Effects of replacing soybean meal with rubber seed meal on digestive enzyme activity, nutrient digestibility and retention in tilapia ( O reochromis niloticus × O reochromis aureus )

A feeding trial was conducted to evaluate the effects of replacing soybean meal ( SBM ) with rubber seed meal ( RSM ) on digestive enzyme activity, nutrient digestibility and retention in juvenile tilapia ( Oreochromis niloticus  ×  Oreochromis aureus ). Five isonitrogenous and isoenergetic diets were formulated with 0 (control), 100, 200, 300 and 400 g kg −1 RSM replacing graded levels of SBM respectively. Each diet was randomly assigned to triplicate groups of 30 fish (initial average weight 5.2 g) per aquarium in a rearing system maintained at 29 ± 1°C for 8 weeks. The hepatic protease and lipase activities gradually decreased with increasing dietary RSM level, but no significant differences were observed among the low inclusion level (0–200 g kg −1 ) groups. The apparent digestibility coefficients of dry matter, crude protein, crude lipid and ash showed a similar trend as the hepatic protease and lipase activities. The retentions of protein and individual essential amino acid (except lysine, threonine and leucine) in fish fed diet with 200 g kg −1 RSM were similar to those in fish fed the control diet. These results indicate that dietary RSM inclusion level up to 200 g kg −1 did not markedly affect the digestive enzyme activity, nutrient digestibility and retention in tilapia, whereas these were depressed by the inclusion of 400 g kg −1 RSM .