers grain component of the free-choice diets BGDG and DGCS; CS = corn silage component of the free-choice diets BGCS and DGCS. 2Ingredient not included in the diet. View Large All feeds were delivered at 0900 h once daily, and were provided for ad libitum intake, which ensured a 5 to 10% daily refusal. For free-choice diets, the location of each dietary ingredient within the pen was switched every 7 d throughout the experiment to eliminate the confounding effect of feed type and bunk location. Before the start of the experiment, all heifers were adapted to the TMR diet, with free-choice diets being available from d 1 to 52 of the study. Housing. Animals were assigned in groups of 10 to 1 of 8 pens to ensure uniformity in BW among pens. Pens measured 21 × 27 m, with 15 m2 of concrete in front of the feed bunk and 12.6 m2 of pen space available for each heifer. Cattle were provided with a continuous supply of fresh water and a bedded (whole barley straw) area away from the feed bunk. The amount of bedding added to the pens was recorded. In addition, visual observations were made (using video camera) over the first 21 d of the experiment to verify whether the heifers consumed the bedding. Each pen contained 2 feeding tubs equipped with an electronic monitoring system (GrowSafe Systems, Airdrie, Alberta, Canada) for automatic recording of feed intake and feeding behavior. Each tub measured 0.91 m × 0.53 m × 0.38 m, and allowed only 1 heifer to eat at a time. Tubs were mounted on 2 load cells and had an antenna encased in the rim that recorded electromagnetic signals from ear transponders (Allflex Canada, St-Hyacinthe, Québec, Canada) with a range of approximately 0.5 m. A reader panel recorded individual transponder numbers, feed weight, and the time of day every 2 s over the experiment. The system was monitored daily to ensure it was functioning correctly. Each treatment was offered in 2 of the 8 pens. The 16 cannulated heifers were assigned equally to the 8 pens (2 cannulated heifers per pen). All heifers were adapted to their feeding system for at least 7 d before commencing the experiment. Data Collection Feed Analysis and Intake. Samples of TMR, BG, DG, and CS were collected weekly for determination of DM. Values were used to calculate weekly composition of the diets offered on a DM basis. A subsample of each was stored and composited at 3-wk intervals for chemical analyses. Feed offered was recorded daily for each pen over the length of the experiment, whereas orts were removed, weighed, and sampled weekly for DM determination. Therefore, DMI was determined weekly for each diet and pen as DM offered minus DM refused to ensure that the automatic monitoring system was functioning correctly. The feed and orts DM was determined by oven drying at 55°C for 48 h. All the analyses were performed on each sample in duplicate, and the analysis was repeated when the CV was greater than 5%. The NDF and ADF contents were determined by methods described by Van Soest et al. (1991), with amylase and sodium sulfite used in the NDF procedure. The concentration of CP (N × 6.25) in feed was quantified by flash combustion with gas chromatography and thermal conductivity detection (Carlo Erba Instruments, Milan, Italy). Feeding Behavior. The electronic feed bunk monitoring system (GrowSafe Systems) allowed the collection and storage of animal behavior data for the 80 heifers 24 h/d from d 1 to 49 of the experiment. Distinct feeding events were pooled into meals as described by González et al. (2009). To pool feeding visits into meals, the meal criterion was calculated for each individual animal using the method of Yeates et al. (2001); the meal criterion was defined as the longest nonfeeding interval (min) accepted as part of a meal. The meal criterion allowed the determination of meal frequency (the number of times per day that a nonfeeding interval length exceeded the meal criterion), the meal size (the average feed consumed per meal; kg of DM/meal), and the meal duration and meal time, calculated as the sum of the length of all visits within a meal (min/meal) or within a day (min/d), respectively, including time out of the feeder within a meal. Frequency of visits was calculated as the number of feeding visits per day (no./d) and per meal (no./meal). Daily feeding time (min/d) was calculated as the time spent at the feeders within a day, without including the time in which heifers were absent from the feeders within a meal. Feeding rate was determined as daily DMI divided by daily feeding time (g of DM/min). Additionally, total DMI and proportion of each component in the total DMI were calculated daily, as was the day-to-day variability over the experiment, calculated as the standard deviation. All calculations were determined for each animal using an in-house program developed in SAS (SAS Inst. Inc., Cary, NC). BW Gain. Heifers were weighed 2 h before feed delivery at the beginning and the end of the study. The ADG of each heifer was determined by dividing BW gain by the number of days on feed. Feed efficiency was calculated individually as G:F (kg of BW gain divided by kg of DMI over the experiment). Ruminal pH. Ruminal pH was continuously measured for 4 periods during the experiment (1 to 4 d, 7 to 14 d, 21 to 28 d, and 42 to 49 d) using the wireless indwelling LRC pH system (Penner et al., 2006). The system consisted of a pH probe (model PHCN-37, Omega Engineering, Stamford, CT) enclosed in a protective shield that allowed the ruminal liquid to percolate freely but prevented the electrode from contacting the ruminal epithelium. Weights were attached to each probe to ensure that they remained in the ventral sac of the rumen. The indwelling electrode measured and recorded the ruminal pH every 60 s over the measurement period. Each electrode was standardized using pH 4.0 and 7.0 standards at the beginning and end of each session. The pH data were first summarized by day and then averaged across each measurement period as mean, maximum, and minimum pH; area under the curve; and proportion of time in which pH was below 5.8, 5.5, and 5.2 as an index of severity of ruminal acidosis. The area under the curve was calculated by multiplying the absolute value of deviations in pH by the time (min) spent below the established threshold for each measure, and was then divided by 60 and expressed as pH unit × hour. VFA Measurements. Ruminal content samples were taken from cannulated animals on d 7 and 42 before feed delivery and on d 4 and 49 at 2 h after feed delivery. A composite sample of ruminal contents (250 mL per site) was obtained from the reticulum, the dorsal and ventral sacs, and the feed mat for each animal and was strained through polyester monofilament fabric (Pecap 7-335/47 mesh opening-335 μm, Tetko Inc., Scarborough, Ontario, Canada). For each heifer, 5 mL of filtrate was preserved for subsequent determination of VFA and lactate by adding 1 mL of 25% (wt/vol) metaphosphoric acid. Samples were stored at −20°C in sealed plastic vials until analysis. Ruminal VFA were quantified using gas chromatography with crotonic acid as an internal standard (model 5890, Hewlett Packard, Little Falls, DE) and a capillary column (30 m × 0.32 mm i.d., 1-μm phase thickness, bonded polyethylene glycol, Phenomenex