Feed enzymes have gained favor for use in animal feeds for improving phosphorus utilization, improving energy digestibility, as well as reducing waste loading into the environment. The utility of feed enzymes in aquafeeds has been increasing particularly for warm water species, but information is still limited for cold water, carnivorous species. Phytases are utilized to improve digestion of phosphorus bound as phytate-P in plan-based ingredients. Xylanases are utilized to digest xylan polysaccharide from hemicellulose. Cellulases are utilized to digest cellulose to glucose. Making these components bioavailable to rainbow trout could potentially improve both feed efficiencies as well as reduce fecal waste loading.
An experiment was performed to test the efficacy of phytase, xylanase and a blend of xylanases and cellulases in a plant-based rainbow trout feed compared to a commercial-style trout feed totaling nine diets. The positive control feed contained 45% crude protein and 15% crude fat from a blend of animal and plant sources with a target 0.54% digestible P. The negative control diet was formulated to target the same protein and fat levels but with 0.18% digestible P and 0.38% phytate-P. Phytase (ABVista Quantum Blue) was supplemented 2500 FTU/kg feed in diets 3-9. Diets 4, 5 and 6 were supplemented with graded levels of xylanase (ABVista EconaseXT) at 2000, 4000 and 8000 BXU/kg feed. Diet 7,8 and 9 were supplemented with NSPase blend of cellulase and xylanse (ABVista VistaPre-T) to target xylanase levels of 3500, 7000 and 14000 BXU/kg feed. Feeds were extruded, floating ~4mm pellets with feed enzymes top-coated prior to final oil top-coating. Fish were reared in 15 ⁰C in triplicate tanks of 20 fish weighing on average 26.1 g and fed to apparent satiation for 12 weeks.
Rainbow trout grew less when fed the negative control diet. Supplementation with phytase improved growth to levels attained with the positive control diet. Supplementing with phytase also improved P digestibility in the plant-based diets. Xylanase (EconaseXT) supplementation improved feed conversion ratios for fish fed the plant-based diet. Condition indices were largely unaffected by dietary treatment. Feed nutrient digestibility differed between diets with the positive control diet having higher dry matter, fat, and energy digestibility but lower protein digestibility than the plant-based diet. Fecal particle size differed between the positive control diet which was smaller for all size distributions compared to the plant-based diets. Feed enzyme addition did not greatly influence fecal particle size distributions.