Evaluation of the nutritional value of ethanol yeast in practical-type diets as an alternative protein source for rainbow trout Oncorhynchus mykiss

Abstract

Ethanol yeast (EY) is a single-cell protein obtained as a co-product during the production of fuel ethanol and may have potential as an alternative protein source for rainbow trout. The objective of the current study was to determine if EY could replace fish meal (FM) without negatively impacting growth performance of juvenile rainbow trout. Three experiments were conducted to evaluate the use of EY. In Exp. 1 a digestibility trial was done to determine EY apparent digestibility coefficients (ADCs) for protein, lipid, energy, DM, and apparent availability coefficients (AACs) for amino acids. In Exp. 2 a feeding trial was conducted where a control diet (42% digestible protein and 20% crude lipid) was compared to diets where FM digestible protein was replaced by EY at varying levels (25, 37.5, 50, 62.5, 75, 87.5, and 100%). Diets were fed twice daily to rainbow trout to apparent satiation in a 15°C recirculating system. There were 4 replicate tanks per diet (30 fish/tank). Experiment 3 was conducted to determine if a mycotoxin inhibitor (Biofix Plus) could improve performance of rainbow trout when fed higher levels of EY. The experiment was a 2x3 factorial where FM was replaced with EY (0, 50 and 100%) with or without Biofix Plus. There were three replicate tanks per diet (15 fish/tank). Results from Exp. 1 showed that Ethanol yeast ADCs for protein, DM, lipid, energy and AAC for and sum of amino acids were quantified at 98, 65, 100, 70, 81 and 81%, respectively. Results from Exp. 2 showed that fish growth was not different from the control diet at the 25% and 37.5% replacement levels. However, reduced growth (P < 0.001) and poorer feed conversion (P < 0.001) were measured when EY replaced more than 37.5% of dietary FM (11.2% EY inclusion). Results from Exp. 3 found no effect of Biofix Plus on performance of rainbow trout. There was reduced growth (P=0.001) in the 50 and 100% replacement diets. Apparent digestibility coefficients suggested that EY nutrients were highly digestible. However, growth was reduced at EY inclusion levels that were greater than 11.2%.