Inc., Torrance, CA) using flame-ionization detection. Lactic acid concentration was determined by gas chromatography after derivatization with boron trifluoride-methanol as described by Supelco (1998). Blood Variables. Blood samples were taken from the cannulated heifers via jugular venipuncture immediately before feed delivery on d 0, 7, 42, and 49 of the study. Blood was collected in 10-mL vacuum tubes containing Na-heparin (Vacutainer No. 6480, Becton Dickinson, Franklin Lakes, NJ), and samples were analyzed within 2 h. Blood pH, partial pressure of CO2, bicarbonate content, total content of CO2 (equivalent to the content of bicarbonate plus carbonic acid), and the serum anion gap (the sum of free cations, Na+ + K+, minus the sum of free anions, Cl− + HCO3−, in serum) were determined immediately using an electrolyte and blood gas analyzer (Vetstat, Idexx Laboratories, Westbrook, ME). Packed cell volume was determined using blood collected into a 7-mL vacuum tube containing EDTA (Vacutainer No. 6450, Becton Dickinson). The blood was transferred to a microhematocrit capillary tube, the end was sealed, and the tube was centrifuged at room temperature for 6 min at 12,000 × g in a hematocrit centrifuge and read with a microcapillary reader (model MH, International Equipment Co., Boston, MA). Statistical Analyses Data were analyzed using the MIXED procedure of SAS for repeated measures over time, considering heifer as the subject with the univariate or first-order antedependence covariance structure, according to the smallest Bayesian information criterion. In addition, a set of 4 pens was used as the blocking factor. In the analysis of feed intake, feeding behavior, feed cost of BW gain, pH profile, and blood and rumen fermentation variables, the model included treatment, time, and their interactions as fixed effects, whereas the effect of the pen was considered a random factor. The meal criterion, SD of DMI, ADG, and G:F were calculated individually over the duration of the experiment, so the model included treatment as a fixed effect, and pen and pen within treatment as random effects. For all statistical analyses, significance was declared at P ≤ 0.05 and trends at 0.05 < P < 0.10, using the Bonferroni multiple comparison test to separate means. RESULTS Treatment differences were of more interest than time differences within a treatment. Therefore, daily means within each treatment are not presented, but treatment × time interactions are discussed where applicable. Feed Intake and Behavior A treatment × time interaction (P < 0.01) was observed for DMI, with all heifers increasing their DMI from wk 1 to 7, with the exception of those fed BGDG, in which DMI did not change during the experiment (Table 2). Heifers fed CSDG had greater DMI than those fed TMR (P = 0.04) or BGCS (P < 0.01), and heifers fed BGDG had greater DMI (P = 0.01) than those fed BGCS. In addition, the day-to-day variability in DMI over the experiment tended to be greater (P = 0.06) in heifers fed CSDG than in those fed the TMR (CSDG = 2.53 vs. TMR = 1.98 kg of DM/d; SEM = 0.094). A treatment × time interaction (P < 0.01) was observed for heifers fed free-choice diets (Figure 1), in which cattle fed CSDG maintained the same consumption pattern over the experiment, but those offered the BGCS or BGDG treatments increased BG (P < 0.05) as a proportion of DMI over time and decreased (P < 0.05) the proportion of DMI of either CS or DG, respectively. Table 2. Feed intake and feeding behavior of heifers consuming TMR, BGCS, BGDG, and CSDG treatments determined individually (n = 20) over the experiment Item  Treatment1  SEM  P-value2  TMR  BGCS  BGDG  CSDG  Trt  Time  Trt × time  DMI, kg of DM/d  8.97bc  8.33c  9.37ab  9.71a  0.254  <0.01  <0.01  <0.01  Meal characteristics                           Size, kg of DM/meal  1.13b  1.24ab  1.37a  1.37a  0.065  0.03  <0.01  0.31   Meal duration, min/meal  15.3c  24.8ab  20.6bc  27.8a  1.65  <0.01  <0.01  0.07   Meal time, min/d  119.0c  164.6ab  137.9bc  194.6a  9.54  0.01  <0.01  <0.01   Frequency, No./d  8.53a  7.15b  7.34b  7.52b  0.332  0.02  <0.01  0.07   Feeding time, min/d  83.3b  74.2b  73.1b  124.5a  4.19  <0.01  <0.01  <0.01   Feeding rate, g of DM/min  112.9b  119.7ab  134.0a  81.1c  4.43  <0.01  <0.01  0.01   Frequency of visits, No./d  18.5b  33.5a  30.6a  36.7a  1.46  0.01  <0.01  <0.01  Item  Treatment1  SEM  P-value2  TMR  BGCS  BGDG  CSDG  Trt  Time  Trt × time  DMI, kg of DM/d  8.97bc  8.33c  9.37ab  9.71a  0.254  <0.01  <0.01  <0.01  Meal characteristics                           Size, kg of DM/meal  1.13b  1.24ab  1.37a  1.37a  0.065  0.03  <0.01  0.31   Meal duration, min/meal  15.3c  24.8ab  20.6bc  27.8a  1.65  <0.01  <0.01  0.07   Meal time, min/d  119.0c  164.6ab  137.9bc  194.6a  9.54  0.01  <0.01  <0.01   Frequency, No./d  8.53a  7.15b  7.34b  7.52b  0.332  0.02  <0.01  0.07   Feeding time, min/d  83.3b  74.2b  73.1b  124.5a  4.19  <0.01  <0.01  <0.01   Feeding rate, g of DM/min  112.9b  119.7ab  134.0a  81.1c  4.43  <0.01  <0.01  0.01   Frequency of visits, No./d  18.5b  33.5a  30.6a  36.7a  1.46  0.01  <0.01  <0.01  a–cMeans in the same row with different superscripts are different (P < 0.05). 1Treatments were a total mixed ration (TMR) and 3 free-choice diets consisting of components offered separately: barley grain and corn silage (BGCS), barley grain and wheat dry distillers grain (BGDG), and corn silage and wheat dry distillers grain (CSDG). 2Fixed effects were treatment (Trt), time, and treatment within time (Trt × time). View Large Table 2. Feed intake and feeding behavior of heifers consuming TMR, BGCS, BGDG, and CSDG treatments determined individually (n = 20) over the experiment Item  Treatment1  SEM  P-value2  TMR  BGCS  BGDG  CSDG  Trt  Time  Trt × time  DMI, kg of DM/d  8.97bc  8.33c  9.37ab  9.71a  0.254  <0.01  <0.01  <0.01  Meal characteristics                           Size, kg of DM/meal  1.13b  1.24ab  1.37a  1.37a  0.065  0.03  <0.01  0.31   Meal duration, min/meal  15.3c  24.8ab  20.6bc  27.8a  1.65  <0.01  <0.01  0.07   Meal time, min/d  119.0c  164.6ab  137.9bc  194.6a  9.54  0.01  <0.01  <0.01   Frequency, No./d  8.53a  7.15b  7.34b  7.52b  0.332  0.02  <0.01  0.07   Feeding time, min/d  83.3b  74.2b  73.1b  124.5a  4.19  <0.01  <0.01  <0.01   Feeding rate, g of DM/min  112.9b  119.7ab  134.0a  81.1c  4.43  <0.01  <0.01  0.01   Frequency of visits, No./d  18.5b  33.5a  30.6a  36.7a  1.46  0.01  <0.01  <0.01  Item  Treatment1  SEM  P-value2  TMR  BGCS  BGDG  CSDG  Trt  Time  Trt × time  DMI, kg of DM/d  8.97bc  8.33c  9.37ab  9.71a  0.254  <0.01  <0.01  <0.01  Meal characteristics                           Size, kg of DM/meal  1.13b  1.24ab  1.37a  1.37a  0.065  0.03  <0.01  0.31   Meal duration, min/meal  15.3c  24.8ab  20.6bc  27.8a  1.65  <0.01  <0.01  0.07   Meal time, min/d  119.0c  164.6ab  137.9bc  194.6a  9.54  0.01  <0.01  <0.01   Frequency, No./d  8.53a  7.15b  7.34b  7.52b  0.332  0.02  <0.01  0.07   Feeding time, min/d  83.3b  74.2b  73.1b  124.5a  4.19  <0.01  <0.01  <0.01   Feeding rate, g of DM/min  112.9b  119.7ab  134.0a  81.1c  4.43  <0.01  <0.01  0.01   Frequency of visits, No./d  18.5b  33.5a  30.6a  36.7a  1.46  0.01  <0.01  <0.01  a–cMeans in the same row with different superscripts are different (P < 0.05). 