J. Anim. Sci. 2002. 80:2726-2739
© 2002 American Society of Animal Science
Effects of restricted versus conventional dietary adaptation on feedlot performance, carcass characteristics, site and extent of digestion, digesta kinetics, and ruminal metabolism1,2
W. T. Choat*,
C. R. Krehbiel*,3,
M. S. Brown
,
G. C. Duff
,4,
D. A. Walker and
D. R. Gill*
* Department of Animal Science, Oklahoma State University, Stillwater 74078;
and
Division of Agriculture, West Texas A&M University, Canyon 79016;
and
Clayton Livestock Research Center, Clayton, NM 88415
3 Correspondence:
208 Anim. Sci. Bldg. (phone: 405/744-8857; fax: 405/744-7390; E-mail:
kclinto{at}okstate.edu).
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Abstract
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Three experiments were conducted to determine effects of restricting intake of the final finishing diet as a means of dietary adaptation compared with diets increasing in grain over a period of 20 to 22 d on overall cattle performance, carcass characteristics, digestibility, digesta kinetics, and ruminal metabolism. In Exp. 1, 84 Angus x Hereford yearling steers (initial BW = 418 ± 29.0 kg) were fed for 70 d. Restricting intake during adaptation had no effect (P > 0.10) on overall ADG:DMI, but decreased (P < 0.05) DMI compared with ad libitum access to adaptation diets, which resulted from differences during the initial 28 d of the experiment. In Exp. 2, 150 mixed crossbred steer calves (initial BW = 289 ± 22.9 kg) were fed for an average of 173 d. Restricting intake decreased (P < 0.01) overall daily gain (1.51 vs 1.65 kg/d) and DMI (8.68 vs 9.15 kg/d) compared with ad libitum fed steers; however, ADG:DMI was not influenced (P > 0.10) by adaptation method. Experiment three used eight ruminally and duodenally fistulated steers (initial BW = 336 ± 20 kg) in a completely random design. Total tract digestibility, digesta kinetics and ruminal metabolism were determined. Restricting intake reduced (P < 0.10) daily DMI variation from d 1 through 7, 8 through 14, and 22 through 28 compared with ad libitum feeding of three adaptation diets. Restricted steers had reduced (adaptation method x period interaction, P < 0.05) intakes and fecal excretions of ADF and greater OM digestibilities on d 4 through 7, 11 through 14, and 18 through 21. Digesta kinetics and ruminal metabolism were generally not affected (P > 0.10) by adaptation method. Our results suggest that restricted-feeding of the final diet as a means of dietary adaptation can be used in finishing cattle with few problems from acidosis or related intake variation. In light-weight steers (Exp. 2), disruptions in intake during the adaptation period might have resulted in restriction for an extended period, which decreased (P < 0.01) hot carcass weight compared with calves fed ad libitum. Effects of limit feeding during the initial 28 d of the feeding period on site and extent of digestion, digesta kinetics, and ruminal metabolism were minimal, supporting few differences in performance across the finishing period for yearling cattle.
Key Words: Adaptation • Beef Cattle • Carcass Quality • Performance • Restricted Feeding
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Introduction
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Traditionally, adapting cattle to high-grain diets has been accomplished by using sequential diets with an increasing grain concentration. This is done so that ruminal microorganisms can gradually adjust to a ruminal environment with a lower pH in an attempt to minimize subacute acidosis and intake variation that can occur with overeating of grain. Restricting feed intake of high-concentrate diets has improved feed efficiency in programmed feeding studies (Zinn, 1986; Loerch, 1990; Knoblich et al., 1997), in studies where cattle were restricted on a percentage of ad libitum (Plegge, 1986; Hicks et al., 1990) and in studies where cattle were fed to some maximum level relative to their maintenance energy requirements (Xiong et al., 1991; Bartle and Preston, 1992). Restricting intake of high-concentrate diets has also shown potential reductions in subacute acidosis when fluctuation in feed intake was reduced (Soto-Navarro et al., 2000), but little information is available on the use of restricting intake of the final finishing diet as a means of dietary adaptation to a finishing program (Bierman and Pritchard, 1996; Weichenthal et al., 1999). We hypothesized that restricting intake of the final finishing diet would reduce DMI and increase digestibility during adaptation and improve overall feed efficiency by cattle, compared with ad libitum feeding of adaptation diets. Therefore, our objective was to compare feedlot performance, carcass characteristics, intake variation, digestibility, digesta kinetics, and ruminal metabolism of steers limit-fed a final 90 to 92.5% concentrate diet during the adaptation period to ad libitum feeding of traditional adaptation diets.
