Hales K E, Shackelford S D, Wells J E, King D A, Pyatt N A, Freetly H C, Wheeler T L
J Anim Sci. 2016 May;94(5):2097-102. doi: 10.2527/jas.2015-0225.
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 additional 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 ( = 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 ( > 0.19) for animal performance or carcass traits. Final BW did not differ among treatment ( = 0.37); however, final BW had a tendency to be 2% greater ( = 0.07) when the 13.5% CP diet was fed. Dry matter intake was not different between steers fed 0 or 300 mg/d of RAC ( = 0.20), yet DMI was 12% greater for cattle fed the 13.5% CP compared with steers fed the 17.5% CP diet ( < 0.01). Daily gain did not differ for cattle fed different levels of RAC or CP ( > 0.16). The G:F was 3.6% greater for cattle fed 300 vs. 0 mg/d of RAC ( = 0.04). The G:F was 8.7% greater for cattle fed the 17.5% diet vs. the 13.5% CP diet ( < 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 of RAC ( = 0.36) or for steers fed the 13.5% diet vs. 17.5% CP diet ( = 0.93). Dressing percentage was 1.5% greater for cattle fed 300 vs. 0 mg/d of RAC ( = 0.05) but was not different between cattle fed the different CP levels in the diet ( = 0.16). Longissimus area, adjusted 12th-rib fat, and marbling score did not differ across RAC or CP treatments ( > 0.26). Additionally, no differences in USDA yield grade or percentage of cattle grading USDA Choice were detected for RAC or CP treatments ( > 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.
盐酸莱克多巴胺（RAC）被用于育肥行业，以提高日增重、改善饲料效率并增加热胴体重（HCW）。然而，关于是否需要在日粮中添加额外蛋白质以最大化RAC在肉牛中的益处，相关研究甚少。我们实验的目的是确定在RAC基础上添加粗蛋白（CP）是否会改善动物生产性能和胴体特性。因此，在育肥期结束时，使用含13.5%或17.5% CP且添加0或300毫克RAC的育肥日粮进行了为期30至33天的实验。肉牛（n = 438；初始体重387.8 ± 1.9千克）按照2×2析因设计随机分配到4种处理中的1种（共16个栏，每个处理4个栏）。对于动物生产性能或胴体性状，未检测到RAC与CP水平之间的交互作用（P > 0.19）。各处理间的末重无差异（P = 0.37）；然而，饲喂13.5% CP日粮时，末重有高出2%的趋势（P = 0.07）。饲喂0或每天300毫克RAC的肉牛干物质采食量无差异（P = 0.20），但与饲喂17.5% CP日粮的肉牛相比，饲喂13.5% CP日粮的肉牛干物质采食量高出12%（P < 0.01）。不同RAC或CP水平的肉牛日增重无差异（P > 0.16）。与饲喂0毫克/天RAC的肉牛相比，饲喂300毫克/天RAC的肉牛料重比高出3.6%（P = 0.04）。与饲喂13.5% CP日粮的肉牛相比，饲喂17.5% CP日粮的肉牛料重比高出8.7%（P < 0.01），这可归因于饲喂17.5% CP日粮的肉牛干物质采食量降低。饲喂0或每天300毫克RAC的肉牛热胴体重无差异（P = 0.36），饲喂13.5%或17.5% CP日粮的肉牛热胴体重也无差异（P = 0.93）。与饲喂0毫克/天RAC的肉牛相比，饲喂300毫克/天RAC的肉牛屠宰率高出1.5%（P = 0.05），但日粮中不同CP水平的肉牛屠宰率无差异（P = 0.16）。不同RAC或CP处理间的背最长肌面积、第12肋处调整脂肪厚度和大理石花纹评分无差异（P > 0.26）。此外，对于RAC或CP处理，未检测到美国农业部产量等级或美国农业部精选级肉牛百分比的差异（P > 0.26），这也支持了在相同脂肪水平下，饲喂RAC的肉牛品质等级不受影响这一观点。我们的数据表明，过量蛋白质并未增强对RAC的反应，此外，除了料重比略有增加外，未观察到其他人报道的RAC带来的生产性能改善。
