Determination Of The Methionine Requirement Of Finishing Double-Muscled Belgian Blue Bulls With A Three-Step Method

A three-step technique was used to determine total amino acids (AA) and the first limiting AA requirements in finishing double-muscled Belgian Blue (dmBB) bulls. In a first experiment, three dmBB bulls (505 ± 21 kg) received a low metabolizable protein diet containing 25% meadow hay and 75% concentrate. Net energy supply was adequate for maximizing daily gains because of continuous infusion of dextrose into the duodenum. The intestinal apparent disappearance of essential AA (EAA) averaged 70.8% and was the lowest for histidine (61.3%) and the highest for arginine (79.9%). In a second experiment, four dmBB bulls (517 ± 16 kg) received the same diet supplemented with duodenal infusion of dextrose and four doses of Na-caseinate (17, 33, 50 and 66% of metabolizable dietary AA) in a 4 × 4 Latin square design. Nitrogen retention for the basal diet alone and the four increasing supplements of Na-caseinate averaged 61, 64, 74, 75 and 78 g d–1, respectively. A supply of 720 g d–1 of metabolizable AA was defined as optimising the N utilization for animal growth. Based on patterns of plasma concentrations, methionine and phenylalanine were probably the limiting AA. In a third experiment, five dmBB bulls (513 ± 60 kg) fed the basal diet received duodenal infusion of dextrose and AA, equivalent to the second dose in exp. 2 except for the supply of metabolizable methionine (12.8, 15.1, 17.6, 20.1, 22.6 and 25.1 g d–1) that varied in a 6 × 6 Latin square design with one missing animal. On the basis of N retention, the metabolizable methionine requirement was estimated to 22.8 g d–1 and corresponded to 360 mg of metabolizable methionine per gram of N retained.