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Materials and Methods
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Experiment 1
Medium-framed Angus x Hereford yearling steers (n = 84; initial BW = 418 ± 29.0 kg) were used to determine effects of restricting intake of the final diet as a means of dietary adaptation compared with diets increasing in grain concentration over 20 d on cattle performance and carcass characteristics. Steers grazed winter wheat pasture for 185 d prior to placement in a feedlot at the Clayton Livestock Research Center (CLRC), Clayton, NM, on May 11, 1999. Wheat pasture was located adjacent to the feedlot at the CLRC. On d 1 of the experiment, steers were implanted with Synovex S (Fort Dodge Animal Health, Fort Dodge, IA), administered a clostridial vaccine (Ultrabac 7; Pfizer Animal Health, Exton, PA), weighed and allotted to eight pens with 10 or 11 steers/pen. Animals were handled and cared for according to a protocol reviewed and approved by the New Mexico State University Institutional Animal Care and Use Committee. Pens (12.2 m x 35.0 m) were uncovered, soil-surfaced, and contained 11 m of bunk space and automatic fence line waters (one/pen). Four pens each were assigned to one of two dietary adaptation treatments fed once daily (0800): 1) ad libitum feeding of four adaptation diets over 20 d with steam-flaked corn concentration increasing from 46 to 66% of diet DM or 2) restricted intake of the final diet with programmed increases in feed offered until steers reached ad libitum access. Increasing energy of the four adaptation diets was accomplished by increasing the percentage of steam-flaked corn and decreasing the percentage of roughage in each sequential diet (Table 1
). Feed samples were analyzed every 28 d for DM according to standard procedures (AOAC, 1996). Steers fed adaptation diets for ad libitum intake were initially offered 1.5% of BW (DM basis), and daily feed was increased by 0.45 kg/steer when a pen’s bunk was slick at the morning feed call. Steers limit-fed the final diet were initially offered 1.25% of BW (DM basis) or approximately 5.2 kg of DM/steer, which was increased by 0.23 kg/steer daily when a pen’s bunk was slick at the morning feed call. Intakes for both treatment groups were similar beginning on approximately d 27 (Figure 1).
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Figure 1. Daily DM intake of yearling steers for 70 d when steers were adapted to the finishing diet with ad libitum intake of four adaptation diets increasing in percent concentrate from 70 to 90, or steers were adapted to the finishing diet with restricted feeding of the final 90% concentrate diet.
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Steers were weighed individually before feeding (0700) on two consecutive days initially; average used as initial BW. Steers were weighed once every 28 d throughout the remainder of the trial. Final live weights were calculated by dividing hot carcass weight by a common dressing percentage (62%). All weights were analyzed as initially recorded (unshrunk). Feed intake was measured and feed efficiency (kg of BW gain/100 kg of DMI) was calculated every 28 d. Variation in DMI during the grain adaptation period was calculated separately for each treatment by two methods (Stock et al., 1995a,b). In the first method, residual intake (for each pen) was calculated as estimated daily DMI minus the average DMI for all days within the concentrate period for that pen. In the second method, residual intake was calculated as estimated daily DMI (for each pen) minus the average DMI for all pens within treatment for each day. Intake variation was calculated on intake residuals within a pen across all days in the adaptation period (pen DMI variation), or on intake residuals within the day among all pens within the treatment (daily DMI variation). Steers were harvested when 70% appeared to grade Choice based upon subjective evaluation of body composition. Steers were harvested at a commercial facility after 70 d. Hot carcass weight (HCW) was determined following harvest, and carcasses were evaluated by trained personnel after a 24-h chill for the following measurements: subcutaneous fat depth at the 12th rib, longissimus muscle area, percentage kidney, pelvic, and heart fat, yield grade; marbling score, and quality grade (USDA, 1997).
Data were analyzed as a completely random design using the MIXED procedure of SAS (SAS Inst. Inc., Cary, NC). Interim feedlot performance was analyzed using repeated measures over time and included treatment, time, and the treatment x time interaction in the model. The model statement for overall feedlot performance and carcass characteristics included treatment along with a random statement "experimental unit (treatment)" to account for interexperimental-unit variation (Littell et al., 1998). The default covariance structure (variance component, VC) was chosen based on fit statistics. Pen served as the experimental unit for feedlot performance and animal was used for variables measured on individual carcasses. Results are discussed as significant if P < 0.05 and as tendencies if P > 0.05 and P < 0.10.
