Weight gain, feed conversion efficiency and plasma free lysine as response criteria in evaluating supplements of lysine plus threonine and lysine plus tryptophan to deficient diets for rats.

Two experiments were conducted on growing male SPF-rats to compare weight gain, feed conversion efficiency and plasma free lysine concentration as response criteria in evaluating adequacy of lysine plus threonine and lysine plus tryptophan supplements to the deficient diets. Two basal semisynthetic diets were prepared limiting in lysine and threonine (Expt. 1) and lysine and tryptophan (Expt. 2). The addition of graded supplements to the basal diets of L-lysine X HCl alone (0.2; 0.4; 0.6; 0.8 and 1.0% of diet) induced imbalance of amino acids resulting in low level of daily weight gain and feed conversion efficiency. Plasma free lysine concentration started to grow linearly from the first supplement of L-lysine X HCl. If rats were fed the diets containing identical supplements of L-lysine X HCl in combination with two supplements of L-threonine (0.2 and 0.4% of diet, Expt. 1) or L-tryptophan (0.05 and 0.1% of diet, Expt. 2), plasma free lysine started to increase before supplements of amino acids were adequate to support maximum weight gain and feed conversion efficiency. this difference in response seems to be caused by different feeding regiment during the growth period of the experiments (ad libitum) and training period prior to blood sampling (feeding twice daily).