Effects of phytase pretreatment of soybean meal and phytase-sprayed in diets on growth, apparent digestibility coefficient and nutrient excretion of rainbow trout (Oncorhynchus mykiss Walbaum)

Rainbow trout were fed a diet containing phytase-sprayed and phytase-pretreated soybean meal with different phytase levels. The single factor random block design was used to analyze the effects on rainbow trout of dietary phytase supplementation on growth performance, nutritional ingredient digestibility and nutrient excretion. After 90 days, the results showed that feed conversion ratio (FCR) and protein efficiency ratio (PER) were significantly improved and specific growth rate (SGR) was not affected by spraying phytase, but SGR, FCR and PER were not significantly improved by phytase pretreatment. A digestibility trial conducted after the feeding trial showed that apparent digestibility coefficient (ADC) of diet protein and minerals was increased with phytase supplementation. However, there was a negative effect of phytase on the ADC of lipid. The excretion experiment showed that the supplementation of phytase resulted in decreased nutrient excretion in feces, but lipid excretion was slightly increased with phytase supplementation. In addition, the results of P excretion and ADC of P analyzed by t-test showed that phytase pre-treatment method should be a more rational method than the spraying method. The results of SGR, ADC of P and P excretion analyzed by quadratic regression indicated that 2,000–3,000 U/kg levels by the spraying method could be a rational range of phytase supplementation, and about 1,000 U/kg should be an optimal level by the pretreatment method. Thus, use of phytase in rainbow trout feeds can have economic and environmental benefits.