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Digestibility, feeding value and limiting amino acids in high-fibre and fibre-reduced sunflower cakes fed to tilapia (Oreochromis niloticus) Maina, Joyce Gichiku


Four experiments were conducted at the University of Nairobi, in Kenya, to determine the effect of reducing the amount of fibre in sunflower cake on nutrient digestibility and feed utilization in tilapia (O. niloticus), and to compare this low-fibre cake with a commercially available high-fibre sunflower cake. The extent to which protein from a high-fibre and a fibre-reduced sunflower cake could replace fishmeal protein in tilapia diets, and the effects of supplementing diets made from a low-fibre sunflower cake with amino acids lysine, methionine, and threonine on growth, feed intake, and feed utilization were also investigated. Also of interest was to compare digestibility and feeding value of Kenyan omena fishmeal with that of Low-Temperature (LT) anchovy fishmeal. Tilapia (O. niloticus) fingerlings were used in all the experiments. Water temperatures and dissolved oxygen concentrations were maintained above 26 °C and 5.5 mg/litre respectively. Dehulling of sunflower seeds was done using a manual dehuller. Crude fibre levels in the dehulled cakes were all below 15% (DM basis). Protein from the low-fibre and high-fibre sunflower cakes was well digested by tilapia. The apparent digestibilities of protein in the sunflower cakes and the fishmeals were not significantly different. Reduction of fibre in sunflower cake had no effect on the digestibility of protein. Digestibility of energy in the sunflower cakes was low. Apparent digestibility coefficient for energy (ADC-E) and digestible energy concentration (DE) were higher in the low-fibre sunflower cake than in the high fibre cake, but the differences were only significant for DE. There were no differences in the apparent digestibilities of protein, energy and organic matter between omena and anchovy fishmeals. In Experiment 2, the feeding value of a high-fibre and a low-fibre sunflower cake, omena and anchovy fishmeals was evaluated at two dietary protein levels (20% and 30%). There was no significant interaction between protein level and protein source. Fish fed at the 30% protein level gained more weight and had better feed conversion efficiency (FCE) than those fed at the 20% level. There were no significant differences in weight gain between fish fed diets based on anchovy and omena fishmeals and the low-fibre sunflower cake. Fish fed diets based on the high-fibre cake gained significantly (P < 0.05) less weight than those fed diets based on anchovy fishmeal. The low-fibre and high-fibre sunflower cakes were tested over a wide range of dietary inclusion in Experiment 3, each supplying 30%, 60%, and 80% of the dietary protein. The extent to which body fatty acids in tilapia reflect dietary fatty acids was also investigated. The low-fibre and high-fibre sunflower cakes could comprise up to 60% and 30% of the dietary protein respectively without compromising the performance of the fish. The inclusion of higher levels of the cakes in the diets caused a depression in feed intake, which resulted in lower weight gains of the fish fed these diets compared to those fed the control diet. Body fatty acid composition closely reflected dietary fatty acid composition. In Experiment 4, a basal diet in which a fibre-reduced sunflower cake provided 80% of the dietary protein was supplemented with amino acids lysine, methionine and threonine. The levels of these amino acids in the basal diet were 1.17%, 0.75% and 1.05% for lysine, methionine and threonine respectively, while the stipulated requirements (NRC, 1993) are 1.54%, 0.8% and 1.2% respectively. There was a trend to improved growth rate and FCE in fish fed diets supplemented with lysine and threonine, but the improvement did not attain statistical significance. Methionine, added alone or together with threonine did not elicit any response in fish.

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