Growth and Residue Studies in Young Chickens Fed l-O-Ethylthreonine

A series of experiments was conducted to measure the effect of feeding l -O-ethylthreonine ( l -OET) on 1) the growth of young chickens, 2) the relationship of OET to isoleucine in the chicken and 3) the disposition of 14 C-labeled l -OET in chicken tissues. The maximum level of l -OET that could be fed to chickens without depressing growth was found to be 0.018% in the diet. Feeding levels of 0.05% resulted in a growth depression of approximately 40% and an apparent neurological disorder that was reversed by single oral doses of isoleucine or by supplementing the diet with 1.0% isoleucine. Supplementing a diet containing 0.05% l -OET with 0, 0.25%, 0.5%, 1% and 2% isoleucine resulted in growth depressions of 46%, 30%, 17%, 9% and 16%, respectively. The increased growth depression observed at the upper level of isoleucine supplementation was attributed to an imbalance of amino acids rather than an effect of l -OET. The studies with 14 C-labeled l -OET demonstrated that the radioactivity and probably the intact OET molecule were rapidly and firmly incorporated into the tissue proteins. Less than 0.2% of a single oral dose of 14 C-labeled l -OET was expired as CO2 and not more than 35% of the administered dose could be accounted for in the excreta over a six-day period. Tissue residue analyses demonstrated that the majority of 14 C was associated with the muscle protein.