Abstract The research studied the effect of phytase to diets based on full fat soyabean and groundnut meal. Phytase addition significantly reduced phytate content in diets based on 25%, 50%, and 100% soya bean meal (P<0.05). Phytate hydrolysis was more effective with significant reduction achieved at 250 FTU/g compared to diets without phytase (Tukey, P<0.05). Significant interaction was observed for nutrient digestibility (factorial, P<0.05). Crude protein digestibility showed significant improvement with soyabean and phytase (Tukey, P<0.05). Regardless of phytase level, increasing soyabean level from 50% to 100% significantly improved digestibility of fat in the diet (Tukey, P<0.05). Phytase at 250 FTU/g showed the highest fat digestibility compared to other diets, including control (Tukey, P<0.05). Phosphorus digestibility was significantly improved with phytase at 1000 FTU/g compared to other phytase diet (Tukey, P<0.05). Weight gain declined with increasing substitution of fish meal by soya bean (Duncan, P<0.05), and irrespective of phytase levels (Tukey, P<0.05). However, significant improvement in weight gain of fish was observed with phytase addition to diets compared to diets without phytase (Tukey, P<0.05). Regardless of soyabean level, phytase at 250 FTU/g (P1) showed higher growth performance (Tukey, P>0.05) compared to 0 FTU/g (P0), 750 FTU/g (P3) and 1000 FTU/g (P4). In experiment 2, phytase effectively degraded phytate in groundnut meal with most phytate reduction (Tukey, P<0.05) achieved with phytase at 250 FTU/g compared to control and other phytase diet (Tukey, P<0.05). Phytase supplementation significantly improved crude protein, energy and lipid digestibility (Tukey, P<0.05). Phosphorus digestibility showed improvement with phytase addition compared to diet without phytase (Tukey, P>0.05). Phytate degradation improved growth performance of fish with phytase at 250 FTU/g compared to diets without phytase (Tukey, P<0.05). In conclusion, improvement in phosphorus utilization by phytase improved nutrient availability and growth performance of fish fed both diets.

Keywords: Phytase, Nutrient availability, soyabean meal, groundnut meal, Growth, Clarias gariepinus