FREE AMINO-ACID SUPPLEMENTATION TO STEERS - EFFECTS ON RUMINAL FERMENTATION AND PERFORMANCE

Three studies were conducted to evaluate amino acid utilization by cat tle. In Exp. 1, five steers (580 kg) were fed 86% rolled corn diets wi th mixtures of amino acids containing up to 6 g/d DL-Met, 24 g/d L-Lys , 6 g/d L-Thr, and 3 g/d L-Trp. Treatments had little effect on rumina l fermentation, diet digestibility, N flow to the duodenum, or microbi al efficiency. Ruminal concentrations of Met and Lys increased linearl y (P < .05) with amino acid supplementation, whereas Thr responded qua dratically, and Trp was not altered. In Exp. 2, four steers (414 kg) w ere used to measure effects of dietary monensin or laidlomycin propion ate in high-grain diets supplemented with amino acids. Ionophores had no significant effect on ruminal fermentation or outflows of amino aci ds from the rumen. In Exp. 3, 100 steers (287 kg initial BW) were fed diets containing 1% of a nonprotein N source. Treatments were 1) no su pplemental N (UREA), 2) UREA plus soybean meal (SBM), 3) UREA plus 2 g /d DL-Met, 8 g/d L-Lys, 2 g/d L-Thr, and 1 g/d L-Trp, or 4) UREA plus 4 g/d DL-Met, 16 g/d L-Lys, 4 g/d L-Thr, and 2 g/d L-Trp. During the g rowing period (diets based on whole-plant mile silage), gains were hig her for SBM-supplemented steers than for UREA steers and intermediate for steers supplemented with amino acids. Few significant differences in performance were observed among treatments during the finishing pha se (diets based on dry-rolled corn) or for the entire experiment, but cattle fed SBM or amino acids tended to be fatter and have better marb ling scores and quality grades. Amino acids did not greatly alter rumi nal fermentation or cattle performance.