Performance, body composition, carcass characteristics and culture water quality of Nile tilapia, Oreochromis niloticus (l.) fed low tannin sorghum
This study investigated the effects of substituting maize with low-tannin sorghum as feed energy ingredients on growth performance, body composition and carcass characteristics of Nile Tilapia and the effects of feeding sorghum on water quality. A total of 720 fingerlings of initial average weight of 3.85±0.074 g were used. Twelve 60-litre plastic basins were used as holding tanks with each basin receiving 60 fingerlings. The dietary treatments comprised of sorghum replacing maize at 100, 75, 50 and 0 percent. The treatments were assigned to the tanks in a completely randomized design with three replicates. The experiment ran for twelve weeks and feed was provided at 3% body weight. The fingerlings were weighed fortnightly to determine body weight. Initial body composition (moisture, protein, fat, ash and energy) was determined at the start of the experiment. Six fingerlings per replicate were sacrificed at the end of the experiment; 3 of which were for determination of carcass characteristics (weights of head, fins, scales, liver and flesh and skin with bones as percentage of the whole body) and the other 3 for determination of final body composition. Water pH, temperature and dissolved oxygen were determined daily using a water quality kit. The contents of ammonia, total phosphorus, nitrate and nitrite in water were analyzed weekly using spectrophotometry. Data were subjected to analysis of variance using the General Linear Model (GLM) of SAS. Polynomial contrasts were used to determine linear and quadratic effects of increasing levels of sorghum on all parameters investigated. Fingerlings attained an average mean weight gain (MWG) of 9.67±1.037 grams, feed conversion ratio (FCR) of 4.78±0.492; relative growth rate (RGR) of 250.21±22.789; specific growth rate (SGR) of 1.54±0.099 and condition factor (Cf) of 4.24±0.079. No significant differences were observed for mean MWG, FCR, RGR, or Cf (P > 0.05) when the levels of sorghum in the diets was increased. Average initial values of crude protein (CP), crude fat, and total ash were 546.88±1.501, 193.87±0.926 and 137.61±0.431 g/kg and final values were 542.77±1.115, 286.74±1.302 and 148.63±0.482 g/kg respectively. There were no differences (P > 0.05) in final body composition of the fish among the four treatments. Carcass characteristics, including percentage head, fin, liver and viscera weights did not differ with increasing dietary sorghum levels (P > 0.05). No significant differences were observed in morning or afternoon temperature, dissolved oxygen (DO) and pH. Similarly the amount of sorghum added had no effect on unionized ammonia, nitrate, nitrite or total phosphorus (P > 0.05) levels in the holding waters. The results imply that low tannin sorghum can serve as an alternative energy ingredient in fish feeds without compromising fish performance, body composition or carcass characteristics.