Utilization of Australian grain legumes by salmonids

A large range of ingredients have been considered as potential replacements for fish meal but grain legumes have attracted most attention in the past. The main factors that limit the use of grain legumes in animal diets are the number of antinutritional factors (ANFs) that may adversely affect animal performance. Due to the high sensitivity of salmonids to ANFs and the importance of salmonid production in Australia, salmonids have been used as reliable biological tools for screening the most suitable Australian grain legumes. In this study, the four most abundant Australian grain legumes: chick peas, faba bean, field peas and lupin were targeted as potential alternatives to fish meal in salmonid diets. Chemical, biological and immunological measurements were used to assess their relative performance. The suitability of the four grains (raw and processed) was tested with rainbow trout and the effects of body weight (small, medium and large) and adaptation period on nutrient digestibility were evaluated. Maximum inclusion of the most suitable grain legume (lupin) was determined and the effects of exogenous enzymes on the nutrient utilization and fish performance were also investigated. The chemical composition and the digestibility of nutrients in lupin were most suited for rainbow trout compared with the other three grains. Also the concentrations of the two most important ANFs (trypsin inhibitor and phytic acid) were lowest in lupin. The concentration of trypsin inhibitor significantly decreased follwing the combination of both soaking and heating the grains. Fish body size did not affect nutrient digestibility of the grains. Over the adaptation period in vivo dry matter digestibility improved only for small fish size and in vivo crude protein digestibility increased for small and medium fish size. In addition, during the adaptation period in vivo crude protein digestibility was improved for the grains that contained higher trypsin inhibition (chick pea and field pea). There was a poor correlation relationship between in vitro and in vivo digestibility for processed grains, however there was a significant relationship for the raw grains. Lupin was selected as the most promising grain legume and the subject of further experiments. Due to the high concentration of neutral detergent fibre (NDF) and relatively low protein content of the whole grain, dehulling was applied to reduce the NDF and increase the protein content. A dose response experiment was conducted to establish the highest possible inclusion level of dehulled lupin (DL) in rainbow trout diets. Fish performed well at up to 40% inclusion of DL in the diet. However, growth and the energy efficiency ratio significantly decreased at 50% inclusion of DL. It was concluded that the fish were unable to effectively utilize the non-protein energy content of DL at this level of inclusion. Supplementation of the diets that contained 50% DL with different exogenous enzymes: EnergexTm, Biofeed plusTM and Alpha galactosidaseTm (separate or in a mixture) did not improve energy utilization by fish. Surprisingly, the growth performance of the fish that received 50% DL in the diet was marginally better than the group of fish that received a fish meal based diet. This was partly related to higher feed intake (ad lib vs set ration), but significantly better energy efficiency ratio suggested improved use of the carbohydrate fraction of the lupin. The DL that was used in the two experiments was from the same source but it had been stored under suitable conditions for about nine months. Having established the suitability of DL at 50% inclusion level in the rainbow trout diet, the potential of DL as a fish meal replacement was tested for Atlantic salmon. The effect of feeding at the most suitable time, based on the diel rhythm of feed intake on nutrient utilization and growth performance was tested when the fish received the diets that contained 30% DL. Fish were fed with the diets that contained 40% and 45% crude protein both in the morning and in the afternoon or diets that contained 40% crude protein in the morning and 45% crude protein in the afternoon (mixed diets) and vise versa. Results did not show any effect of feeding time on nutrient utilization. However, the growth of fish that received the mixed diets was comparable to groups that received 45% crude protein diets. The suitability of DL as a fish meal replacement for both rainbow trout and Atlantic salmon was shown in the current study. Additional studies are needed to improve the energy utilization and to minimize the dry matter waste of the diets that contain high levels of DL.