Utilization of free versus protein-bound lysine by channel catfish in production ponds

Second-year channel catfish, Ictalurus punctatus, were stocked intensively in production ponds and fed for 14 weeks to compare the bioefficacy of crystalline versus protein-bound lysine as supplements to a lysine-deficient, all-plant commercial diet. A lysine-deficient basal diet, in which a combination of peanut meal, cottonseed meal and corn gluten meal was the primary protein source was supplemented with graded levels of lysine by either adding crystalline L-lysine HC1 or by replacing increments of the peanut meal, cotton seed meal and corn gluten meal mixture with soybean meal. Supplementation of the basal diet with free lysine or protein-bound lysine from soybean meal significantly improved the growth of channel catfish. Supplementation of lysine in protein-bound form allowed for significantly higher weight gain than supplementing with crystalline lysine. Slope ratio analysis of response data showed that the efficiency of utilization of protein-bound lysine relative to free lysine was 219%, based on weight gain. Dress-out percentage and fillet yield were significantly lower in fish fed the basal diet but improved with the lysine supplementation. Economic analysis showed that supplementation of protein-bound lysine from soybean meal allowed greater return above feed cost than supplementation with free lysine from lysine HCl. In a subsequent experiment, second year channel catfish were fed the basal diet supplemented with free lysine or the soybean meal diet, each supplemented with chromic oxide (Cr2O3), in fiberglass circular tanks to compare the rate of stomach excretion of lysine in free or protein-bound form. Rate of change in the ratio of lysine to chromic oxide in stomach contents over time was the criterion for evaluation. The ratio of lysine to chromic oxide decreased faster for fish fed free-lysine than for fish fed protein-bound lysine. This indicated that lysine in free form leaves the stomach faster than protein-bound lysine which may be the reason that free lysine is used less efficiently than protein-bound lysine.