Determination of the methionine requirement of growing double-muscled Belgian Blue bulls with a three-step method

The three-step technique was used to determine the requirements of total am ino acids (TAA) and the first-limiting amino acid (AA) in growing double-mu scled Belgian Blue bulls (BBb). In Exp. 1, three double-muscled BBb weighin g initially 306 +/- 28 kg received a basal diet consisting of 30% meadow ha y and 70% concentrate that was poor in digestible protein but had adequate NE because of continuous infusion of dextrose into the duodenum. The intest inal apparent digestibility of essential AA (EAA) was defined according to their duodenal and ileal flows. It averaged 72% but varied between 60% for Met and 79% for Arg. In Exp. 2, five double-muscled BBb (334 +/- 22 kg) rec eived the same diet supplemented with duodenal infusions of dextrose and fo ur doses of Na-caseinate (28, 56, 84, and 112% of intestinal digestible die tary AA) in a 4 x 4 Latin square design with one additional animal. Nitroge n retention for the basal diet alone and the four increasing supplements of Na-caseinate reached 49, 61, 70, 80, and 86 g/d, respectively. Nitrogen ut ilization improved from 34.3% without Na-caseinate supplementation to a max imum of 40.6%, with the third dose supplying 788 g/d of apparently digestib le AA. Based on patterns of plasma concentrations, Met, Phe, and Arg were p robably the limiting AA when animals optimized N utilization. In Exp. 3, si x double-muscled BBb (315 +/- 25 kg) fed the basal diet received duodenal i nfusions of dextrose and AA, equivalent to the third dose in Exp. 2, except for digestible Met (9.3, 14.4, 18.4, 22.4, 26.4, and 30.4 g/d) in a 6 x 6 Latin square design. The Met requirement was close to 26.4 g/d on the basis of N retention.