Effect of dietary zinc level on growth, enzyme activity and body trace elements of hybrid tilapia,Oreochromis niloticus × O. aureus, fed soya bean meal-based diets

Abstract

A feeding experiment was conducted to determine the dietary zinc (Zn) requirement of hybrid tilapia fed on a diet with soya bean meal as the sole protein source. The quantity of phytic acid in the experimental diet was 15.5 g kg−1. Juvenile hybrid tilapia were fed on diets containing 31–227 mg Zn kg−1 in triplicates for 6 weeks. Haematology of the fish was not affected by various dietary Zn levels. Fish fed on a diet containing 31 mg kg−1 endogenous Zn showed the lowest growth rates, feed utilization, and body and plasma Zn levels. Weight gain (WG), plasma Zn level and superoxide dismutase (SOD) activity increased when a higher quantity of dietary Zn of 127 mg kg−1 was administered to the experimental fish. Beyond this level, the values of these parameters were relatively stable. On the other hand, within the dietary Zn range tested, whole-body Zn and ash increased with higher dietary Zn levels. Analysis using a broken-line model showed that the dietary Zn requirements of hybrid tilapia fed on soya bean meal-based diets containing 15.5 g kg−1 endogenous phytic acid were 115, 115 and 105 mg kg−1 based on WG, whole-body Zn retention and plasma Zn level, respectively.