Replacement of fish meal in diets for Australian silver perch, Bidyanus bidyanus: IV. Effects of dehulling and protein concentration on digestibility of grain legumes

Abstract

Three digestibility experiments were conducted using juvenile silver perch. The first factorial experiment evaluated four ingredients; field peas ( Pisum sativum), faba beans ( Vicia faba), chick peas ( Cicer arietinum) and vetch ( Vicia sativa) with and without hulls. The second and third experiments determined digestibility of field pea, faba bean and lupin protein concentrates. Apparent digestibility coefficients (ADCs) were determined using indirect procedures. Test diets comprised 29.7% of the test ingredient, 69.3% of a practical reference diet and 1.0% chromic oxide as the inert indicator. Faeces were collected from fish by passive settlement techniques. Two-factor ANOVA of the data from the first experiment revealed a significant interaction between ingredient type and the presence/absence of hulls for dry matter, protein and energy digestibility. Dry matter and energy ADC's for whole grains ranged from 43–58% and 51–59%, respectively. Dehulling significantly improved the dry matter and energy digestibility of all grains except faba beans. Dehulled vetch had the highest dry matter (81%) and energy (81%) digestibility of all grains, but diets containing 30% vetch were poorly accepted by silver perch. Protein digestibility for all legumes was high, ranging from 71% to 91% for whole grains and 80–97% for dehulled grains. Protein digestibility was significantly improved by dehulling field peas, faba beans and vetch, but dehulling did not affect protein digestibility of chick peas. Dry matter and energy ADC's for protein concentrates were much higher than either ADC's for whole or dehulled grains. Protein digestibility was highest for the field pea concentrate (97%), but protein ADCs for concentrates of faba beans and lupins were of a similar value to that of their respective dehulled grains. Silver perch have demonstrated they can efficiently digest the protein from the dehulled and refined legumes evaluated in this study. Further, both dry matter and energy digestibility of most grains was dramatically improved after dehulling or protein concentration, indicating processes such as these are highly desirable for legumes prior to their inclusion in diets for silver perch. The cost effectiveness of using protein concentrates in aquaculture diets requires further evaluation. Of the grain legumes investigated in this study, dehulled field peas appear the most promising for juvenile silver perch.