Experiment 2
One hundred fifty mixed crossbred steer calves (initial BW = 289 ± 22.9 kg) were used to determine effects of restricting intake of the final diet as a means of dietary adaptation compared with diets of increasing grain concentration over 22 d on cattle performance and carcass characteristics. All steers were removed from wheat pasture and transported approximately 160 km to the Willard Sparks Beef Research Center, Stillwater, OK, on March 28, 2000. Upon arrival, steers were individually weighed on three consecutive days (d -1, 0, and 1); on d -1, steers were individually ear-tagged for identification. Steers were given ad libitum access to hay and water on d -1 and 0. On d 1, steers were processed, blocked by the average individual weight taken on d -1 and 0 and allotted to one of 30 pens (10 pens/block, 5 steers/pen). Blocks 1 and 2 were fed for 165 d, and Block 3 was fed for 180 d. At processing, all steers were vaccinated with Covexin 8 (Schering-Plough Animal Health, Kenilworth, NJ), and Frontier 4 Plus (Intervet Animal Health, Millsboro, DE) and treated for internal and external parasites using Ivomec-Plus injectable (Merial Animal Health, Duluth, GA). Animals were handled and cared for according to a protocol reviewed and approved by the Oklahoma State University Institutional Animal Care and Use Committee. Pens (12.8 m x 4.6 m) were partially covered, soil surfaced, and contained 4.6 m of bunk space with one fence line automatic water tank shared between two pens. Fifteen pens each were assigned to one of two adaptation treatments (Table 2) fed once daily (0800): 1) ad libitum feeding of three adaptation diets over 22 d, with the concentration of dry-rolled corn (DRC) increasing from 52 to 80% (DM basis) or 2) restricted intake of the final diet with programmed increases until steers reached ad libitum intake. Methods for feed sampling, DM determination and intake variation were the same as those used in Exp. 1. Steers fed adaptation diets for ad libitum intake were initially offered feed at 2.0% of BW (DM basis), and daily feed was increased by 0.45 kg/steer when a pen’s bunk was slick at the morning feed call. Steers limit-fed the final diet were initially fed 1.25% of BW (DM basis) or approximately 3.7 kg of DM/steer, and DMI was increased by 0.23 kg/steer daily until ad libitum intake was reached (Figure 2).
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Figure 2. Daily DM intakes of crossbred steer calves for 70 d when calves were adapted to the finishing diet with ad libitum intake of three adaptation diets increasing in percent concentrate from 65 to 92, or calves were adapted to the finishing diet with restricted feeding of the final 92% concentrate diet.
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Steers were weighed individually before feeding on three consecutive days at arrival, every 2 wk for the first 28 d, and every subsequent 28 d for the duration of the experiment. Initial BW = average of the first 3 d. Feed intake was measured and feed efficiency (kg of gain/100 kg of DMI) was calculated every 28 d. Final live weights were calculated by dividing HCW by the average dressing percentage for steers harvested on the same day. Average dressing percentages were 64 for Blocks 1 and 2 (Harvest 1), and 62 for Block 3 (Harvest 2). Steers from Blocks 1 and 2 were harvested after 165 d on feed, and Block 3 was harvested after 180 d on feed. Carcasses were evaluated as described in Exp. 1. In addition, presence of liver abscesses (Brink et al., 1990) was determined.
Data were analyzed as a randomized complete block design with two treatments in three blocks using the MIXED procedure of SAS. Interim feedlot performance was analyzed using repeated measures over time and included treatment, time, block, treatment x block, time x block, and the treatment x time x block interaction in the model. The model for overall feedlot performance and carcass characteristics included statements for treatment and block along with a random statement "experimental unit (treatment)" to account for interexperimental-unit variation (Littell et al., 1998). The default covariance structure (VC) was chosen based on fit statistics. Pen served as the experimental unit for feedlot performance and animal was used for variables measured on individual carcasses. Results are discussed as significant if P < 0.05 and as tendencies if P > 0.05 and P < 0.10.
Experiment 3 (Metabolism Experiment)
Eight ruminally and duodenally cannulated crossbred steers (initial BW = 336 ± 20 kg) were selected to be used in a completely random design. The objective of this study was to determine the effects of restricting intake of the final diet as a means of dietary adaptation compared with diets of increasing grain concentration over 21 d on total and compartmental digestion, digesta kinetics, and ruminal metabolism. Four steers were randomly assigned to each of two adaptation treatments: 1) ad libitum feeding of three adaptation diets over 21 d, with DRC concentration increasing from 52 to 80% (DM basis) or 2) restricted intake of the final diet with programmed increases in intake until ad libitum intake was achieved. Diets are presented in Table 2
. Initial intake of ad libitum steers was set at 2.0% of BW (DM basis) and intake was increased 0.45 kg/steer daily when feed on the previous day was completely consumed. Initial intake of restricted steers was calculated using the Level 1 model (NRC, 1996). The intake scaler of the Level 1 model was manipulated so that steers consuming the final diet would gain similar weight to ad libitum steers consuming 2.0% of BW (DM basis) of the 65% concentrate diet (Table 2); therefore, initial intake of restricted steers was set at approximately 1.65% of BW (DM basis). Intake of restricted steers was increased 0.23 kg/steer daily when feed on the previous day was completely consumed. Steers were housed in individual pens (5 m x 4 m) in a barn with slatted concrete floors under continuous lighting and had free access to fresh water. All surgical procedures, post-surgical care, and experimental protocol had been reviewed and approved by the Oklahoma State University Institutional Animal Care and Use Committee.
