Effects of dietary nitrogen source and concentration in high-grain diets on finishing steer performance and nutrient digestion

Two experiments were designed to evaluate dietary N source and concentration on finishing steer performance and nutrient digestion. In Exp. 1, 100 steers were used in a randomized complete block design experiment with 2 x 2 + 1 factorially arranged treatments. Diets contained 1.93 or 2.24% N supplemented by urea or soybean meal (SBM), or 2.24% N supplemented by cottonseed meal (CSM). Steers fed SBM-supplemented diets gained 13% faster (P <.01) and were 9% (P <.01) more efficient converting feed to gain than steers receiving urea. Steers fed diets containing 2.24% N were 4% ( P <.05) more efficient than those fed diets containing 1.93% N. Steers fed CSM-supplemented diets gained 6% (P < .10) less efficiently than steers receiving SBM. Increasing dietary N with urea from 1.93 to 2.24% decreased carcass weights 3%, whereas increasing dietary N with SBM increased carcass weights 3%. Carcass-adjusted gains were reduced 8% by increasing urea from.9 to 1.5% but increased 7% by increasing SBM from 6.1 to 10.5% of DM. In Exp. 2, four ruminally and duodenally cannulated steers (390 kg) were used in a 4 x 4 Latin square design experiment to evaluate urea and SBM supplementation on digestion. Diets contained no supplemental N, 1.84% N with urea or SBM as the supplement, or 2.16% N with SBM as the supplement. Total tract starch digestion, duodenal microbial N flow, and efficiency of microbial protein synthesis in the rumen were higher ( P <.10) in steers fed SBM- than in those fed urea-supplemented diets. Supplementation with SBM increased metabolizable protein supply and dietary energy utilization.