True Digestibility and Growth Response of Coated Essential Amino Acid Supplementation in Plant-Based Feeds for Rainbow Trout (Oncorhynchus mykiss)

In order to assess the efficacy of using essential amino acids to support the inclusion of high plant based proteins in diets for rainbow trout, a 56-day growth and feed utilisation study was conducted with juvenile (60g) rainbow trout (Oncorhynchus mykiss) under controlled conditions. Fish were fed diets where a predominately white fishmeal based control diet was compared to diets where full fat soyabean meal (FFS) as well as maize gluten meal (MGM) were incorporated respectively at significant levels (up to 75% of the fishmeal). Ten essential amino acids (EAAs) supplemented these diets in both standard and coated forms to evaluate their potential to rebalance the dietary EAA profiles more precisely. All diets were adjusted to be isonitrogenous and isolipidic. In addition to the growth trial, a low purified protein diet was fed to the fish separately (at a mean weight of 200g) to collect faeces for determining endogenous protein loss as well as the individual experimental diets described. As such it was possible to calculate the true digestibility for essential amino acids. The results indicated that corrective amino acid fortification of high plant diets, especially MGM could significantly (P<0.05) improve trout performance and raise their nutritional value. This was confirmed by final body weight, specific growth rate, feed conversion ratio and protein utilisation values. The true digestibility of the EAAs were similar and the values of each digestible EAA but coated EAAs were slightly superior in their digestibility coefficients. Coated EAAs improved digestibility coefficients by 2-5% and FCR by 8% compared to crystalline forms and also improved their utilisation leading to better trout performance with respect to growth, feed conversion efficiency and Nitrogen (crude protein) retention (P<0.05). The findings are discussed in the context of sustainable feeds and the circular bioeconomy for fish farming.