METABOLIZABLE PROTEIN AND AMINO-ACID-REQUIREMENTS OF GROWING CATTLE

Metabolizable protein and amino acid requirements for growing cattle w ere estimated using data from 11 research trials. A total of 543 steer s were individually fed a high-roughage diet supplemented with protein at several levels above a urea supplement control. The mean weight fo r all animals was 253 kg, with a range in mean initial to final weight s of 200 to 316 kg, respectively. Daily gain ranged from -.04 to .89 k g. Metabolizable protein for each treatment group was calculated at th e point at which the protein requirement was met. The sum of dietary e scape protein (basal and supplemental) and calculated microbial protei n represented metabolizable protein supplied per test protein source a nalyzed in each trial. Daily gain was regressed against calculated met abolizable protein flow using weighted regression analysis (r2 = .69, n = 45) to determine the metabolizable protein requirements for mainte nance (3.8 x BW.75 g/d, where BW is expressed in kilograms) and growth (305 g/kg of live weight gain). Calculated metabolizable amino acid r equirements as a percentage of metabolizable protein for a 253-kg anim al gaining .49 kg/d were as follows: methionine, 3.0%; total sulfur am ino acids, 5.8%; lysine, 8.0%; tryptophan, 1.0%; threonine, 5.2%; vali ne, 5.7%; isoleucine, 5.6%; leucine, 6.9%; phenylalanine, 3.9%; and hi stidine, 1.6%. The proposed requirements were based on live animal gai n and intake of metabolizable protein and should represent the needs o f the growing beef animal.