Soybean antinutritional factors and their relative importance in limiting the use of soybean meal in salmonid diets
Historical inclusion rates of fish meal in aquaculture feeds are unsustainable because of limited supply and increasing costs. Alternative plant protein sources must be identified. The goal of this project was to systematically examine the antinutritional factors in soybean meal and their effects on rainbow trout and Atlantic salmon. A Managed Research Program, funded by the United Soybean Board, Indiana Soybean Alliance, Illinois Soybean Association, Iowa Soybean Association and Ohio Soybean Council, was established that included participants from seven US institutions. The University of Idaho, in collaboration with the US Fish and Wildlife Service Fish Technology Center, and Michigan State University evaluated the effects of trypsin inhibitors (TI) in both species and the effects of extrusion conditions on TI activity. Purdue University evaluated the effects of soybean lectins in diets fed to both species and the interactive effects of feeding lectins, trypsin inhibitors and oligosaccharides to rainbow trout. The Ohio State University examined the effects of soy saponins on rainbow trout. The Universities of Maine and Wisconsin evaluated the effects of soybean isoflavones, with emphasis on genistein, on rainbow trout and Atlantic salmon. The University of Maine also conducted taste tests of trout and salmon to characterize flavors of salmonids fed soy-based diets. Kentucky State University conducted an economic analysis on feeding high soybean diets to salmonids in order to develop least cost diet formulations utilizing the highest possible levels of soybean meal. The approach undertaken was a relatively complete series of studies that described the effects of soybean antinutritional factors on rainbow trout and Atlantic salmon.
The key findings of the Soy-in-Aquaculture Program indicated which particular soybean antinutritional factors have been limiting the amount of soybean meal fed to salmonids. Lectins and trypsin inhibitors were both identified as exerting antinutritional effects on rainbow trout and Atlantic salmon when included in the diet at levels corresponding to a 35-40% soybean meal diet. In a study with rainbow trout, feeding trypsin inhibitors resulted in a 12% reduction in growth and feeding lectins resulted in a 14% reduction in growth. Saponins did not exert antinutritional effects when fed to rainbow trout, and were shown to have a potential benefit by enhancing the immune response. There were no observed negative effects (growth or reproduction) as a result of feeding soy genistein to rainbow trout; however, when fed to Atlantic salmon, smoltification was inhibited. Fillets from fish fed soybean meal were lighter in color, but there were no detectable differences in flavor when sampled by an untrained panel. Additional findings of the Soy-in-Aquaculture Program indicated that a higher extruder barrel temperature and shorter retention time may be the optimum settings when extruding high-soybean meal diets for salmonids. Finally, using least-cost modeling, soy inclusion rates were calculated to be 15 and 25% in Atlantic salmon and rainbow trout diets, respectively. The results of these studies identified specific antinutritional factors limiting the amount of soy ingredients used in salmonid diets. These results show farmers and processors which antinutritional factors need to be reduced in soy ingredients.