Limiting amino acids for growing Holstein steers limit-fed soybean hull-based diets

Studies were conducted to determine limiting amino acids (AA) for cattle li mit-fed soybean hull-based diets. Ruminally cannulated Holstein steers were maintained in metabolism crates, fed the same basal diet (73% soyhulls, 19 % alfalfa, DM basis), and given the same intraruminal infusions (400 g/d ac etate; to supply energy without increasing microbial protein supply). Treat ments were infused abomasally. In Exp. 1, steers (200 kg) were provided 1) water, 2) 10 gld of methionine (MET), or 3) a mixture of 10 essential AA (1 0AA). Nitrogen retention (13.7 g/d) was greatest (P < .05) for steers recei ving 10AA. Steers receiving MET (7.9 g/d) had greater (P < .05) N retention than control steers (5.4 g/d). In Exp. 2, steers (200 kg) were provided 10 AA or 10AA with L-Lys deleted from the mixture. Steers receiving 10AA tende d (P < .09) to have greater N retention (19.0 g/d) than those receiving no lysine (16.3 gld). In Exp. 3, steers (194 kg) were provided 10AA or 10AA wi th L-Thr deleted from the mixture. Nitrogen retention was not affected by r emoval of threonine. In Exp. 4, steers (152 kg) were provided 10AA or 10AA with L-His, L-Trp, L-Arg, L-Phe, or branched-chain AA (L-Leu, L-Ile, and L- Val) removed. Nitrogen retention was reduced (P < .05) by removal of either L-His or the branched-chain AA. For steers limit-fed soybean hull-based di ets, methionine was first-limiting; histidine, at least one of the branched -chain AA, and possibly lysine were also limiting.