Phosphorus Availability In Fish Diets

A feeding study was conducted to quantify the effects of phytase levels (0, 500, 1,000, 2,000, 4,000, and 8,000 units per kg diet) on the utilization of dietary protein and minerals by fingerling channel catfish fed a all-plant-protein diet composed of soybean meal, corn and wheat middlings. After fourteen weeks, average weight gains, feed conversion efficiencies, protein efficiency rations and dietary protein retentions were not significantly different among the groups. The addition of 1,000 U/kg phytase significantly increased the calcium, phosphorus, magnesium and manganese content of bone, relative to fish fed an unsupplemented diet and significantly increased the quantity of total phytate phosphorus in the feces. Adding phytase at the 8,000 U/kg level resulted in a significant increase in the bioavailability of naturally occurring zinc in feed ingredients, and increased the rate of phytate dephosphorylation in the stomach, compared with a diet with no added phytate. The authors concluded that the study demonstrates phytase additions increased the utilization of naturally occurring minerals in feed ingredients, reduces the need for mineral supplementation and decreases the amount of mineral excreted in the feces. The use of phytase in catfish feeds can therefore be expected to provide both economic and environmental benefits.

Yan, W.B., R.C. Reigh and Z.M. Xu. J. 2002. Effects of fungal phytase on utilization of dietary protein and mineral, dephosphorylation of phytic acid in the alimentary tract of channel catfish (Ictalurus punctatus) fed an all-plant-protein diet. World Aquaculture Soc. 33(1):10-22.