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.
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.
Based on the digestibility data, nutrients from GDDY were well digested and absorbed by rainbow trout, yet feeding high levels of GDDY resulted in linear reductions in performance and poorer FCRs. However, these performance factors were not associated with reductions in palatability because fish consumed equivalent amounts of the diets that contained higher inclusion levels of GDDY.
Protein digestibilities among diets were essentially equal and all diets were balanced for available methionine, lysine, threonine and phosphorus. Hence, the reduced performance could not be attributed to any of these factors.
The factors contributing to the reduced fish performance when 34% (based on FCR) or 40% (based on growth) of the fish meal protein was replaced with GDDY protein are not clear at this point but may be attributable to another limiting amino acid or unknown interactive effect of the high nucleic acid or polysaccharide content of GDDY.
Further research is warranted to determine why fish performance decreased with higher inclusion levels of GDDY in spite of similar feed intake among levels.