Effect of constant digestible protein intake and varying digestible energy levels on energy and protein utilization in Nile tilapia

In literature, the variability in the estimated optimal digestible protein to digestible energy ratio (DP/DE) is high. The present study aimed to estimate the optimal DP/DE ratio in Nile tilapia (Oreochromis niloticus) using different criteria (performance, energy and nitrogen balances parameters). Duplicate aquaria were randomly assigned to one of 16 diets. These diets covered a wide range of dietary DP/DE ratio (from 16.7 to 27 g MJ−1). DP levels ranged between 36 and 50% and DE levels between 17.5 and 22 MJ kg−1. Fish were fed restrictively based on a similar digestible protein amount at all 16 diets. Initial fish weight was 6.7 g. Broken line analysis showed that no optimal DP/DE ratio was present for Nile tilapia within the DP/DE ratio range studied. Regression analysis showed that growth declined as DP/DE ratio increased and seemed to level off at high DP/DE ratio (25 g MJ−1). FCR ranged between 0.8 and 1.1 and increased linearly with increasing DP/DE ratio. Decreasing the DP/DE ratio resulted in a linear increase in protein efficiency to a highest value of 53%. However, protein efficiency did not show a plateau or a maximum value. Moreover, decreasing the DP/DE ratio resulted in a very high fat content of the fish (over 16%). In conclusion, an optimal DP/DE ratio in Nile tilapia being fed restrictively seems to be absent or to be below 16 g MJ−1. A maximum protein deposition level is not present in 5-40 g Nile tilapia.