The dietary adaptation began on June 22, 2000 (d 1), and consisted of four 7-d adaptation periods and a later fifth period, which consisted of both treatments consuming the final diet to appetite. Dry matter intake was recorded on a daily basis; all refusals were weighed, DM content was determined (AOAC, 1996), and DM refused was subtracted from the total intake of that steer for that respective adaptation period. Chromic oxide (Cr2O3; 15 g/d) was intraruminally dosed twice daily via gelatin capsules (2/steer) as an indigestible marker of digesta flow throughout the entire experiment.
Sampling
Steers were fed once daily at 0800 h. Feed was subsampled daily throughout the experiment, and at the end of each adaptation period, samples were composited by diet concentrate level and dried at 50°C for 36 h. Fecal grab samples were taken on d 4 through 7 of each 7-d adaptation period and were composited by animal within period. A portion of the composite for each animal was dried in a forced-air oven (50°C, 96 h) and ground to pass a 2-mm screen in a Wiley mill for later determination of DM, OM, Cr, starch, and ADF. A second portion of the fecal composite was frozen, lyophilized at the conclusion of the experiment, and was used for N determination. On d 7 of each adaptation period, at approximately 0730, Co-EDTA (200 mL) and DRC (1 kg) labeled with ytterbium acetate were pulse-dosed intraruminally; 1 kg of Yb-labeled DRC replaced 1 kg of diet (as-is basis). Ytterbium and Co-EDTA have been shown to be reliable external markers in high-concentrate diets for corn and liquids, respectively (Sindt et al., 1993). Labeling procedures for corn were the same as those outlined by Teeter et al. (1984), whereas procedures for preparing Co-EDTA were the same as those described by Uden et al. (1980). Ruminal fluid and particulate matter were collected at 0, 3, 6, 9, 12, 15, 18, 21, and 24 h after dosing. Immediately after collection, 200 mL of ruminal fluid were strained through four layers of cheesecloth and pH was measured using a combination electrode. A 10-mL aliquot was acidified with 0.5 mL of 6 N HCl and frozen (-20°C) for later ammonia N analysis. A second 8-mL aliquot was acidified with 2 mL of 25% (wt/vol) metaphosphoric acid and frozen (-20°C) for later VFA analysis. A third and final 10-mL aliquot was frozen (-20°C) for later Co analysis. Samples of ruminal particulate matter were dried at 50°C for 36 h, and ground to pass a 2-mm screen for later analysis of Yb concentration.
Whole duodenal contents (250 mL) were collected simultaneous to ruminal sampling. Duodenal contents were frozen and lyophilized, ground using a coffee grinder, and composited within animal and period on an equal weight of DM basis.
Four hours after feeding at the end of each adaptation period, steers were weighed and total ruminal contents were removed, weighed, mixed thoroughly, subsampled, and DM determined.
Laboratory Analysis
Ground samples of feed, feces, duodenal contents, and ruminal particulate matter were analyzed for DM and OM based on standard procedures (AOAC, 1996). Nitrogen content of feed and lyophilized feces and duodenal contents was determined by the combustion method (Leco NS2000, St. Joseph, MI; AOAC, 1996). Acid detergent fiber and starch concentrations of feed, feces, and duodenal contents were determined by the methods of Van Soest et al. (1991) and MacRae and Armstrong (1968), respectively. Ruminal and duodenal fluid samples were thawed and centrifuged at 10,000 x g for 10 min. Chromium concentrations of fecal and duodenal composites were quantified using an Inductively Coupled Plasma Spectrophotometer (ICP Spectro Analytical Instruments, Fitchburg, MA). Concentrations of Co in ruminal fluid and concentrations of Yb in ruminal particulate samples were also determined by ICP analysis. The wavelengths used to measure optical emission of Cr, Co, and Yb were 267.7, 228.6, and 265.4, respectively. Ruminal ammonia N was determined using procedures outlined by Broderick and Kang (1980). Volatile fatty acid analysis of ruminal fluid was done using gas chromatography as outlined by Goetsch and Galyean (1983).
Calculations and Statistics
Dilution rate of Co and passage rate of Yb were calculated by regressing the natural log of marker concentration on time after dosing. Ruminal fluid volume was calculated by dividing dose by ruminal concentration extrapolated to 0 h, and fluid retention time was calculated as 1/dilution rate.
Digestibility, digesta flow, and ruminal DM and liquid fill data were analyzed as a completely random design using the Mixed procedure of SAS. Random and repeated statements were used to model the covariance structure in a repeated measures analysis by period. The model included fixed effects of treatment, period, and the treatment x period interaction. The type option in the repeated statement was used to specify the covariate structure; types VC, CS, AR(1), and UN were tested. Specific metabolites were also analyzed as repeated measures using time as the repeated variable rather than period. The model for all metabolites included treatment, period, treatment x period, time, treatment x time, period x time, and treatment x period x time. Means were separated at the highest-level interaction that was significant (P < 0.10) using Fisher’s LSD. Results are discussed as significant if P < 0.05 and as tendencies if P > 0.05 and P < 0.10.
