SUBSTITUTION OF SOYBEAN MEAL WITH VELVET BEAN SEED MEAL IN THE AFRICAN CATFISH, Clarias gariepinus (BURCHELL 1822) DIETS

Abstract

This study investigated the potential of an alternative plant protein source as partial replacement for fish meal in the diets of the African catfish, Clarias gariepinus. The nutritive potentials of two varieties of Mucuna pruriens var. utilis - the ‘white’ Mucuna seeds and the ‘black’ Mucuna seeds were evaluated with respect to their potential as partial replacements for soybean meal, the conventional plant protein source, in practical diets of C. gariepinus. Matured black and white variety of Mucuna seeds were characterized with respect to their proximate composition, Calcium and Phosphorus contents. The results showed that the black variety of Mucuna seeds had higher crude fibre (5.25- 5.30: 3.9-3.92) and total ash (0.09-0.11: 0.08-0.09) contents relative to the white variety (either raw, cooked or autoclaved). Cooking or autoclaving improved the protein content while the crude lipid content decreased. Cooking or autoclaving the seeds had no effect on calcium and phosphorous contents. However, all the values obtained were within already established limits. The water quality parameters (temperature, pH, dissolved oxygen concentration) monitored during the feeding trials were within the acceptable range for the C. gariepinus. This contributed to the low mortality recorded during the feeding trial. Over 90% mortality was however recorded for C. gariepinus fed diets containing raw Mucuna seeds, which was attributed to the toxicity of the antinutrients in the raw seeds (both black and white varieties). When C. gariepinus was fed with the Mucuna seed meal-based diets, the best growth response by was achieved at 15% and 30% levels of inclusion while C. gariepinus fed diets containing 60% Mucuna seed meal dietary inclusion had the lowest and poorest growth response. However, there was no significant difference (P > 0.05) in the growth performance and nutrient utilization of C. gariepinus fed with diets containing the control and 15% Mucuna seed meal. With respect to total weight gain, C. gariepinus fed with the control diet (containing no Mucuna seed meal) performed better than diets containing the cooked or autoclaved Mucuna seed meals at the varying substitution levels. However, dietary inclusion of cooked or autoclaved Mucuna seed meal up to 30% substitution level in this study were not significantly different (P > 0.05) but were similar in all respects to the control diet. Growth depression was more pronounced in C. gariepinus fed with diets in which soybean meal was replaced with cooked or autoclaved Mucuna seed meals at 45% and 60%. The Specific Growth Rate (SGR), Feed Conversion Ratio (FCR), Protein Efficiency Ratio (PER) were similar (P > 0.05) to C. gariepinus fed with the control diet which was significantly different from the other diets. All the test diets elicited higher Erythrocyte Sedimentation Rate (ESR) values (P < 0.05) in C. gariepinus fed with the test diets than those fed the control diet. Packed cell volume (PCV) or Haematocrit (Hct) values were similar (P > 0.05) for C. gariepinus fed with the control and test diets. There were variations in the values for the other haematological parameters and no definite trends were observed. The histology of the liver and kidney tissues of C. gariepinus fed with diets containing Mucuna seed meals were not altered by soybean meal substitution up to 30% replacement level.