Evaluation of grain distillers dried yeast as a fish meal substitute in practical-type diets of juvenile rainbow trout, Oncorhynchus mykiss

•Defined suitable inclusion levels of grain distillers dried yeast for rainbow trout•Quantified nutrient availability of grain distillers dried yeast for rainbow trout•Demonstrated that replacing fish meal with GDDY improves diet phosphorus availability

Grain distillers dried yeast (GDDY) is a single-cell protein obtained as a co-product during the production of fuel ethanol that may have potential as a protein replacement for rainbow trout. The goal of this study was to examine the suitability of GDDY as a replacement for fish meal on a digestible protein basis in rainbow trout diets. An in-vivo digestibility study was performed to determine the nutrient availability of GDDY. Subsequently, a control diet containing 42% digestible protein and 20% lipid was formulated to replace fish meal protein with GDDY protein at eight different levels (0, 25, 37.5, 50, 62.5, 75, 87.5, and 100%). Diets were fed to juvenile rainbow trout stocked into four replicate tanks per dietary treatment (30 fish/tank) and fed twice daily for nine weeks. High GDDY inclusion rates significantly altered rainbow trout growth and feed conversion but not feed intake. There were no significant differences in production performance in fish fed the 25% GDDY and 37.5% GDDY diets when compared to fish fed the control diet, but further dietary fish meal replacement generally decreased fish performance. Further research is warranted to determine why fish performance decreased with higher inclusion levels of GDDY in spite of similar feed intake among levels.