Effects of Feeding 17 α-Methyltestosterone and Withdrawal on Feed Utilization and Growth of Nile Tilapia, Oreochromis niloticus L., Fingerlings

Fingerling Nile tilapia, Oreochromis niloticus, were fed diets containing 17 a-methyltestosterone (MT) at doses of 0 (control), 2, 5, or 8 mg MT/kg diet for 20 weeks (Phase I), followed by an 8-week withdrawal period (Phase 11). At the end of Phase I, fish receiving 2 mg MT /kg diet exhibited significantly higher (P < 0.05) weight gain than fish fed the control diet. Likewise, food conversion ratio (FCR), protein efficiency ratio (PER), and productive protein value (PPV) of fish fed 2 mg MT/kg diet were significantly better than those of control fish (2.82 versus 3.40, 1.14 versus 0.94, and 18.15 versus 14.98, respectively). The anabolic property of the dietary steroids became more visible after 10 weeks. Increasing the dietary concentration of MT above 2 mgkg diet reduced fish growth and feed utilization efficiency, but not below that of control fed fish. The results of the present study indicated an increase in moisture, protein, fat, and ash deposition in fish body, with the decrease of MT levels in the diet. Fish receiving 2 mg MT/kg diet had greater retention values (by 47.7%, 44.9%, 20.8% and 35.1%) than did the control group for moisture, protein, fat, and ash, respectively. Hormone withdrawal reduced fish growth and feed efficiency values to below the level of control in all the treatments, and greater declines (P < 0.05) occurred for fish fed the diet containing 2 mg MT/kg. Hormone withdrawal negated MT stimulated increases in moisture protein, fat, and ash retention in fish body that occurred in Phase I. Results demonstrated that incorporation of MT into diets of Nile tilapia fingerling offers little potential for improving either growth or food utilization efficiency after hormone withdrawal if treatment and withdrawal period similar to those used in this study are employed.