Amino acid supplementation of low-protein sorghum-soybean meal diets for 20- to 50-kilogram swine.

Five growth experiments (28 or 35 d in duration) with growing pigs (24 kg initially) were conducted to assess the value of added threonine, tryptophan, methionine, and isoleucine in low-protein, lysine-fortified, sorghum-soybean meal diets. Basal 12, 13, and 14% CP diets were fortified with lysine.HCl to contain .62% digestible lysine, the lysine content of the 16% CP control diet included in all experiments. The additions of .12% threonine (.54 vs .42% dietary threonine) to the 12% CP diet in Exp. 1 improved (P < .01) ADG and gain/feed, but additions of .05% tryptophan or .10% methionine were without effect. Interactions of threonine, methionine, and tryptophan additions were nonsignificant (P > .30). Increasing lysine from .71 to .86% or increasing threonine from .54 to .65% in 12% CP diets of Exp. 2 improved (P < .07) gain/feed but did not affect ADG. Neither the addition of .05% isoleucine nor the addition .46% NaHCO3 to the 12% CP diet in Exp. 3 affected performance. All 12% CP diets in Exp. 1 to 3 resulted in performance below that obtained on the 16% CP diet. Addition of threonine to 13% CP (.47 vs .55% threonine, Exp. 4) or 14% CP (.51 vs .58% threonine, Exp. 5) diets tended (P < .12) to improve gain/feed but did not affect ADG. Performance on the 14% CP diet with added threonine and on the 16% CP diet was equivalent. These data suggest a minimum of 14% CP is needed in lysine-threonine fortified, sorghum-soybean meal diets of growing pigs for maximum performance.