1Treatments were a total mixed ration (TMR) and 3 free-choice diets consisting of components offered separately: barley grain and corn silage (BGCS), barley grain and wheat dry distillers grain (BGDG), and corn silage and wheat dry distillers grain (CSDG). 2Fixed effects were treatment (Trt), time, and treatment within time (Trt × time). View Large Figure 1. View largeDownload slide Dietary component preference (as % of total DMI) of each free-choice treatment, obtained individually each week over the experiment (n = 20). Treatments consisted of components offered separately: barley grain and corn silage (BGCS), barley grain and wheat dry distillers grain (BGDG), and wheat dry distillers grain and corn silage (CSDG). The treatment × time interaction was significant for CS (P < 0.01), BG (P = 0.01), and DG (P < 0.01) dietary components. Different letters (a, b) within the same dietary component and treatment are different (P < 0.05). Figure 1. View largeDownload slide Dietary component preference (as % of total DMI) of each free-choice treatment, obtained individually each week over the experiment (n = 20). Treatments consisted of components offered separately: barley grain and corn silage (BGCS), barley grain and wheat dry distillers grain (BGDG), and wheat dry distillers grain and corn silage (CSDG). The treatment × time interaction was significant for CS (P < 0.01), BG (P = 0.01), and DG (P < 0.01) dietary components. Different letters (a, b) within the same dietary component and treatment are different (P < 0.05). The meal criterion was shorter (P < 0.01) in heifers fed TMR than those fed CSBG (TMR = 1,007.1 vs. CSBG = 1,793.0 s; SEM = 161.3). Meal size increased (P < 0.01) over the experiment for all treatments, but it was smaller (P = 0.03) in heifers fed TMR than in those fed BGDG or CSDG (Table 2). Daily feeding time showed a treatment × time interaction (P < 0.01), in which heifers fed BGCS and BGDG decreased the time they spent feeding over the experiment, whereas those fed TMR and CSDG maintained the same feeding time over the course of the experiment. Feeding rate also showed a treatment × time interaction (P = 0.01), where all heifers increased their feeding rate over the experiment with the exception of those fed CSDG. The frequency of visits per day, like the frequency of visits per meal, showed a treatment × time interaction (P < 0.01). Heifers fed TMR and BGDG did not alter (P > 0.10) the number of feeder visits over the experiment, whereas those fed BGCS and CSDG visited the feed bunk more frequently over the course of the study. The analyses of feeding behavior on each dietary component separately showed the same trends and effects with the data sorted by meal or by day; therefore, discussion regarding feeding behavior will be presented by day. Regarding the feeding behavior of heifers offered CS as a dietary component, a treatment × time interaction (P < 0.01) was observed for all but a few calculated feeding behavior variables (Table 3). The proportion of CS consumed in the final diet was constant over the experiment when it was offered with DG, but decreased (P < 0.01) when it was offered with BG (wk 1 = 24.4 vs. wk 7 = 16.9% of total DMI; SEM = 1.99). The daily DMI of CS increased over the experiment when it was offered with DG but remained constant when it was offered with BG. The daily feeding time remained constant over the experiment when CS was offered with DG but decreased over the experiment with BG (wk 1 = 39.2 vs. wk 7 = 24.1 min/d; SEM = 5.30). The feeding rate showed a trend (P = 0.08) for a treatment × time interaction. Feeding rate was less over the course of the experiment when CS was offered with DG than when CS was offered with BG. The frequency of daily visits to the CS feeder had a greater increase over the study when CS was offered with DG (wk 1 = 14.7 vs. wk 7 = 20.3 visits/d; SEM = 1.04) than when it was offered with BG (wk 1 = 10.5 vs. wk 7 = 13.4 visits/d; SEM = 1.04). Table 3. Feeding behavior of heifers consuming corn silage (CS), barley grain (BG), and wheat distillers grains (DG) when offered separately in the free-choice dietary treatments (n = 20) Item  Treatment1  SEM  P-value2  BGCS  BGDG  CSDG  Trt  Time  Trt × time  CS                        % of DMI, kg of CS/total DMI  20.4  —3  36.6  1.99  0.02  0.046  <0.01   DMI, kg/d  1.65  —  3.52  0.16  0.01  0.04  <0.01   Feeding time, min/d  30.6  —  67.5  5.30  0.03  0.047  <0.01   Feeding rate, g of DM/min  62.7  —  54.8  2.40  0.02  <0.01  0.08   Frequency of visits, No./d  13.5  —  19.9  1.04  0.03  <0.01  <0.01  BG                        % of DMI, kg of BG/total DMI  79.6  62.0  —  1.99  0.03  <0.01  0.01   DMI, kg/d  6.70  5.79  —  0.18  0.01  <0.01  0.11   Feeding time, min/d  43.7  41.9  —  1.82  0.48  <0.01  <0.01   Feeding rate, g of DM/min  163.7  147.2  —  6.08  0.06  <0.01  <0.01   Frequency of visits, No./d  20.1  17.4  —  0.97  0.30  <0.01  <0.01  DG                        % of DMI, kg of DG/total DMI  —  38.1  63.4  1.97  0.01  <0.01  <0.01   DMI, kg/d  —  3.59  6.19  0.34  0.046  0.47  <0.01   Feeding time, min/d  —  31.2  57.0  2.15  <0.01  <0.01  <0.01   Feeding rate, g of DM/min  —  122.3  114.7  4.82  0.28  <0.01  <0.01   Frequency of visits, No./d  —  13.2  16.8  0.74  <0.01  <0.01  <0.01  Item  Treatment1  SEM  P-value2  BGCS  BGDG  CSDG  Trt  Time  Trt × time  CS                        % of DMI, kg of CS/total DMI  20.4  —3  36.6  1.99  0.02  0.046  <0.01   DMI, kg/d  1.65  —  3.52  0.16  0.01  0.04  <0.01   Feeding time, min/d  30.6  —  67.5  5.30  0.03  0.047  <0.01   Feeding rate, g of DM/min  62.7  —  54.8  2.40  0.02  <0.01  0.08   Frequency of visits, No./d  13.5  —  19.9  1.04  0.03  <0.01  <0.01  BG                        % of DMI, kg of BG/total DMI  79.6  62.0  —  1.99  0.03  <0.01  0.01   DMI, kg/d  6.70  5.79  —  0.18  0.01  <0.01  0.11   Feeding time, min/d  43.7  41.9  —  1.82  0.48  <0.01  <0.01   Feeding rate, g of DM/min  163.7  147.2  —  6.08  0.06  <0.01  <0.01   Frequency of visits, No./d  20.1  17.4  —  0.97  0.30  <0.01  <0.01  DG                        % of DMI, kg of DG/total DMI  —  38.1  63.4  1.97  0.01  <0.01  <0.01   DMI, kg/d  —  3.59  6.19  0.34  0.046  0.47  <0.01   Feeding time, min/d  —  31.2  57.0  2.15  <0.01  <0.01  <0.01   Feeding rate, g of DM/min  —  122.3  114.7  4.82  0.28  <0.01  <0.01   Frequency of visits, No./d  —  13.2  16.8  0.74  <0.01  <0.01  <0.01  1Treatments were free-choice diets consisting of components offered separately: barley grain and corn silage (BGCS), barley grain and wheat dry distillers grain (BGDG), and corn silage and wheat dry distillers grain (CSDG). 2Fixed effects were treatment (Trt), time, and treatment within time (Trt × time). 