Utilization of digestible nitrogen and energy from four agricultural ingredients by juvenile silver perch Bidyanus bidyanus

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A comparative slaughter, growth assay was carried out using juvenile silver perch to evaluate different inclusion contents of peanut meal, canola meal, meat meal and dehulled field peas. Each ingredient was combined with a nutritionally balanced basal diet composed mainly of fishmeal (27%), soya bean meal (21%), wheat (28%) and sorghum (11%) such that between 15% and 75% of the basal diet was wholly replaced by the test ingredient. In addition, the basal diet was replaced with 15%, 30% or 45% of an inert filler (diatomaceous earth) in order to compare diets containing test ingredients and the inert filler. Fish were fed respective test diets twice a day for 56 days under a slightly restricted feeding regime (90% of apparent satiation) to negate any palatability problems. Weight gain of silver perch decreased steadily as the basal diet was systematically replaced with diatomaceous earth, confirming the limiting contribution to weight gain from the basal diet under a restricted feeding regime. Silver perch fed diets containing a mixture of the basal diet and either peanut meal, meat meal, canola meal or up to 60% field peas gained more weight than fish fed diets containing similar contents of the inert filler, indicating silver perch were able to utilize these ingredients to support growth. Regression analysis was applied to investigate protein and energy retention and models were fitted with 95% confidence and prediction intervals. Inspection of these relationships indicated various outliers which greatly affected the fitted models. We postulate that these outliers represent test diets which contain ingredients that are poorly utilized, or poorly utilized at particular inclusion contents. Removal of these outliers greatly improved the fit of each model. Using this approach, the predicted digestible protein (DP) content that gave maximum protein deposition in silver perch was 41.1%. The DP requirement for maintenance was 0.61 g DP kg BW )0.6 day )1 and the efficiency of DP for growth above maintenance was constant (0.45) after diets containing 45% or more of peanut meal and 75% of field peas were removed from the fitted model. The digestible energy (DE) requirement for maintenance was 36.79 kJ kg BW )0.6 day )1 and the efficiency of digestible energy for growth above maintenance was constant (0.68) after diets containing 75% of field peas and 75% of canola were removed from the fitted model. Adherence of other diets containing test ingredients to the slope of each regression suggests that silver perch are capable of utilizing any of the protein sources tested at all but the inclusion contents described above. Confirmation of this approach under different feeding regimes is required.