Digestibility and postprandial ammonia excretion in Nile tilapia (Oreochromis niloticus) fed diets containing different oilseed by-products

The present study was undertaken to evaluate the potential for using oilseed by-products (soybean, copra and palm kernel meals) as partial replacements of fishmeal in feeds for Nile tilapia (Oreochromis niloticus). Nutrient digestibility and postprandial ammonia excretion rates were examined. A fishmeal-based diet served as control against three test diets in which 30 % of each of the oilseed by-products was included.

Diets were randomly assigned to triplicate groups of fish (similar to 1 kg bulk weight) for the digestibility trials which spanned a total of 9 days. The partial inclusion of oilseed meals did not significantly affect apparent protein digestibility, although lipid, ash and dry matter digestibilities were significantly affected (p <0.05).

Fish fed the soybean meal diets significantly reduced their feed intake and showed lower growth and feed utilization efficiencies over the trial period. The inclusions of the plant proteins caused a reduction in ammonia excretion rates with the palm kernel meal diet recording the lowest mean excretion rates of 117 mg kg(-1) day(-1) which was twofold lower than the highest mean daily ammonia excretion rate of the fish group fed the fishmeal-based control diets.

Overall, the study confirmed the potential of using copra and palm kernel meals to partially replace fishmeal in Nile tilapia diets based on their effects on short-term growth and feed utilization, nutrient digestibilities and lower ammonia excretion rates, while soybean meal in an unrefined form is not a promising replacement for fishmeal in tilapia diets.