Dietary calcium, phytate and zinc interactions in channel catfish

Abstract

Interactions among dietary calcium, phytate and zinc were investigated by feeding eight purified egg-white diets containing 0.5 or 2.0% Ca, 0.5 or 1.5% phytate and 20 or 200 mg Zn/kg in a factorial arrangement to triplicate groups of fingerling channel catfish ( Ictalurus punctatus) in aquaria for 12 weeks. A significant ( P<0.01) effect of dietary calcium on performance was observed as catfish fed diets containing 2.0% Ca generally had reduced weight gain and feed efficiency values. The influences of the various dietary factors on zinc bioavailability were reflected in bone zinc concentration at the end of the feeding trial. In diets containing 20 mg Zn/kg, the higher (1.5%) phytate level alone caused a significant ( P<0.01) reduction in catfish bone zinc; whereas the higher (2.0%) calcium level alone did not result in reduced bone zinc. However, a combination of the higher phytate and calcium levels reduced catfish bone zinc to the lowest level. Catfish fed diets containing 200 mg Zn/kg had significantly higher bone zinc levels than those fed diets containing 20 mg Zn/kg. At the higher dietary zinc level, changes in calcium and phytate did not affect bone zinc levels since excess dietary zinc was available. Results from this study indicate that dietary phytate reduces zinc bioavailability to the channel catfish, and this effect can be exacerbated in the presence of high dietary calcium. Also, a supplemental zinc level of 200 mg Zn/kg diet provides sufficient available zinc to the channel catfish even when the diet contains high calcium and phytate concentrations.