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Effects of dietary protein concentration and ractopamine hydrochloride on performance and carcass characteristics of finishing beef steers

Hales, K. E., Shackelford, S. S., Wells, J. E., King, D. A., Pyatt, N. A., Freetly, H. C., Wheeler, T. L.
Journal of animal science 2016 v.94 no.5 pp. 2097-2102
USDA, beef cattle, beta-adrenergic agonists, carcass weight, cattle feeding, dietary protein, dressing percentage, dry matter intake, feed additives, feed conversion, feedlots, finishing, industry, liveweight gain, marbling, steers
Ractopamine hydrochloride (RAC) is used in the feedlot industry to increase daily gain, improve feed efficiency, and increase HCW. However, little work has been done to determine whether addition- al protein is needed in the diet to maximize the benefit of RAC in beef cattle. Objectives of our experiment were to determine if feeding additional CP in conjunction with RAC would improve animal performance and carcass characteristics. Therefore, an experiment was conducted using finishing diets containing 13.5 or 17.5% CP with 0 or 300 mg of RAC for 30 to 33 d at the end of the finishing period. Beef steers (n = 438; 387.8 ± 1.9 kg initial BW) were randomly assigned to 1 of 4 treatments in a 2 × 2 factorial arrangement (16 pens total, 4 pens/treatment). No interactions between feeding RAC and CP level were detected (P > 0.19) for animal performance or carcass traits. Final BW did not differ among treatment (P = 0.37); however, final BW had a tendency to be 2% greater (P = 0.07) when the 13.5% CP diet was fed. Dry matter intake was not different between steers fed 0 or 300 mg/d(-1) of RAC (P = 0.20), yet DMI was 12% greater for cattle fed the 13.5% CP compared with steers fed the 17.5% CP diet (P < 0.01). Daily gain did not differ for cattle fed different levels of RAC or CP (P > 0.16). The G:F was 3.6% greater for cattle fed 300 vs. 0 mg/d(-1) of RAC (P = 0.04). The G:F was 8.7% greater for cattle fed the 17.5% diet vs. the 13.5% CP diet (P < 0.01), which can be attributed to the decreased DMI for cattle fed the 17.5% CP diet. Hot carcass weight was not different for steers fed 0 or 300 mg/d(-1) of RAC (P = 0.36) or for steers fed the 13.5% diet vs. 17.5% CP diet (P = 0.93). Dressing percentage was 1.5% greater for cattle fed 300 vs. 0 mg/d(-1) of RAC (P = 0.05) but was not different between cattle fed the different CP levels in the diet (P = 0.16). Longissimus area, adjusted 12th-rib fat, and marbling score did not differ across RAC or CP treatments (P > 0.26). Additionally, no differences in USDA yield grade or percentage of cattle grading USDA Choice were detected for RAC or CP treatments (P > 0.26), which also supports the idea that quality grade of cattle fed RAC at the same level of fatness is not impacted. Our data indicate excess protein did not enhance the response to RAC, and furthermore, the improved performance from RAC reported by others was not observed other than a small increase in G:F.