3Ingredient not included in the diet. View Large Table 3. Feeding behavior of heifers consuming corn silage (CS), barley grain (BG), and wheat distillers grains (DG) when offered separately in the free-choice dietary treatments (n = 20) Item  Treatment1  SEM  P-value2  BGCS  BGDG  CSDG  Trt  Time  Trt × time  CS                        % of DMI, kg of CS/total DMI  20.4  —3  36.6  1.99  0.02  0.046  <0.01   DMI, kg/d  1.65  —  3.52  0.16  0.01  0.04  <0.01   Feeding time, min/d  30.6  —  67.5  5.30  0.03  0.047  <0.01   Feeding rate, g of DM/min  62.7  —  54.8  2.40  0.02  <0.01  0.08   Frequency of visits, No./d  13.5  —  19.9  1.04  0.03  <0.01  <0.01  BG                        % of DMI, kg of BG/total DMI  79.6  62.0  —  1.99  0.03  <0.01  0.01   DMI, kg/d  6.70  5.79  —  0.18  0.01  <0.01  0.11   Feeding time, min/d  43.7  41.9  —  1.82  0.48  <0.01  <0.01   Feeding rate, g of DM/min  163.7  147.2  —  6.08  0.06  <0.01  <0.01   Frequency of visits, No./d  20.1  17.4  —  0.97  0.30  <0.01  <0.01  DG                        % of DMI, kg of DG/total DMI  —  38.1  63.4  1.97  0.01  <0.01  <0.01   DMI, kg/d  —  3.59  6.19  0.34  0.046  0.47  <0.01   Feeding time, min/d  —  31.2  57.0  2.15  <0.01  <0.01  <0.01   Feeding rate, g of DM/min  —  122.3  114.7  4.82  0.28  <0.01  <0.01   Frequency of visits, No./d  —  13.2  16.8  0.74  <0.01  <0.01  <0.01  Item  Treatment1  SEM  P-value2  BGCS  BGDG  CSDG  Trt  Time  Trt × time  CS                        % of DMI, kg of CS/total DMI  20.4  —3  36.6  1.99  0.02  0.046  <0.01   DMI, kg/d  1.65  —  3.52  0.16  0.01  0.04  <0.01   Feeding time, min/d  30.6  —  67.5  5.30  0.03  0.047  <0.01   Feeding rate, g of DM/min  62.7  —  54.8  2.40  0.02  <0.01  0.08   Frequency of visits, No./d  13.5  —  19.9  1.04  0.03  <0.01  <0.01  BG                        % of DMI, kg of BG/total DMI  79.6  62.0  —  1.99  0.03  <0.01  0.01   DMI, kg/d  6.70  5.79  —  0.18  0.01  <0.01  0.11   Feeding time, min/d  43.7  41.9  —  1.82  0.48  <0.01  <0.01   Feeding rate, g of DM/min  163.7  147.2  —  6.08  0.06  <0.01  <0.01   Frequency of visits, No./d  20.1  17.4  —  0.97  0.30  <0.01  <0.01  DG                        % of DMI, kg of DG/total DMI  —  38.1  63.4  1.97  0.01  <0.01  <0.01   DMI, kg/d  —  3.59  6.19  0.34  0.046  0.47  <0.01   Feeding time, min/d  —  31.2  57.0  2.15  <0.01  <0.01  <0.01   Feeding rate, g of DM/min  —  122.3  114.7  4.82  0.28  <0.01  <0.01   Frequency of visits, No./d  —  13.2  16.8  0.74  <0.01  <0.01  <0.01  1Treatments were free-choice diets consisting of components offered separately: barley grain and corn silage (BGCS), barley grain and wheat dry distillers grain (BGDG), and corn silage and wheat dry distillers grain (CSDG). 2Fixed effects were treatment (Trt), time, and treatment within time (Trt × time). 3Ingredient not included in the diet. View Large When animals were offered free-choice treatments, the proportion of BG ingested showed a treatment × time interaction (P = 0.01), in which BG intake increased over the experiment when it was offered with CS or DG (Figure 1). However, the variability of the proportion of BG consumed over the experiment was less (P = 0.02) when BG was offered with CS than with DG (BGCS = 5.74 vs. BGDG = 8.33 kg of BG/total DMI; SEM = 0.771).The daily DMI of BG (P = 0.01) was greater when BG was offered with CS than with DG and increased (P < 0.01) over the experiment with CS or DG (Table 3). The feeding behavior of heifers offered free-choice diets containing DG showed a treatment × time interaction (P < 0.01) for all the calculated parameters (Table 3). The proportion of DG chosen was constant over the experiment when it was offered with CS, but decreased with BG (wk 1 = 43.1 vs. wk 7 = 32.8% of total DMI; SEM = 1.97; Figure 1). In addition, the variability of the proportion of DG ingested over the experiment was greater (P < 0.01) when it was offered with BG than with CS (BGDG = 8.33 vs. CSDG = 4.88 kg of DG/total DMI; SEM = 0.598). The daily DMI increased when it was offered with CS but decreased when it was offered with BG. The daily feeding time decreased when DG was offered with BG (wk 1 = 41.2 vs. wk 7 = 23.2 min/d; SEM = 2.15) but remained constant with CS. The frequency of visits per day to the DG feeder increased when DG was offered with CS (wk 1 = 14.3 vs. wk 7 = 18.6 visits/d; SEM = 0.74) but remained constant with BG. Rumen Fermentation Profile Heifers fed CSDG exhibited a greater mean ruminal pH (P = 0.01) than those fed the other diets and a greater minimum pH (P = 0.01) than those fed TMR or BGCS (Table 4). Heifers fed CSDG had shorter (P < 0.05) duration of ruminal pH <5.8 and <5.5 than those fed the other diets. Similarly, cattle fed CSDG had a smaller (P = 0.05) area under the curve at pH 5.8 than those fed the TMR or BGDG. Table 4. Daily ruminal fluid pH profile determined in 16 feedlot cattle (n = 4) consuming TMR, BGCS, BGDG, and CSDG treatments over 4 measurement periods (from d 1 to 4, 7 to 14, 21 to 28, and 42 to 49) Item  Treatment1  SEM  P-value2  TMR  BGCS  BGDG  CSDG  Trt  Time  Trt × time  Daily pH                           Mean  5.66b  5.86b  5.72b  6.25a  0.110  0.01  0.83  0.62   Maximum  6.63  6.83  6.54  6.90  0.147  0.28  0.39  0.22   Minimum  4.96b  5.00b  5.18ab  5.44a  0.092  0.01  0.51  0.75  Time at pH, % of the day                           <5.8  0.62a  0.48a  0.58a  0.16b  0.084  <0.01  0.89  0.73   <5.5  0.44a  0.33a  0.42a  0.06b  0.091  0.03  0.78  0.70   <5.2  0.22  0.18  0.23  0.01  0.073  0.22  0.65  0.58  Area, h∙pH/d                           <5.8  7.17a  5.56ab  7.05a  0.96b  1.711  0.05  0.81  0.58   <5.5  3.30  2.65  3.36  0.21  1.428  0.15  0.78  0.52   <5.2  0.91  0.83  0.95  0.01  0.702  0.36  0.94  0.41  Item  Treatment1  SEM  P-value2  TMR  BGCS  BGDG  CSDG  Trt  Time  Trt × time  Daily pH                           Mean  5.66b  5.86b  5.72b  6.25a  0.110  0.01  0.83  0.62   Maximum  6.63  6.83  6.54  6.90  0.147  0.28  0.39  0.22   Minimum  4.96b  5.00b  5.18ab  5.44a  0.092  0.01  0.51  0.75  Time at pH, % of the day                           <5.8  0.62a  0.48a  0.58a  0.16b  0.084  <0.01  0.89  0.73   <5.5  0.44a  0.33a  0.42a  0.06b  0.091  0.03  0.78  0.70   <5.2  0.22  0.18  0.23  0.01  0.073  0.22  0.65  0.58  Area, h∙pH/d                           <5.8  7.17a  5.56ab  7.05a  0.96b  1.711  0.05  0.81  0.58   <5.5  3.30  2.65  3.36  0.21  1.428  0.15  0.78  0.52   <5.2  0.91  0.83  0.95  0.01  0.702  0.36  0.94  0.41  a,bMeans in the same row with different superscripts are different (P < 0.05). 1Treatments were a total mixed ration (TMR) and 3 free-choice diets consisting of components offered separately: barley grain and corn silage (BGCS), barley grain and wheat dry distillers grain (BGDG), and corn silage and wheat dry distillers grain (CSDG). 2Fixed effects were treatment (Trt), time, and treatment within time (Trt × time). View Large Table 4. Daily ruminal fluid pH profile determined in 16 feedlot cattle (n = 4) consuming TMR, BGCS, BGDG, and CSDG treatments over 4 measurement periods (from d 1 to 4, 7 to 14, 21 to 28, and 42 to 49) Item  Treatment1  SEM  P-value2  TMR  BGCS  BGDG  CSDG  Trt  Time  Trt × time  Daily pH                           Mean  5.66b  5.86b  5.72b  6.25a  0.110  0.01  0.83  0.62   Maximum  6.63  6.83  6.54  6.90  0.147  0.28  0.39  0.22   Minimum  4.96b  5.00b  5.18ab  5.44a  0.092  0.01  0.51  0.75  Time at pH, % of the day                           <5.8  0.62a  0.48a  0.58a  0.16b  0.084  <0.01  0.89  0.73   <5.5  0.44a  0.33a  0.42a  0.06b  0.091  0.03  0.78  0.70   <5.2  0.22  0.18  0.23  0.01  0.073  0.22  0.65  0.58  Area, h∙pH/d                           <5.8  7.17a  5.56ab  7.05a  0.96b  1.711  0.05  0.81  0.58   <5.5  3.30  2.65  3.36  0.21  1.428  0.15  0.78  0.52   <5.2  0.91  0.83  0.95  0.01  0.702  0.36  0.94  0.41  Item  Treatment1  SEM  P-value2  TMR  BGCS  BGDG  CSDG  Trt  Time  Trt × time  Daily pH                           Mean  5.66b  5.86b  5.72b  6.25a  0.110  0.01  0.83  0.62   Maximum  6.63  6.83  6.54  6.90  0.147  0.28  0.39  0.22   Minimum  4.96b  5.00b  5.18ab  5.44a  0.092  0.01  0.51  0.75  Time at pH, % of the day                           <5.8  0.62a  0.48a  0.58a  0.16b  0.084  <0.01  0.89  0.73   <5.5  0.44a  0.33a  0.42a  0.06b  0.091  0.03  0.78  0.70   <5.2  0.22  0.18  0.23  0.01  0.073  0.22  0.65  0.58  Area, h∙pH/d                           <5.8  7.17a  5.56ab  7.05a  0.96b  1.711  0.05  0.81  0.58   <5.5  3.30  2.65  3.36  0.21  1.428  0.15  0.78  0.52   <5.2  0.91  0.83  0.95  0.01  0.702  0.36  0.94  0.41  a,bMeans in the same row with different superscripts are different (P < 0.05). 