Digestibility and haematological indices in Nile tilapia (Oreochromis niloticus) fed diets based on food processing byproducts and plant-derived ingredients

In order to determine the nutritional quality of food processing by-products and plant-derived ingredients in the diet of Nile tilapia (Oreochromis niloticus), six ingredients were studied for apparent digestibility and blood indices in 360 tilapia fingerlings (30.2 ± 1.54 g). Each experimental diet was prepared using 70% reference mash and 30% test ingredient, on an ‘as isÂ' basis. Ingredients assessed were fishmeal (Rastrineobola argentea) (reference diet, RD), kidney bean leaf meal (KBLM), spent brewer’s grain (SBG), spent brewer’s yeast (SBY), sweet potato leaf meal (SPLM) and wheat middlings (WM). In this six weeks experiment, apparent digestibility (AD) of diets and ingredients was assessed for crude protein (CP), crude lipid (CL), gross energy (GE) and dry matter (DM). Apparent digestibility of crude protein (ADCP, %) was highest for diet SPLM (83.1%) and SBG (83.0%), followed by RD (81.6%), SBY (77.7%), KBLM (73.1%) and WM (69.7%). Mean AD for indispensable amino acids (ADIAA), including lysine and methionine, followed a similar pattern, i.e. it was highest for diet SPLM (87.1%), followed by SBG (85.3%) and RD (fishmeal) (83.9%) and lowest in WM (72.9%). The four main haematological indices (white blood cell count (WBC), red blood cell count (RBC), haemoglobin (Hb), haemocrit (Hct)) differed somewhat between the diets. RD displayed lower RBC than diet SPLM. Diet SBY showed the lowest WBC and KBLM and WM the highest. Diet SPLM had high (p<0.05) RBC and Hb, whereas KBLM had the lowest values for RBC, Hct and Hb, indicating possible negative effects of kidney bean leaf meal on blood physiology in tilapia. Mean corpuscular volume (MCV), mean concentration of haemoglobin (MCH) and mean corpuscular haemoglobin concentration (MCHC) did not differ between the diets. Overall, the assessment of nutritional quality of the ingredients showed that SBY, SBG and SPLM had acceptable protein content, displayed high AD of nutrients and indispensable amino acids and revealed acceptable haematological indices in fish. Hence SBY, SBG and SPLM can be considered valuable ingredients in tilapia diets that may contribute to sustainable domestic fish production. Diets containing KBLM and WM displayed low digestibility and high WBC values, thus not recommended for tilapia diets.