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Results
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Experiment 1
Overall live weight gains were not affected (P = 0.43) by treatment (2.10 vs 2.02 kg/d for ad libitum vs restricted steers, respectively; Table 3). However, interim daily gains revealed differences due to treatment, when separated into 28-d intervals (adaptation x period interaction, P = 0.003). Daily gain by steers restricted in intake was reduced by 37.1% during the first 28 d of the feeding period compared with steers fed ad libitum. Adaptation method had no affect (P = 0.33) on daily gain from d 29 through 56; however, restricting intake improved (P = 0.03) daily gains during the final 14 d on feed. As expected, overall DMI was greater (P < 0.01) for ad libitum than for restricted steers. This response seemed to result from differences observed during the first 28 d of the feeding period (adaptation x period interaction, P < 0.01) when restricted steers were consuming 77.6% of the DM consumed by ad libitum steers (Figure 1). Overall gain:feed (kg of BW gain/100 kg of DMI) was similar (P = 0.14) for restricted compared with ad libitum steers (19.6 vs 20.8 kg/100 kg DMI). Interim gain:feed was similar among treatments through d 56; however, restricted steers were more (P = 0.03) efficient during the last 14 d of the feeding period compared with steers fed ad libitum. Intake variation on a daily basis (Table 4) was greater (P < 0.01) for ad libitum fed steers on d 11 through 15, 16 through 20, and 21 through 25 compared with steers restricted in intake during the adaptation period. Similarly, pen intake variation was greater (P = 0.06) for ad libitum fed steers during the entire adaptation phase (d 1 through 25) compared with restricted steers. Adaptation method did not influence (P > 0.10) carcass characteristics measured (Table 5).
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Table 3. Effect of restricting intake of the final diet vs ad libitum dietary adaptation on feedlot performance (Exp. 1)
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Table 4. Dry matter intake variation during adaptation to a high-concentrate diet using two separate methods (Exp. 1)
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Table 5. Effect of restricting intake of the final diet vs ad libitum dietary adaptation on carcass characteristics (Exp. 1)
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Experiment 2
Overall daily gain was greater (P < 0.01) for calves fed ad libitum compared with restricted steers (Table 6). Intermittent daily gains (adaptation x period interaction, P < 0.01) for calves restricted in intake during adaptation were reduced (P < 0.01) by 37.1% during the first 28 d of the feeding period compared with calves fed ad libitum, which was identical to the decrease observed in Exp. 1. Restricted steers also tended (P = 0.08) to have lower daily gains from d 113 through 140 compared with ad libitum-fed steers. Overall DMI was greater (P < 0.01) for ad libitum compared with restricted steers (9.15 vs 8.68 kg/d). As designed, interim DMI (adaptation x period interaction, P < 0.01) data indicated that DMI by restricted steers was 69.7% of ad libitum during the initial 28 d of the feeding period (P < 0.01), which accounted for a large portion of the differences observed in overall DMI. Restricted steers reached ad libitum DMI between d 29 and 56 (Figure 2) and had greater (P < 0.01) DMI compared with ad libitum steers from d 85 through 112. Steers fed ad libitum had greater (P = 0.02) DMI from d 113 through 140 compared with restricted steers. Overall and interim gain:feed (kg of gain /100 kg of DMI) was similar among treatments; however, restricted steers tended (P = 0.06) to be more efficient from d 29 through 56 compared with steers fed ad libitum. In contrast to the previous study, daily DMI variation (Table 7) was greater (P < 0.01) for restricted steers from d 9 through 17 compared with steers fed ad libitum. Pen DMI variation (Table 7) was decreased (P < 0.01) with restricted feeding on d 1 through 8, 9 through 17, and 18 through 22 compared with ad libitum feeding, which was similar to results of Exp. 1. Final weight (Table 6) as well as HCW (Table 8) was decreased (P < 0.01) by restricted intake compared with ad libitum (566 vs 543 kg and 359 vs 344 kg, respectively), whereas, carcass characteristics did not differ (P > 0.10).
