Pretreatment of Soybean Meal with Phytase for Salmonid Diets to Reduce Phosphorus Concentrations in Hatchery Effluents

Abstract

Performances of rainbow trout (Oncorhynchus mykiss) fed experimental diets containing phytase-treated or untreated soybean meal with graded levels of supplemental phosphorus (P) were compared with the performance of rainbow trout fed a standard commercial feed. Growth rates and feed conversions (feed fed/weight gain) of fish fed diets containing phytase-treated soybean meal were equal to or significantly better (P < 0.05) than those of fish fed a commercial diet. Whole-carcass P concentrations of fish fed phytase-treated diets were not significantly different from those of fish fed commercial diets (P > 0.05). Pretreatment with the enzyme phytase increased P availability in soybean meal by hydrolyzing phytin P to an available inorganic form. Converted phytin P replaced supplemental P in the diet and was utilized effectively by rainbow trout. The lower dietary P levels resulted in a 65–88% reduction of P in the effluent. Juvenile fish (1.9 g/ fish, mean initial weight) fed a commercial feed released 4.54 g P/kg weight gain while fish fed a phytase-treated diet with no P supplementation released 1.61 g P/kg weight gain. Larger fish (16.97 g/fish, mean initial weight) fed a phytase-treated diet with no supplemental P released only 0.29 g P!kg weight gain to the effluent. Effluent P concentrations were determined by a modified mass balance strategy.