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 concent ration on finishing steer performance and nutrient digestion. In Exp. 1, 100 steers were used in a randomized complete block design experime nt with 2 x 2 + 1 factorially arranged treatments. Diets contained 1.9 3 or 2.24% N supplemented by urea or soybean meal (SBM), or 2.24% N su pplemented by cottonseed meal(CSM). Steers fed SBM-supplemented diets gained 13% faster (P < .01) and were 9% (P < .01) more efficient conve rting feed to gain than steers receiving urea. Steers fed diets contai ning 2.24% N were 4% (P < .05) more efficient than those fed diets con taining 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 increas ing 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 ruminal ly and duodenally cannulated steers (390 kg) were used in a 4 x 4 Lati n square design experiment to evaluate urea and SBM supplementation on digestion. Diets contained no supplemental N, 1.84% N with urea or SB M as the supplement, or 2.16% N with SBM as the supplement. Total trac t starch digestion, duodenal microbial N flow, and efficiency of micro bial protein synthesis in the rumen were higher (P < .10) in steers fe d SBM- than in those fed urea-supplemented diets. Supplementation with SBM increased metabolizable protein supply and dietary energy utiliza tion.