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Table 6. Effect of restricting intake of the final diet vs ad libitum dietary adaptation on feedlot performance (Exp. 2)
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Table 7. Dry matter intake variation during adaptation to a high-concentrate diet using two separate methods (Exp. 2)
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Table 8. Effect of restricting intake of the final diet vs ad libitum dietary adaptation on carcass characteristics (Exp. 2)
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Metabolism Experiment
Because of problems associated with initial adaptation, one steer on the restricted-intake treatment was removed from the experiment following sampling in period 2, and samples were collected from only six of the remaining seven animals in the fifth period (n = 3) due to health problems that were not associated with experimental treatments. Daily DMI variation (Table 9) was greater for ad libitum fed steers from d 1 through 7 (P = 0.04), 8 through 14 (tendency, P = 0.07), and 22 through 28 (P < 0.01) compared with restricting intake of the final diet. Animal DMI variation (Table 10) was not affected (P > 0.10) by adaptation method. Adaptation method had no affect (P > 0.10) on intake and duodenal flow of OM, starch and N, or fecal excretion of starch and N (Table 10). Fecal excretion of OM tended (P = 0.09) to be reduced in restricted steers (adaptation method x period interaction) compared with steers fed ad libitum. An adaptation method x period interaction (P < 0.05) was observed for intake, duodenal flow and fecal excretion of ADF. Ad libitum fed steers had greater ADF intake and fecal excretion (P < 0.05) on d 4 through 7, 11 through 14, and 18 through 21 compared with steers limit-fed the final diet. Duodenal flow of ADF was greater for ad libitum-fed steers on d 4 through 7 and 11 through 14 compared with steers restricted in intake (P < 0.05). Fecal OM excretion was greater for ad libitum steers on d 4 through 7, 11 through 14, and 18 through 21 (P < 0.05) compared with restricted steers. An adaptation method x period interaction (P < 0.05) was observed for total tract OM digestibility. Restricted steers had greater digestibility of OM (P < 0.05) on d 4 through 7 (80.1 vs 66.4%), 11 through 14 (81.1 vs 66.3%) and 18 through 21 (81.9 vs 72.2%) compared with ad libitum-fed steers. Adaptation method had no affect (P > 0.10) on total tract digestibility of ADF, starch, or N, or ruminal or postruminal digestibilities of OM, ADF, starch, or N. Adaptation method had no affect (P > 0.10) on full BW, DM and fluid fill, passage rate of particulate matter, fluid dilution rates, retention time or ruminal volume (Table 11). Similarly, adaptation method did not influence (P > 0.10) ruminal metabolites (data not shown).
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Table 9. Dry matter intake variation during adaptation to a high-concentrate diet using two separate methods (Exp. 3)
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Table 10. Effect of restricting intake of the final diet vs ad libitum dietary adaptation on ruminal, post ruminal, and total tract digestibilities
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Discussion
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The results of the initial performance experiment concur with results of Bierman and Pritchard (1996) and Weichenthal et al. (1999). In their studies, restricting intake during adaptation did not influence daily gain but reduced DMI, resulting in improved feed efficiency by steers limit-fed the final diet compared with steers fed adaptation diets for ad libitum intake. Although overall feed efficiency was not statistically improved in either of the present performance experiments, restricting intake of the final diet as a method of adaptation appears to be efficacious for adapting cattle to high-concentrate diets. This method of adaptation may produce other benefits, such as simplified bunk management, reduced feed waste (Lake, 1986), as well as the potential for decreased manure and nutrient output. The present metabolism study revealed that restricting intake of the final diet reduced fecal excretion of OM and N by 50 and 35%, respectively, through d 21.
In Exp. 1 and 2, it is important to note that the decreased intake variation with restricted-feeding most likely resulted from differences in feed delivery. Both day-to-day (Exp. 1) and pen-to-pen (Exp. 1 and 2) feed intake variation were decreased with restricted feeding. Soto-Navarro et al. (2000) reported that constant daily feed intake by restricted steers reduced pH area below 6.2 compared with a 10% fluctuation in daily feed intake, which might suggest that the potential for subacute acidosis was reduced. Although pen-to-pen variation in feed intake was decreased by restricting intake in Exp. 1 and 2 of our study, it should be noted that variation among animals within a pen might have been significant (Stock et al., 1995b). However, in our metabolism study animal-to-animal variation was not affected by restricted feeding of the final diet during adaptation.