1Treatments were a total mixed ration (TMR) and 3 free-choice diets consisting of components offered separately: barley grain and corn silage (BGCS), barley grain and wheat dry distillers grain (BGDG), and corn silage and wheat dry distillers grain (CSDG). 2Fixed effects were treatment (Trt), time, and treatment within time (Trt × time). View Large Treatment only affected VFA profiles in samples taken after feeding (Table 5). Heifers fed TMR had a greater total VFA concentration than those fed the other diets. In addition, total VFA concentration was less (P = 0.01) on d 4 than on d 49 of the experiment (107.3 vs. 130.3 mM; SEM = 7.14). Animals fed CSDG had greater (P = 0.01) proportions of acetate and butyrate and a smaller (P < 0.01) proportion of propionate than those fed the other diets. In addition, propionate was less (P = 0.03) on d 4 than on d 49 of the experiment (37.2 vs. 39.5 mol/100 mol; SEM = 1.12). There was a trend for a treatment × time interaction (P = 0.08), where only heifers fed CSDG had a greater acetate-to-propionate ratio on d 4 compared with d 49. The branched-chain VFA showed a trend (P = 0.05) for a treatment × time interaction, where it only decreased in heifers fed CSDG from d 4 to d 49, but remained unchanged over the experiment for other diets. On average across all diets, lactate concentration decreased (P < 0.01) from 0.19 to 0.02 mM (SEM = 0.053) between d 4 and 49 of the experiment. Table 5. Ruminal fermentation profile determined in 16 feedlot cattle (n = 4) consuming TMR, BGCS, BGDG, and CSDG treatments, on d 7 and 42 before feeding and d 4 and 49 after feeding Item  Treatment1  SEM  P-value2  TMR  BGCS  BGDG  CSDG  Trt  Time  Trt × time  Before feeding                           Total VFA concentration, mM  119.6  110.9  105.2  88.3  14.15  0.53  0.03  0.50   VFA proportion, mol/100 mol                            Acetate  42.2  46.6  47.5  55.7  4.14  0.15  0.90  0.89    Propionate  42.8  41.5  37.5  28.5  4.30  0.11  0.79  0.91    Butyrate  9.16  6.78  9.16  10.2  0.806  0.06  0.68  0.66    Branched-chain VFA3  1.71  2.32  2.32  2.42  0.465  0.54  0.29  0.53   Acetate-to-propionate ratio  1.00  1.16  1.46  2.15  0.452  0.10  0.95  0.90   Lactate concentration, mM  0.06  0.06  0.15  0.07  0.320  0.20  0.06  0.25  After feeding                           Total VFA concentration, mM  147.8a  113.1b  115.5b  99.0b  7.14  <0.01  0.01  0.94   VFA proportion, mol/100 mol                            Acetate  44.6b  46.6b  46.5b  54.2a  1.21  0.01  0.25  0.44    Propionate  44.9a  41.6a  40.1a  26.8b  1.12  <0.01  0.03  0.46    Butyrate  6.35b  6.90b  7.97b  12.7a  0.89  <0.01  0.63  0.68    Branched-chain VFA3  1.49  1.96  1.99  2.67  0.308  0.06  0.84  0.05   Acetate-to-propionate ratio  0.99b  1.12b  1.18b  2.05a  0.068  <0.01  0.03  0.08   Lactate concentration, mM  0.08  0.12  0.12  0.10  0.053  0.87  <0.01  0.82  Item  Treatment1  SEM  P-value2  TMR  BGCS  BGDG  CSDG  Trt  Time  Trt × time  Before feeding                           Total VFA concentration, mM  119.6  110.9  105.2  88.3  14.15  0.53  0.03  0.50   VFA proportion, mol/100 mol                            Acetate  42.2  46.6  47.5  55.7  4.14  0.15  0.90  0.89    Propionate  42.8  41.5  37.5  28.5  4.30  0.11  0.79  0.91    Butyrate  9.16  6.78  9.16  10.2  0.806  0.06  0.68  0.66    Branched-chain VFA3  1.71  2.32  2.32  2.42  0.465  0.54  0.29  0.53   Acetate-to-propionate ratio  1.00  1.16  1.46  2.15  0.452  0.10  0.95  0.90   Lactate concentration, mM  0.06  0.06  0.15  0.07  0.320  0.20  0.06  0.25  After feeding                           Total VFA concentration, mM  147.8a  113.1b  115.5b  99.0b  7.14  <0.01  0.01  0.94   VFA proportion, mol/100 mol                            Acetate  44.6b  46.6b  46.5b  54.2a  1.21  0.01  0.25  0.44    Propionate  44.9a  41.6a  40.1a  26.8b  1.12  <0.01  0.03  0.46    Butyrate  6.35b  6.90b  7.97b  12.7a  0.89  <0.01  0.63  0.68    Branched-chain VFA3  1.49  1.96  1.99  2.67  0.308  0.06  0.84  0.05   Acetate-to-propionate ratio  0.99b  1.12b  1.18b  2.05a  0.068  <0.01  0.03  0.08   Lactate concentration, mM  0.08  0.12  0.12  0.10  0.053  0.87  <0.01  0.82  a,bMeans in the same row with different superscripts are different (P < 0.05). 1Treatments were a total mixed ration (TMR) and 3 free-choice diets consisting of components offered separately: barley grain and corn silage (BGCS), barley grain and wheat dry distillers grain (BGDG), and corn silage and wheat dry distillers grain (CSDG). 2Fixed effects were treatment (Trt), time, and treatment within time (Trt × time). 3Includes isobutyrate and isovalerate. View Large Table 5. Ruminal fermentation profile determined in 16 feedlot cattle (n = 4) consuming TMR, BGCS, BGDG, and CSDG treatments, on d 7 and 42 before feeding and d 4 and 49 after feeding Item  Treatment1  SEM  P-value2  TMR  BGCS  BGDG  CSDG  Trt  Time  Trt × time  Before feeding                           Total VFA concentration, mM  119.6  110.9  105.2  88.3  14.15  0.53  0.03  0.50   VFA proportion, mol/100 mol                            Acetate  42.2  46.6  47.5  55.7  4.14  0.15  0.90  0.89    Propionate  42.8  41.5  37.5  28.5  4.30  0.11  0.79  0.91    Butyrate  9.16  6.78  9.16  10.2  0.806  0.06  0.68  0.66    Branched-chain VFA3  1.71  2.32  2.32  2.42  0.465  0.54  0.29  0.53   Acetate-to-propionate ratio  1.00  1.16  1.46  2.15  0.452  0.10  0.95  0.90   Lactate concentration, mM  0.06  0.06  0.15  0.07  0.320  0.20  0.06  0.25  After feeding                           Total VFA concentration, mM  147.8a  113.1b  115.5b  99.0b  7.14  <0.01  0.01  0.94   VFA proportion, mol/100 mol                            Acetate  44.6b  46.6b  46.5b  54.2a  1.21  0.01  0.25  0.44    Propionate  44.9a  41.6a  40.1a  26.8b  1.12  <0.01  0.03  0.46    Butyrate  6.35b  6.90b  7.97b  12.7a  0.89  <0.01  0.63  0.68    Branched-chain VFA3  1.49  1.96  1.99  2.67  0.308  0.06  0.84  0.05   Acetate-to-propionate ratio  0.99b  1.12b  1.18b  2.05a  0.068  <0.01  0.03  0.08   Lactate concentration, mM  0.08  0.12  0.12  0.10  0.053  0.87  <0.01  0.82  Item  Treatment1  SEM  P-value2  TMR  BGCS  BGDG  CSDG  Trt  Time  Trt × time  Before feeding                           Total VFA concentration, mM  119.6  110.9  105.2  88.3  14.15  0.53  0.03  0.50   VFA proportion, mol/100 mol                            Acetate  42.2  46.6  47.5  55.7  4.14  0.15  0.90  0.89    Propionate  42.8  41.5  37.5  28.5  4.30  0.11  0.79  0.91    Butyrate  9.16  6.78  9.16  10.2  0.806  0.06  0.68  0.66    Branched-chain VFA3  1.71  2.32  2.32  2.42  0.465  0.54  0.29  0.53   Acetate-to-propionate ratio  1.00  1.16  1.46  2.15  0.452  0.10  0.95  0.90   Lactate concentration, mM  0.06  0.06  0.15  0.07  0.320  0.20  0.06  0.25  After feeding                           Total VFA concentration, mM  147.8a  113.1b  115.5b  99.0b  7.14  <0.01  0.01  0.94   VFA proportion, mol/100 mol                            Acetate  44.6b  46.6b  46.5b  54.2a  1.21  0.01  0.25  0.44    Propionate  44.9a  41.6a  40.1a  26.8b  1.12  <0.01  0.03  0.46    Butyrate  6.35b  6.90b  7.97b  12.7a  0.89  <0.01  0.63  0.68    Branched-chain VFA3  1.49  1.96  1.99  2.67  0.308  0.06  0.84  0.05   Acetate-to-propionate ratio  0.99b  1.12b  1.18b  2.05a  0.068  <0.01  0.03  0.08   Lactate concentration, mM  0.08  0.12  0.12  0.10  0.053  0.87  <0.01  0.82  a,bMeans in the same row with different superscripts are different (P < 0.05). 