Influence of supplemental phytase on fish meal replacement by soybean and its effects on growth and body composition of rainbow trout (Oncorhynchus mykiss)

Influence of phytase enzyme and replacement of fishmeal protein by soybean meal protein on growth and body composition of rainbow trout (Oncorhynchus mykiss) specimens each with a mean weight of 121.75~c5.95g were studied. Experimental diets by factorial method 3*3, containing three levels of replacement (0.0, 35 and 70%) and three levels of microbial phytase (0.0, 1000, 2000 units/kg diet) were formulated and prepared. Nine treatments and three replicates of random groups of 20 fish per each 2 cubic meter (2*2*0.5m) tank were used in the study. All diets contained 36-38% protein and about 4000 cal/g energy. During the 9 weeks period of the study, fish were fed to satiation level, three times daily. Statistical results indicated that the increase of replacement level to 35% had no significant effect on growth index (P greater than or equal to 0.05). Effect of replacement on body composition was not significant, except for crude energy which decreased in 70% level (P≥0.05). Effect of phytase enzyme on growth index was positive and 2000 units/kg diet of phytase showed the best result (P≥0.05), but phytase levels had no significant effect on body composition. Result also showed that there was a significant interaction between phytase enzyme and meal rep lacement (P≥0.05). We suggest using 2000 units/kg diets of phytase with at least 35% replacement of fish meal protein by soybean meal protein as a suitable formula for growing rainbow trout