The effect of crude protein withdrawal and the use of β-agonists on feedlot performance, carcass merit, and theoretical nitrogen retention and excretion for heavy yearling steers
Date
2011
Authors
Peterson, Ryan Kenneth, author
Wagner, John Joseph, 1958-, advisor
Engle, Terry E., committee member
Callan, Robert J., committee member
Journal Title
Journal ISSN
Volume Title
Abstract
Eight hundred and sixty-four crossbred yearling steers were used in a 3 x 4 factorial experiment to study the effects of β - agonist (BA) administration and crude protein (CP) withdrawal during the last 28 d on feed. Three BA treatments were evaluated: no BA, 200 mg Optaflexx (OPT) per head daily for 28 d, and 75 mg Zilmax (ZIL; 7.56 g/ton DM basis) per head daily for 20 d. The 4 CP withdrawal treatments included: 1) 13.5% CP, 3.5% crude protein equivalent (CPE) from non-protein nitrogen (NPN) from d 0 through slaughter (13.5/3.5); 2) 13.5/3.5 until 28 d remained on feed followed by 12.5% CP, 3.5% CPE from NPN (12.5/3.5); 3) 13.5/3.5 until 28 d remained on feed followed by 12.5% CP, 2.5% CPE from NPN (12.5/2.5); and 4) 13.5/3.5 until 28 d remained on feed followed by 11.5% CP, 1.5% CPE from NPN (11.5/1.5). There were no interactions (P > 0.32) between BA and CP withdrawal treatment; therefore, only main effects are discussed. Average slaughter weights and ADG were greater (P < 0.01) for steers treated with BA as compared with control steers. There were no differences between the slaughter weights and ADG for the OPT versus the ZIL steers. Beta-agonist treatment had no effect on daily dry matter intake. Feed-to-gain ratio was improved by 25% for steers fed BA during the final 28 d as compared with controls. More than a 35% improvement (P < 0.001) in gain-to-feed ratio (G:F) for the final 28 d was observed for steers fed BA as compared with controls. Differences between OPT and ZIL were not significant. Net energy recovery was increased 19% during the final 28 d on feed for steers fed BA as compared with controls. Least squares means for HCW, adjusted for live weight at treatment initiation, was 9.21 kg heavier (P < 0.01) for ZIL steers as compared with controls. The advantage (P < 0.05) in HCW for ZIL as compared with OPT was 5.81 kg. Although not statistically significant, the numerical advantage in HCW for OPT as compared with controls was 3.4 kg. Dressing percentage was increased for steers fed ZIL versus control (64.4 versus 63.4%, P < 0.001) and OPT (P < 0.01) but not increased for OPT versus control (63.2 versus 63.4%; P = 0.67). No BA treatment differences were observed for fat depth measured at the 12th rib. Ribeye area for control carcasses (avg = 32.87 cm2) was lower (P < 0.06) than OPT (avg = 33.53 cm2) which was in turn lower (P < 0.01) than ZIL (avg = 34.59 cm2). Kidney, pelvic, and heart fat was lower for ZIL as compared with control (P < 0.05) or OPT (P < 0.10). Yield grades calculated from carcass measurements were similar for OPT and ZIL (3.01 versus 2.97), and both were reduced (P < 0.05) as compared with control (avg = 3.12). Carcasses from OPT and ZIL treatments were more likely (P < 0.05) to qualify for the USDA Yield Grade 1 and 2 categories and less likely (P < 0.05) to qualify for the USDA Yield Grade 3 category as compared with control. There were no differences in the distribution of USDA Yield Grade 4 and 5 carcasses among control, OPT, and ZIL treatments. Marbling score was similar for OPT and ZIL carcasses averaging Small11 and Small09, respectively. Marbling score for the control carcasses was significantly higher (P < 0.01) than for BA treatment and averaged a Small32. The likelihood of an individual carcass within a pen of grading low Choice and above was reduced (P < 0.01) for the OPT (avg = 53.9%) and ZIL (avg = 49.7%) treatments as compared with controls (avg = 61.8%). Slaughter weight, ADG during the final 28 d, and ADG d 1 through slaughter was lower for the 12.5/3.5 treatments as compared with the 12.5/2.5 (P < 0.05) and 11.5/1.5 (P < 0.10) treatments. Crude protein withdrawal treatment had no effect on HCW and all other carcass variables evaluated. Lower slaughter weight by the 12.5/3.5 treatment and the effect of this reduced weight on ADG were likely a function of reduced gut fill due to a reduction in DMI by steers fed the 12.5/3.5 diets during the final 28 d on feed as compared with the 13.5/3.5 (P < 0.05), 12.5/2.5 (P < 0.0.01), and 11.5/1.5 (P < 0.10) treatments. Nitrogen retention was 5.54, 5.43, 6.97, and 6.90% of N intake for the 13.5/3.5, 12.5/3.5, 12.5/2.5, and 11.5/1.5 CP treatments, respectively. Nitrogen excretion was 5,282; 4,654; 4,875; and 4,391 g/hd for the 13.5/3.5, 12.5/3.5, 12.5/2.5, and 11.5/1.5 CP treatments, respectively. Nitrogen excretion was greatest (P < 0.001) for the 13.5/3.5 CP treatment as compared with the remaining treatments. Differences between the 12.5/3.5 versus 12.5/2.5 (P < 0.05) and between the 12.5/2.5 versus 11.5/1.5 (P < 0.0001) treatments were also significant. Using the 12.5/2.5 or 11.5/1.5 CP treatment diets as compared with the 13.5/3.5 CP diet did not reduce feedlot performance, HCW, or USDA Quality and Yield Grade during the final 28 d. Considering the source and cost of non-protein nitrogen, significant savings could be obtained from using the 12.5/2.5 diet or the 11.5/1.5 diet as compared with using the 13.5/3.5 diet during the last 28d on feed.
Description
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Subject
beta-agonist
zilmax
undegradable intake protein
optaflexx
degradable intake protein