1Treatments were a total mixed ration (TMR) and 3 free-choice diets consisting of components offered separately: barley grain and corn silage (BGCS), barley grain and wheat dry distillers grain (BGDG), and corn silage and wheat dry distillers grain (CSDG). 2Fixed effects were treatment (Trt), time, and treatment within time (Trt × time). 3Includes isobutyrate and isovalerate. View Large Blood Variables There were no differences (P > 0.10) among treatments in any of the blood variables measured (Table 6). Partial pressure of CO2 increased (P = 0.02) from 38.7 to 44.3 kPa (SEM = 1.44) between d 0 and 49. Blood concentrations of Na+ (P = 0.01) and Cl− (P = 0.06) were less on d 7 compared with the other days. Table 6. Blood variables determined in 16 feedlot cattle (n = 4) consuming TMR, BGCS, BGDG, and CSDG treatments, on d 0, 7, 42, and 49 of the experiment, before feed delivery Item  Treatment1  SEM  P-value2  TMR  BGCS  BGDG  CSDG  Trt  Time  Trt × time  pH  7.46  7.45  7.44  7.46  0.009  0.39  0.11  0.31  pCO2,3 kPa  40.8  42.8  43.0  40.4  1.44  0.48  0.03  0.46  Total CO2, mM  27.9  28.5  27.9  27.5  1.082  0.93  0.12  0.81  Anion gap, mM  17.3  17.5  17.5  17.1  1.304  0.98  0.38  0.65  Na+, mM  142.0  144.1  142.6  143.3  1.068  0.56  0.01  0.15  K+, mM  4.07  3.99  4.07  3.94  0.154  0.78  0.15  0.98  Cl−, mM  102.1  103.5  102.3  103.8  1.647  0.55  0.06  0.51  HCO3−, mM  26.7  27.2  26.6  26.3  1.045  0.93  0.14  0.82  Packed cell volume, mL/100 mL  of blood  42.9  41.2  44.1  43.7  1.857  0.75  0.92  0.52  Item  Treatment1  SEM  P-value2  TMR  BGCS  BGDG  CSDG  Trt  Time  Trt × time  pH  7.46  7.45  7.44  7.46  0.009  0.39  0.11  0.31  pCO2,3 kPa  40.8  42.8  43.0  40.4  1.44  0.48  0.03  0.46  Total CO2, mM  27.9  28.5  27.9  27.5  1.082  0.93  0.12  0.81  Anion gap, mM  17.3  17.5  17.5  17.1  1.304  0.98  0.38  0.65  Na+, mM  142.0  144.1  142.6  143.3  1.068  0.56  0.01  0.15  K+, mM  4.07  3.99  4.07  3.94  0.154  0.78  0.15  0.98  Cl−, mM  102.1  103.5  102.3  103.8  1.647  0.55  0.06  0.51  HCO3−, mM  26.7  27.2  26.6  26.3  1.045  0.93  0.14  0.82  Packed cell volume, mL/100 mL  of blood  42.9  41.2  44.1  43.7  1.857  0.75  0.92  0.52  1Treatments were a total mixed ration (TMR) and 3 free-choice diets consisting of components offered separately: barley grain and corn silage (BGCS), barley grain and wheat dry distillers grain (BGDG), and corn silage and wheat dry distillers grain (CSDG). 2Fixed effects were treatment (Trt), time, and treatment within time (Trt × time). 3Partial pressure of CO2. View Large Table 6. Blood variables determined in 16 feedlot cattle (n = 4) consuming TMR, BGCS, BGDG, and CSDG treatments, on d 0, 7, 42, and 49 of the experiment, before feed delivery Item  Treatment1  SEM  P-value2  TMR  BGCS  BGDG  CSDG  Trt  Time  Trt × time  pH  7.46  7.45  7.44  7.46  0.009  0.39  0.11  0.31  pCO2,3 kPa  40.8  42.8  43.0  40.4  1.44  0.48  0.03  0.46  Total CO2, mM  27.9  28.5  27.9  27.5  1.082  0.93  0.12  0.81  Anion gap, mM  17.3  17.5  17.5  17.1  1.304  0.98  0.38  0.65  Na+, mM  142.0  144.1  142.6  143.3  1.068  0.56  0.01  0.15  K+, mM  4.07  3.99  4.07  3.94  0.154  0.78  0.15  0.98  Cl−, mM  102.1  103.5  102.3  103.8  1.647  0.55  0.06  0.51  HCO3−, mM  26.7  27.2  26.6  26.3  1.045  0.93  0.14  0.82  Packed cell volume, mL/100 mL  of blood  42.9  41.2  44.1  43.7  1.857  0.75  0.92  0.52  Item  Treatment1  SEM  P-value2  TMR  BGCS  BGDG  CSDG  Trt  Time  Trt × time  pH  7.46  7.45  7.44  7.46  0.009  0.39  0.11  0.31  pCO2,3 kPa  40.8  42.8  43.0  40.4  1.44  0.48  0.03  0.46  Total CO2, mM  27.9  28.5  27.9  27.5  1.082  0.93  0.12  0.81  Anion gap, mM  17.3  17.5  17.5  17.1  1.304  0.98  0.38  0.65  Na+, mM  142.0  144.1  142.6  143.3  1.068  0.56  0.01  0.15  K+, mM  4.07  3.99  4.07  3.94  0.154  0.78  0.15  0.98  Cl−, mM  102.1  103.5  102.3  103.8  1.647  0.55  0.06  0.51  HCO3−, mM  26.7  27.2  26.6  26.3  1.045  0.93  0.14  0.82  Packed cell volume, mL/100 mL  of blood  42.9  41.2  44.1  43.7  1.857  0.75  0.92  0.52  1Treatments were a total mixed ration (TMR) and 3 free-choice diets consisting of components offered separately: barley grain and corn silage (BGCS), barley grain and wheat dry distillers grain (BGDG), and corn silage and wheat dry distillers grain (CSDG). 2Fixed effects were treatment (Trt), time, and treatment within time (Trt × time). 3Partial pressure of CO2. View Large Animal Performance Average daily gain was not different among feeding treatments (Table 7). However, heifers fed the CSDG treatment tended (P = 0.07) to have a smaller G:F than those fed other treatments. Table 7. Performance variables of heifers consuming TMR, BGCS, BGDG, and CSDG treatments determined individually (n = 20) over the experiment Item  Treatment1  SEM  P-value  TMR  BGCS  BGDG  CSDG  Treatment  ADG, kg of BW/d  1.86  1.82  1.78  1.67  0.082  0.45  G:F, kg of BW/kg of DMI  0.22a  0.23a  0.20a  0.18b  0.009  0.07  Item  Treatment1  SEM  P-value  TMR  BGCS  BGDG  CSDG  Treatment  ADG, kg of BW/d  1.86  1.82  1.78  1.67  0.082  0.45  G:F, kg of BW/kg of DMI  0.22a  0.23a  0.20a  0.18b  0.009  0.07  a,bWithin a row, means without a common superscript differ (P < 0.07). 1Treatments were a total mixed ration (TMR) and 3 free-choice diets consisting of components offered separately: barley grain and corn silage (BGCS), barley grain and wheat dry distillers grain (BGDG), and corn silage and wheat dry distillers grain (CSDG). View Large Table 7. Performance variables of heifers consuming TMR, BGCS, BGDG, and CSDG treatments determined individually (n = 20) over the experiment Item  Treatment1  SEM  P-value  TMR  BGCS  BGDG  CSDG  Treatment  ADG, kg of BW/d  1.86  1.82  1.78  1.67  0.082  0.45  G:F, kg of BW/kg of DMI  0.22a  0.23a  0.20a  0.18b  0.009  0.07  Item  Treatment1  SEM  P-value  TMR  BGCS  BGDG  CSDG  Treatment  ADG, kg of BW/d  1.86  1.82  1.78  1.67  0.082  0.45  G:F, kg of BW/kg of DMI  0.22a  0.23a  0.20a  0.18b  0.009  0.07  a,bWithin a row, means without a common superscript differ (P < 0.07). 