Digestibility and Growth Response of Rainbow Trout (Oncorhynchus mykiss) Fed Coated and Non-Coated Crystalline Essential Amino Acids as Supplements in Plant-Based Feeds

Abstract

The efficacy of using essential amino acids to support the inclusion of high plant based proteins in diets for rainbow trout was undertaken over a 56-day growth and feed utilisation study with juvenile (60g) rainbow trout (Oncorhynchus mykiss). Fish were fed diets where a predominantly 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) were supplemented in 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 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 control (animal protein blend/fishmeal protein) reference diet fed trout (Ref) gave superior and significant (P<0.05) growth performance with a 146% overall increase in body weight. Conversely, FFS (Full Fat Soyabean meal) fed trout was the most inferior performing group amongst the experimental diets, with only a 78% increase in weight gain over the same period. Supplementation of this basic formulation with a mixture of essential amino acids in crystalline form resulted in a moderate but significant (P<0.05) improvement in overall growth performance (86.80%) and, in particular for MGM+EAAS (Maize Gluten and Essential Amino Acid, uncoated) fed trout when in protected coated form, yielding an SGR of 125% (P<0.05). The true digestibility of the EAAs was similar to 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.