The basis for the large depression in performance by intake-restricted steers during the initial 28 d of the feeding period in Exp. 1 and 2 might be due to differences in ruminal fill; however, no difference in fill was observed during the first 21 d of this metabolism study. In contrast to our results, Bartle and Preston (1992) limited maximum intake of 370-kg steers to 2.1, 2.3, 2.5, and 2.7 x maintenance during wk 1, 2, 3, and 4, respectively, of a high-concentrate adaptation period compared with ad libitum feeding of traditional adaptation diets (i.e., roughage was decreased from 40 to 10% across the 4-wk period). During the 28-d adaptation period, limiting maximum intake reduced DMI by 6.2% compared with ad libitum feeding, but no statistical difference was observed for ADG. Differences in results are most likely due to the greater degree of intake restriction during adaptation in our experiments. During adaptation, DMI was 28.8 and 43.5% lower for intake-restricted steers compared with steers fed ad libitum in Exp. 1 and 2, respectively, and ME allowable ADG predicted from the NRC (1996) Level 1 Model (Table 12) was lower for restricted steers across the adaptation period in both Exp. 1 and 2. For the overall feeding period, the reduction in daily gain observed in Exp. 2 by intake-restricted lighter-weight steers was likely a result of the magnitude and duration of the restriction. The difference in difficulty of adapting calves vs yearlings under the conditions of the present studies is best described from Figures 1 and 2. Daily intake of restricted steers during adaptation in Exp. 1 (Figure 1) increased gradually with few to no interruptions in our programmed increases in intake (i.e., 0.23 kg•steer-1•d-1), and this pattern continued until approximately d 36. In contrast, daily intakes of restricted steer calves in Exp. 2 (Figure 2) showed greater variation and more interruptions occurred, especially during the first few days of adaptation. A general plateau in intake occurred from approximately d 25 through 28. Interestingly, similar variation was observed during the first few days with ad libitum feeding of adaptation diets in Exp. 2, which suggests that the difficulty observed in adaptation was not entirely due to limit-feeding of the final diet, but also due to differences in calves vs yearlings or other unidentified factors. Although not evaluated in the current experiments, limiting maximum intake based on multiples of the maintenance energy requirement (Bartle and Preston, 1992) might have lowered disruptions in intake during the adaptation period in calves.
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Table 12. Calculated ME and metabolizable protein (MP) allowable ADG based on the 1996 Beef Cattle NRC (Level-1 Model)
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Rapid increases in intake of ad libitum-fed steers in Exp. 1 and 2, in an attempt to reach ad libitum intake as quickly as possible, resulted in a sharp decrease in intake toward the end of the adaptation period. Although few comparisons are available in the literature, this ’overconsumption’ by ad libitum steers observed with the traditional method of adaptation might be a common occurrence (Stock et al., 1995b), and appears to be alleviated with restricting intake of the final diet.
It appears that the key to improving feed efficiency with restricted feeding of feedlot cattle is to reduce DMI without a reduction in gain. Obviously, an overall reduction in gain can have a negative impact on final live and carcass weight, as observed in Exp. 2. The length and degree of restriction in Exp. 1 and 2 were minimal for yearlings and moderate for calves in comparison to other limit-feeding experiments (Hicks et al., 1990; Galyean, 1999). For Exp. 1 and 2, restricted steers had similar intakes compared with steers fed ad libitum by approximately 27 and 41 d, respectively, and intakes of restricted steers were not less than 69.7% of ad libitum in both experiments. Calculated ME and metabolizable protein (MP) allowable ADG using the Level 1 Model (NRC, 1996) for the adaptation periods in Exp. 1 and 2 indicated that MP was the first limiting nutrient for the first 5 d in Exp. 1 and throughout the adaptation in Exp. 2 (Table 12). This suggests that increasing the MP concentration during a period of intake restriction might be more important with calves than yearlings. Due to the relatively short adaptation period, nutrient concentrations were not altered in the present experiment so that one diet could be fed throughout adaptation and finishing. In Exp. 1, ME allowable ADG was similar to actual 28-d performance by d 11 through 15 of the adaptation period. In contrast, predicted ME allowable ADG during the 28-d adaptation period was only 67.8 and 61.2% of actual performance for ad libitum and restricted calves, respectively, in Exp. 2. Metabolizable energy and MP allowable ADG predicted from the NRC (1996) Level 1 Model for the overall feeding period are shown in Tables 3 and 6, respectively, for Exp. 1 and 2.
Several mechanisms have been proposed as possible explanations for improved feed efficiency in intake-restricted cattle. However, due to the wide range in limit feeding applications, no single mechanism may be active in every scenario. Due to common low intakes associated with restricted feeding, one would expect that restricted feeding simply increases diet digestibility due to the fact that in most cases digestibility and intake are inversely related. This is consistent with the current metabolism study where total tract digestibility of OM was improved with limit feeding at times when restriction was greatest. Our observations are in general agreement with those of Loerch (1990), Murphy et al. (1994b), and Zinn and Owens (1983). In contrast, Old and Garrett (1987) reported that intake level (ad libitum, 85% of ad libitum, or 70% of ad libitum) did not affect digestibility of a high-concentrate diet. The greater OM digestibility by restricted steers in the current metabolism study (Exp. 3) is largely attributable to lower percentages of roughage found in the finishing diet compared with the initial adaptation diets. This finding is comparable to Loerch (1990) who found that digestibility of an all-concentrate diet fed at restricted intake was greater than that of a corn silage-based diets provided ad libitum. The length of time in which digestibility was improved in the current metabolism study is consistent with the duration of the restriction. Again, it is important to note that the length of restriction in the current experiments was relatively short compared with the existing literature on limit feeding, which might reduce the opportunities for improved feed efficiency.