1Treatments were a total mixed ration (TMR) and 3 free-choice diets consisting of components offered separately: barley grain and corn silage (BGCS), barley grain and wheat dry distillers grain (BGDG), and corn silage and wheat dry distillers grain (CSDG). View Large DISCUSSION Feed Choice and Intake Cattle fed the free-choice treatments showed different preferences for each dietary component depending on the diet offered and, in some cases, changed their diet composition over the course of the experiment. These results suggest that ruminants allowed to choose among ingredients were able to select diets according to their needs or preferences, in agreement with previous studies (Forbes and Provenza, 2000; Atwood et al., 2001; Askar et al., 2006). How ruminants regulate diet selection and food intake has been widely studied and reviewed (Gallouin and Le Magnen, 1987; Forbes, 1995; Illius et al., 2002), with work showing that multiple factors are involved in the control of voluntary intake (Forbes, 2003). These include postingestive feedback mechanisms (Yearsley et al., 2006), learning and feeding motivation (Day et al., 1998), energy balance and nutrient requirements (Stubbs and Tolkamp, 2006), fitness of the animal (Illius et al., 2002), and oxygen efficiency (Ketelaars and Tolkamp, 1996). In our study, when cattle were able to freely choose BG as a dietary component in combination with either CS or DG, they gradually increased the proportion of BG in their diet over the 52-d feeding period. This increase is interpreted as a natural and voluntary adaptation process to a high-concentrate diet, as reported in previous studies (Catanese et al., 2009) and is linked to the requirement of the animal for more energy for fat deposition with advancing maturity (NRC, 2000), as well as adaptation to the consumption of high-grain diets (Tajima et al., 2000; Sun et al., 2010). Previous studies (Britton and Stock, 1987) associated an erratic feed intake from day-to-day, reflected in increased variability of DMI, with metabolic digestive disorders such as subclinical acidosis. In our experiment, providing heifers a diet either as TMR or free-choice did not alter DMI variability, except in the CSDG treatment. However, the greater DMI variability with CSDG was not associated with ruminal acidosis: heifers fed this treatment had the greatest ruminal pH values. Greater DMI variability with CSDG was likely the result of an adaptation process to a diet with unusually high proportions of DG. Cattle offered BG and CS initially consumed a diet containing about 70% BG but gradually increased the proportion of BG in the diet to 80% of total DMI, an amount similar to the formulated TMR, which contained about 85% BG. Previous research has also shown the ability of cattle to consume large proportions of grain when given a free-choice diet (Sahin et al., 2003; Askar et al., 2006; González et al., 2009). However, other studies reported grain consumption below 60% of total DMI in animals offered free-choice diets (Catanese et al., 2009; Commun et al., 2009). These differences are most likely due to differences in animal and forage characteristics among experiments, such as the age of the animals or the physically effective fiber content of the forage. When a choice of BG or DG was offered, without any forage, the proportion of BG ingested also increased over the experiment to 70%, but it did not reach the amount of BG present in either the TMR or the BGCS diets. This difference in the BG proportion consumed between the BGCS and BGDG treatments may reflect the lesser NDF content of DG (27.8%) compared with CS (42.2%) and its smaller particle size. When used as a replacement for forage, wheat-based DG does not stimulate the same degree of rumination and chewing (Penner et al., 2009), reducing saliva production and, consequently, the capacity to neutralize the rapid acid production that arises from the fermentation of grain. Accordingly, the variability of the BG proportion in the diet was greater when it was offered with DG than with CS, indicating an erratic feed intake likely associated with a more acidotic rumen environment (Britton and Stock, 1987). Despite the decreased intake of BG when it was offered with DG than with CS, heifers fed the BGDG treatment had a greater total DMI; they consumed 3.59 kg DM of DG, compared with the 1.65 kg DM of CS consumed by the heifers offered BGCS. The larger particle size of CS compared with DG may account, in part, for the greater intake of DG compared with CS, because of its lesser filling effect (Allen, 1996) and the faster passage rate from the reticulorumen (Jung and Allen, 1995). Other factors, such as fermentative products acting as a negative feedback to control feed intake, are discussed later. Wheat DG was used as the main energy source of the diet for cattle offered CS and DG, comprising up to 63% of total DMI. Even though DG and BG have approximately the same energy value, the proportion of DG consumed was less than the proportion of BG consumed when they were offered with CS, suggesting that factors such as fiber content or increased N content as opposed to rumen fill may have limited DG consumption. In addition, the variability of the DG proportion in the diet was greater when it was offered with BG than with CS, likely associated with a more acidotic rumen environment (Britton and Stock, 1987), as previously explained. There are no previous studies in which finishing feedlot cattle were fed a diet with this proportion of wheat DG. Distillers grains have increased S content (0.37 to 0.85% DM) and at greater inclusion rates (greater than 1%) can lead to S toxicity in cattle (Schingoethe et al., 2009). We did not measure the S content of feeds in the present study, but no signs of S toxicity were observed. The decreased energy content of CS compared with the rest of the dietary components resulted in heifers fed CSDG increasing their DMI to meet energy requirements. Mertens (1994) showed that DMI is positively correlated with NDF concentration when energy limits intake. Feeding Behavior As the energy requirements of heifers increased for fattening, a progressive increase in daily DMI and meal size occurred. In addition, the feeding rate increased over the experiment, likely because of increased BW and bite size throughout the experiment. González et al. (2008) also reported an increase in eating rate of both concentrate and straw as the age of heifers increased. As a result, the duration of eating reported by González et al. (2008) remained constant or decreased over the experiment. Heifers fed the TMR had a shorter meal criterion and less frequent visits than those offered diets free-choice. These differences were likely related to feed distribution, because heifers fed the TMR had 2 feeder tubs within each pen in which the same diet was offered, whereas heifers fed free-choice diets were forced to visit 2 feeder tubs to complete their meals. Consequently, heifers fed the TMR treatment had shorter meal length and meal time than those fed a similar diet free-choice because of the increased time required to switch between feeders. This is supported by the lack of effect on daily feeding time, which does not include the time in which heifers were absent from the feeders. Others have also reported longer meals in heifers fed free-choice diets compared with TMR (Boga et al., 2009). Heifers fed the free-choice treatments also had greater meal size than those fed TMR. Atwood et al. (2001) also reported this effect indicating that greater meal size resulted from the animals being able to choose