SULFUR AMINO-ACID UTILIZATION BY GROWING STEERS

Studies were conducted to quantify sulfur amino acid requirements of g rowing steers. In Exp. 1, six steers (160 kg) were used to determine t he methionine requirement in the presence of excess L-Cys. Treatments were abomasal infusion of water only or water plus 2, 4, 6, 8, or 10 g /d of L-Met. Steers were fed 2.4 kg/d DIM of a diet designed to minimi ze basal Met supply to the small intestine. Continuous ruminal infusio ns of VFA (506 g/d) and abomasal infusions of dextrose (150 g/d) incre ased energy supply to the steers. To prevent amino acids other than Me t from limiting performance, an amino acid mixture containing excess C ys was infused continuously into the abomasum. Break-point analysis es timated maximal N retention at 5.8 g/d supplemental L-Met. The basal a bsorbable Met supply was 2.1 g/d; therefore, the total Met requirement was 7.9 g/d when excess Cys was available. In Exp. 2, five steers (19 5 kg) were used to determine the efficiency of transsulfuration, Treat ments were abomasal infusion of water only or water plus 1.62 or 3.25 g/d L-Cys or 2 or 4 g/d L-Met. Diet and infusions were similar to thos e in Exp. 1, except the amino acid mixture was devoid of Cys and all s teers received 4 g/d L-Met to make total sulfur amino acids, but not n ecessarily Met, limiting. Nitrogen retention increased in response to Met but not Cys infusion and was maximized at 2 g/d supplemental L-Met ; thus, the total Met requirement was near 8.4 g/d when the Cys supply was 2.1 g/d. Supplemental Cys did not spare Met, suggesting that nonp rotein functions of Met may be quantitatively important.