Effects of crude protein concentration in diets of feedlot steers fed to achieve stepwise increases in rate of gain

Two experiments were conducted to determine supplemental CP requirements for feedlot steers prescription-fed to achieve stepwise increases in ADG during the feeding period. In Exp. 1, 106 steers (initial BW = 285.7 ± 3.4 kg) were allotted to 12 pens. Treatments consisted of ad libitum consumption of a 12% CP diet throughout the experiment (AL12%), prescription intake of a 12% CP diet (PI12%), or prescription intake of high-CP diets (PIHCP). The PIHCP diets were formulated to provide the same grams of CP per day as the AL12% diet. Steers in the prescription intake feeding regimens were fed to gain 1.13 kg/d for the first 87 kg of gain and 1.36 kg/d for the next 134 kg of gain and were offered feed for ad libitum consumption for the final 52 kg of gain before slaughter at 559 kg. Overall ADG was lower (P < 0.05) for steers fed PI12% and PIHCP diets than for those fed the AL12% diet (1.48 and 1.50 vs 1.60 kg/d, respectively). Total DM intake was lower (P < 0.08) for steers fed the PI12% and PIHCP diets than for steers fed the AL12% diet (1,359, 1,370, and 1,421 kg, respectively). Dressing percentage, back-fat thickness, and quality grade were lower (P < 0.05) for steers fed the PI12% and PIHCP diets than for steers fed the AL12% diet. In Exp. 2, 142 steers (initial BW = 285.4 ± 3.1 kg) were allotted to 16 pens. System 1 was ad libitum consumption of a 12% CP diet throughout the experiment. Steers in Systems 2, 3, 4, and 5 were subjected to the same prescription intake strategy as steers in Exp. 1. During the three phases of growth (1.13 kg/d, 1.36 kg/d, and ad libitum), steers were fed the following CP concentrations: System 2(12, 14, and 12%); System 3(12, 14, and 14%); System 4(12, 16, and 12%); and System 5(12, 16, and 14%). No differences (P > 0.06) in total steer performance, carcass characteristics or carcass chemical composition were observed among systems. Increasing percentage of CP in diets of prescription-fed steers to provide the same daily intake of CP as steers consuming feed ad libitum was not beneficial.