Cole N A, Defoor P J, Galyean M L, Duff G C, Gleghorn J F
USDA-ARS Conservation and Production Research Laboratory, Bushland, TX 79012, USA.
J Anim Sci. 2006 Dec;84(12):3421-32. doi: 10.2527/jas.2006-150.
As cattle mature, the dietary protein requirement, as a percentage of the diet, decreases. Thus, decreasing the dietary CP concentration during the latter part of the finishing period might decrease feed costs and N losses to the environment. Three hundred eighteen medium-framed crossbred steers (315 +/- 5 kg) fed 90% (DM basis) concentrate, steam-flaked, corn-based diets were used to evaluate the effect of phase-feeding of CP on performance and carcass characteristics, serum urea N concentrations, and manure characteristics. Steers were blocked by BW and assigned randomly to 36 feedlot pens (8 to 10 steers per pen). After a 21-d step-up period, the following dietary treatments (DM basis) were assigned randomly to pens within a weight block: 1) 11.5% CP diet fed throughout; 2) 13% CP diet fed throughout; 3) switched from an 11.5 to a 10% CP diet when approximately 56 d remained in the feeding period; 4) switched from a 13 to an 11.5% CP diet when 56 d remained; 5) switched from a 13 to a 10% CP diet when 56 d remained; and 6) switched from a 13 to an 11.5% CP diet when 28 d remained. Blocks of cattle were slaughtered when approximately 60% of the cattle within the weight block were visually estimated to grade USDA Choice (average days on feed = 182). Nitrogen volatilization losses were estimated by the change in the N:P ratio of the diet and pen surface manure. Cattle switched from 13 to 10% CP diets with 56 d remaining on feed or from 13 to 11.5% CP with only 28 d remaining on feed had lower (P < 0.05) ADG, DMI, and G:F than steers fed a 13% CP diet throughout. Steers on the phase-feeding regimens had lower (P = 0.05) ADG and DMI during the last 56 d on feed than steers fed 13.0% CP diet throughout. Carcass characteristics were not affected by dietary regimen. Performance by cattle fed a constant 11.5% CP diet did not differ from those fed a 13% CP diet. Serum urea N concentrations increased (P < 0.05) with increasing dietary CP concentrations. Phase-feeding decreased estimated N excretion by 1.5 to 3.8 kg/steer and nitrogen volatilization losses by 3 to 5 kg/steer. The results suggest that modest changes in dietary CP concentration in the latter portion of the feeding period may have relatively small effects on overall beef cattle performance, but that decreasing dietary CP to 10% of DM would adversely affect performance of cattle fed high-concentrate, steam-flaked, corn-based diets.
随着牛的成熟，日粮中蛋白质需求占日粮的百分比会下降。因此，在育肥后期降低日粮粗蛋白（CP）浓度可能会降低饲料成本，并减少氮向环境中的排放。选用318头中等体格的杂交阉牛（体重315±5千克），给它们饲喂90%（干物质基础）的精料，即以蒸汽压片玉米为基础的日粮，以评估CP分阶段饲喂对生长性能、胴体特性、血清尿素氮浓度和粪便特性的影响。阉牛按体重进行分组，然后随机分配到36个饲养栏（每个栏8至10头阉牛）。经过21天的适应期后，在每个体重组内将以下日粮处理（干物质基础）随机分配到各个栏中：1）全程饲喂11.5% CP的日粮；2）全程饲喂13% CP的日粮；3）在饲喂期大约还剩56天时，从11.5% CP的日粮转换为10% CP的日粮；4）在还剩56天时，从13% CP的日粮转换为11.5% CP的日粮；5）在还剩56天时，从13% CP的日粮转换为10% CP的日粮；6）在还剩28天时，从13% CP的日粮转换为11.5% CP的日粮。当体重组内大约60%的牛目测评定为美国农业部特选级时（平均饲养天数 = 182天），对该体重组的牛进行屠宰。通过日粮和栏舍表面粪便中氮磷比的变化来估算氮挥发损失。在饲喂期还剩56天时从13% CP日粮转换为10% CP日粮的牛，以及在饲喂期仅剩28天时从13% CP日粮转换为11.5% CP日粮的牛，其平均日增重（ADG）、干物质采食量（DMI）和料重比（G:F）均低于全程饲喂13% CP日粮的阉牛（P < 0.05）。采用分阶段饲喂方案的阉牛在最后56天的饲养期内，其ADG和DMI低于全程饲喂13.0% CP日粮的阉牛（P = 0.05）。日粮方案对胴体特性没有影响。饲喂恒定11.5% CP日粮的牛的生长性能与饲喂13% CP日粮的牛没有差异。血清尿素氮浓度随着日粮CP浓度的增加而升高（P < 0.05）。分阶段饲喂使每头阉牛的估计氮排泄量减少了1.5至3.8千克，氮挥发损失减少了3至5千克。结果表明，在饲喂期后期适度改变日粮CP浓度可能对肉牛的总体生长性能影响相对较小，但将日粮CP降至干物质的10%会对饲喂高浓度、蒸汽压片玉米基础日粮的牛的生长性能产生不利影响。
