Effects of feeding frequency and rate of stomach evacuation on utilization of dietary free and protein-bound lysine for growth by channel catfish Ictalurus punctatus

Previous research has demonstrated that free lysine is utilized less efficiently for growth than protein-bound lysine in practical diets by young channel catfish Ictalurus punctatus. Two experiments were conducted in an attempt to explain this. In experiment 1, diets based on soybean meal or peanut meal-corn gluten meal supplemented with L-lysine HCl were fed to channel catfish (1.7 g average weight) either two or five times daily. The diet containing protein-bound lysine from soybean meal allowed greater weight gains than the diet supplemented with free lysine, and fish fed five times daily grew faster than fish fed twice daily. The interaction between lysine source and feeding frequency was not significant, indicating that feeding frequency did not affect the utilization of free lysine relative to that of protein-bound lysine. In experiment 2, ingesta were collected from the stomachs of fish at various times after feeding. The ratio of lysine to chromic oxide in ingesta from fish fed the free lysine-supplemented diet decreased with time, whereas no change occurred in this ratio in fish fed the protein-bound lysine diet. This indicates that free lysine passed from the stomach more rapidly than protein-bound lysine and suggests that free lysine would be absorbed before protein-bound lysine.