PROTEIN-REQUIREMENTS OF GROWING STEERS LIMIT-FED CORN-BASED DIETS

In Exp. 1, six steers (254 kg) were used in a 6 x 4 incomplete Latin s quare to determine the effects of solvent-extracted soybean meal alone or in combination with rumen-protected methionine and lysine on N bal ance in steers limit-fed a high-corn diet to gain 1.1 kg/d. The basal diet contained (DM basis) 80% rolled corn, 15% alfalfa, and .9% urea ( 13.9% CP), and 2 or 4% soybean meal replaced corn to give CP concentra tions of 14.8 and 15.6%, respectively. Each diet was fed with and with out 5 g/d of Smartamine(TM)-ML (.75 and 2.0 g of rumen-protected methi onine and lysine, respectively). Nitrogen retention increased linearly (P = .09) with level of soybean meal. Rumen-protected methionine and lysine had no effect on N balance. In Exp. 2, seven steers (233 kg) we re used in a 7 x 4 incomplete Latin square experiment to investigate o ptimal levels and sources of CP for steers limit-fed to gain 1 kg/d. T reatments included a negative-control diet (urea; 11.7% CP) and six di ets containing either 13.5, 15.4, or 17.2% CP with either solvent-extr acted or expeller-processed soybean meal. Diets provided 75, 87.5, 100 , or 112.5% of estimated CP requirement for a gain of 1 kg/d. The basa l diet contained 83% rolled corn, 15% alfalfa, and .2% urea. Nitrogen retention increased linearly (P = .006) with soybean meal addition, an d no differences were observed between CP sources. The CP system under predicted the protein requirements of limit-fed steers under our condi tions.