Effect of lysine and threonine supplementation of sorghum gestation diets on nitrogen balance and plasma constituents in first-litter gilts.

Fifteen gravid, crossbred gilts were utilized in an experiment to measure the effect of five isonitrogenous dietary treatments (12% crude protein, sorghum-soybean meal; sorghum; sorghum + lysine; sorghum + threonine; sorghum + lysine + threonine) on N balance, colostrum composition and plasma constituents (protein, urea N and IgG). All gilts were fed a standard corn-soybean meal diet from breeding until switched to the sorghum basal diet, on d 60 of gestation. Treatment diets were imposed at d 70. Nitrogen retention, measured between d 103 and 113 of gestation, was improved (P less than .002) by lysine addition. Nitrogen retention was similar (P greater than .10) for gilts fed either the 12% crude protein, sorghum-soybean meal diet or the lysine + threonine-supplemented diet. Plasma protein levels at farrowing were depressed (P less than .06) in those gilts fed sorghum diets without supplemental lysine and plasma urea N was elevated (P less than .10). Plasma protein and plasma urea N were not affected by threonine supplementation (P greater than .10). In contrast, the depression of plasma IgG was ameliorated (P less than .03) by threonine and unaffected by lysine supplementation. These results confirm the hypothesis that lysine is the first limiting amino acid in sorghum protein for gravid gilts as determined by N retention but indicate that threonine is likely the first limiting amino acid for the production of the specific protein, IgG.