from a variety of foods with different flavors and nutrient content. However, this effect could also be explained by the fact that heifers fed the TMR may not have been able to eat larger meals because of physical or physiological restrictions related to the high-grain content of the diet. It is also possible that TMR-fed heifers had shorter meals indicating they reached satiation more quickly than those heifers that had to consume their diet from 2 feeders, where satiety may have been delayed as a result of the need to switch between dietary components. The previously described differences in dietary ingredient preference, depending on the offered treatment, can be explained by changes in feeding behavior. As the eating rate increased over the experiment for all offered dietary ingredients, observed changes in the diet composition were directly related to changes in the DMI, feeding time, or frequency of visits to the feed bunk. Therefore, heifers fed the BGCS treatment increased the BG proportion progressively over the experiment by maintaining a constant feeding time with increased DMI, whereas consumption of CS decreased over the experiment through a reduction in feeding time at a constant DMI. Similarly, heifers fed BGDG increased consumption of BG over the experiment with a constant feeding time, with an increase in DMI, whereas the DG proportion decreased as a result of a reduction in both feeding time and DMI. This increase in the proportion of BG ingested when it was offered with either CS or DG was accompanied by an increase in the frequency of visits to the feed bunk throughout the experiment. The increase in the frequency of visits without affecting feeding time suggests that heifers ate more BG through more-frequent visits to the feed bunk rather than by increasing the duration of each meal. This could be interpreted as a mechanism used by heifers to eat more BG without negatively affecting the rumen environment, by distributing the daily intake more evenly over time, allowing for intermittent periods of rumination and salivation. Ruminal Fermentation Profile and Blood Variables Ruminal pH, fermentation profile, and blood variables were measured to assess the relationship between diet selection and ruminal and metabolic acidosis. The TMR, BGCS, and BGDG treatments did not differ in fermentation, blood, or pH profiles. On the one hand, this finding suggests that providing heifers a BG and CS diet either as TMR or free-choice did not alter rumen fermentation or blood variables, even though it did alter feeding behavior and the relative proportion of BG and CS consumed. In addition, free-choice provision of BG and DG, without forage, resulted in no noticeable increase in subclinical or clinical acidosis. However, these results must be interpreted with caution because pH variables were measured in only 4 heifers on each diet. Individuals are known to differ widely in their response to diets that increase the risk of subclinical acidosis, with many of the factors responsible for this variation remaining unclear (Bevans et al., 2005; Penner et al., 2009). Although cattle offered BGCS and BGDG free-choice increased proportional intake of BG over the experiment, ruminal pH remained relatively constant, with no measurable treatment by time interaction. This suggests that cattle were able to increase BG intake without negatively affecting ruminal pH. This might have been achieved by increased frequency of visits, but metabolic and physiologic adaptations may also be feasible. Cattle fed CSDG had the greatest ruminal fluid pH despite the fact that concentrations of VFA were similar in heifers fed BGCS and BGDG diets before and after feeding. This may reflect the greater amount of physically effective fiber in CS, which stimulates rumination and salivation. Furthermore, the CSDG diet would have contained the least fermentable carbohydrate among the diets and promoted the greatest CP intake, a factor that may have also modulated pH through increased NH3 in the rumen (Sauvant et al., 1999). Cattle fed the TMR treatment had a greater total VFA concentration after feeding than cattle fed free-choice diets. These differences may have resulted from the greater proportion of BG consumed by heifers fed the TMR. However, considering that ruminal liquid samples were obtained 2 h after feed was offered, this greater VFA concentration could also be a consequence of differences in feeding behavior (i.e., a shorter meal duration for heifers fed TMR or an increase in the frequency of visits of heifers fed free-choice diets). However, as there were no differences in ruminal pH or blood variables among diets, our work suggests that the heifers were capable of self-selecting diets with respect to preventing increased risk of subclinical or clinical acidosis. As expected, cattle consuming CSDG, which had the greatest NDF content, had the greatest and least molar proportions of acetate and propionate after feeding, respectively, compared with diets that contained greater amounts of fermentable carbohydrate. In addition, heifers fed CSDG had the greatest DMI, suggesting that in the rest of treatments the greater propionate could be acting as a negative feedback mechanism to control feed intake, as has been described previously (Farningham and Whyte, 1993; Oba and Allen, 2003). Animal Performance and Feed Cost In the context of this relatively short feeding period, the cattle fed any of the free-choice treatments maintained the same ADG and G:F as those fed TMR, with the only exception being a trend for reduced G:F in heifers fed the CSDG diet. Others have shown similar results in beef cattle fed a free-choice diet versus a TMR (Atwood et al., 2001; Sahin et al., 2003; Askar et al., 2006). The lack of difference in ADG among treatments leads to the conclusion that offering grain and forage components separately enabled the heifers to select their own diet without negatively affecting performance relative to provision of a TMR. Furthermore, neither growth rate nor ruminal fluid pH was affected when the source of forage (CS) was eliminated from the diet, as in the BGDG treatment. This is an important finding because of the growing availability of DG and the potential to reduce the forage component of finishing feedlot diets (Klopfenstein et al., 2008; Wierenga et al., 2010). Conclusions Cattle fed either TMR or free-choice diets had similar ruminal fermentation and growth performance, with the exception of the CSDG diet for which ruminal pH levels were consistently greater, and G:F tended to be less. In addition, when BG was the concentrate component in free-choice diets, heifers were able to gradually increase the intake of BG through more frequent visits to the feeder containing BG, and reducing the time spent eating the alternative dietary ingredient (i.e., CS or DG). 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TI - Feeding behavior and ruminal acidosis in beef cattle offered a total mixed ration or dietary components separately
JF - Journal of Animal Science
DO - 10.2527/jas.2010-3045
DA - 2011-02-01
UR - https://www.deepdyve.com/lp/oxford-university-press/feeding-behavior-and-ruminal-acidosis-in-beef-cattle-offered-a-total-cLsoI1EspP
SP - 520
EP - 530
VL - 89
IS - 2
DP - DeepDyve
ER -