The current metabolism experiment observed no difference (P > 0.05) in OM intake; however, restriction was evident based on total grams of daily intake. The intakes by limit-fed steers as a percentage of steers fed ad libitum were 65, 79, 79, 92, and 93% for days 1 through 7, 8 through 14, 15 through 21, 22 through 28, and 68 through 74, respectively. Owens et al. (1986) stated that as feed intake increases, rate of passage is accelerated, which causes decreased time for digestion and, therefore, digestibility of slowly fermented materials, particularly fiber components, decreases. Thus, one hypothesis might be that restricted feeding simply decreases passage rate. Similarly, across the adaptation period, limit-fed steers had a numerically greater (approximately 17%) ruminal retention time and a numerical reduction (12%) in liquid dilution rate compared with steers fed ad libitum in the current experiment. These results concur with Murphy et al. (1994a) who observed decreased dilution rates when steers were fed an all-concentrate diet at 70% of ad libitum. In contrast, Hicks et al. (1990) observed no difference in passage rate (calculated by Cr concentration) of steers consuming an 80% cracked-corn diet at either ad libitum, 80% of ad libitum for the first 56 d followed by ad libitum access, programmed intake to gain 1.50 kg/d, or programmed intake to gain 1.35 kg/d.
If passage rate is actually decreased with limit feeding, differences in ruminal fill should be more difficult to detect than would be expected in cattle at different levels of intake. This theory is consistent with the results of our metabolism study and the results of Galyean et al. (1979) in which feeding an 84% cracked-corn diet at 1.0, 1.33, 1.67, or 2.0 x maintenance had no affect on ruminal volume. However, these results are inconsistent with those of Merchen et al. (1986) and Murphy et al. (1994b) who observed greater ruminal volumes when sheep were fed at a high level of intake (2.6% of BW) or ad libitum, respectively, compared with sheep restricted in intake (1.6% of BW or 70% of ad libitum).
Rumsey et al. (1970) reported that total VFA concentration increased as intake increased. This is inconsistent with our metabolism study where total VFA concentration was not affected by restricting intake. However, our results do concur with the findings of Merchen et al. (1986) who observed no difference in total VFA concentration in wethers fed at high and low-levels of intake. Murphy et al. (1994a) reported that steers consuming 70% of ad libitum had greater concentrations of ruminal VFA at 3 and 4 h after feeding compared with steers fed ad libitum, which was attributed to decreased ruminal volume in steers fed 70% of ad libitum. The lack of differences in intake, ruminal fill and VFA concentration likely all contributed to the similar pH values and ruminal ammonia N concentration observed in the current metabolism study.
The current metabolism study was conducted in order to quantify differences in performance observed in Exp. 1 and 2. The improvement in digestibility with restricted feeding might be offset due to differences in ruminal fill, but data remain unclear. The ease in which limit feeding was used to adapt yearling steers to the finishing program is supported by the lack of differences observed in ruminal pH along with the reduced day-to-day intake variations observed in the current metabolism study. The minimal differences in performance observed in yearling steers might be explained by a lack of limit-feeding effects on ruminal VFA and ammonia N concentrations, and the short duration in which steers were restricted in intake.
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Implications
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Restricted feeding of the final diet as a means of dietary adaptation can be used in yearling cattle with few problems from acidosis or related intake variation. Care is needed to ensure that the length and degree of restriction is limited so that daily gains are not depressed to a point where increased days on feed are required. This method of adaptation is also efficacious in calves. However, care should be taken in order to avoid disruptions in intake during the adaptation period, which might result in restriction for an extended period of time and ultimately increase the days on feed required to achieve comparable carcass weights. The effects of limit feeding during the initial 28 d of the feeding period on site and extent of digestion, digesta kinetics, and ruminal metabolism appear to be minimal, supporting few differences in performance across the finishing period.
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Footnotes
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1 Approved for publication by the Director of the Oklahoma Agric. Exp. Sta. This research was supported by the Oklahoma and New Mexico Agric. Exp. Sta. 
2 The authors would like to acknowledge Lisa Blan, Kathryn Malcolm-Callis, and Mathew Wiseman from the Clayton Livestock Research Center, Clayton, NM, Roy Ball and Bill Starr from the Willard Sparks Beef Research Center, Steve Welty from the Nutrition and Physiology Research Center, Tim Bodine, Carolyn Lunsford, Donna Perry, Christina Stout, and Joan Summers from the Ruminant Nutrition Laboratory, Stillwater, OK, and Ken Bolenbaugh of Bolenbaugh Farms, Carrier, OK for their contributions to these experiments.
4 Present Address: Univ. of Arizona, Dept. of Anim. Sci., 217 Shanzt Bldg., Tucson, 85721.
Received for publication January 14, 2002.
Accepted for publication May 28, 2002.
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Abstract
Introduction
