Free lysine (L-lysine(.)HCl) is utilized for growth less efficiently than protein-bound lysine (soybean meal) in practical diets by young channel catfish (Ictalurus punctatus)

Two 10-week feeding experiments were conducted with channel catfish fingerlings in aquaria to compare the efficiency of utilization for growth of free versus protein bound lysine in practical diets. In experiment 1, a basal, 26% protein diet, deficient only in lysine, was formulated with a combination of peanut meal and corn gluten meal as the primary protein source. The diet was then supplemented with graded levels of lysine by either adding L-lysine(.)HCl or substituting soybean meal for the peanut meal-corn gluten meal combination to produce two series of diets with increasing concentrations of either free lysine or protein-bound lysine. In experiment 2, the basal diet contained sesame meal as the primary protein source and was supplemented with free or protein-bound lysine as in experiment 1. Slope ratio analysis of response data showed that the efficiency of utilization of protein-bound lysine in soybean meal relative to free lysine from lysine-HCl was 196 and 163% for experiments 1 and 2, respectively, based on weight gain. A subsequent feeding experiment was conducted in which the peanut meal-corn gluten meal basal diet was supplemented with lysine alone or with lysine plus histidine, isoleucine, threonine, and tryptophan to match the essential amino acid composition of the diet in which soybean meal provided the protein. Supplementation of the basal diet with the additional amino acids did not improve fish weight gain relative to that from supplementation with lysine alone. Overfortification of the basal diet with free lysine improved weight gain of fish to equal that obtained with the diet containing soybean meal. Loss of lysine due to leaching of diets in water was 12.7 and 2.0% of the original lysine concentration for free lysine and protein-bound lysine, respectively. When leaching losses are subtracted from the lysine concentrations in the test diets, the efficiency of utilization of protein-bound lysine is still higher than that of free lysine, 175 and 146%, based on weight gain, for experiments 1 and 2, respectively. These results indicate that protein-bound lysine is more efficiently utilized than free lysine in practical type diets for young channel catfish.