Supplementation of fishmeal and plant protein-based diets for rainbow trout with a mixture of sodium formate and butyrate

Two experiments were conducted to evaluate the effects of adding an organic acid salt blend (OAB, 10 g acid moiety kg−1 of a mixture of sodium formate and butyrate, ratio 2:1) and plant proteins (soybean meal and pea protein concentrate) to diets for rainbow trout (Oncorhynchus mykiss) reared in freshwater. The experiments were designed according to a 2 × 3 factorial design, where the factors were: two protein sources (fish or plant meals) and three methods of applying OAB to the diets (none, before, or after extrusion). In Exp. 1, duplicate groups of 0.95-kg trout were used to assess the effect of OAB on digestibility of nutrients. The supplementation of OAB before extrusion in the plant protein-based diet significantly reduced the digestibility of dry matter, organic matter, crude fat, and most amino acids. Supplementation of OAB after extrusion reduced the digestibility of crude fat both in the fishmeal and plant protein-based diets. Partly replacing fishmeal with plant proteins reduced the digestibility of dry matter, organic matter, crude fat, phosphorous and several amino acids. In Exp. 2, triplicate groups of 0.23-kg trout were used to evaluate the effect of OAB on growth performance and gut health. Supplementation of OAB in both fishmeal and plant protein-based diets before extrusion significantly increased feed conversion ratio, and middle intestine to body weight ratio. Partly replacing fishmeal with plant proteins significantly increased feed conversion ratio. There was no effect of dietary treatment on morphology in the distal intestine. In conclusion, supplementing diets with a sodium formate and butyrate blend did not improve growth rate or feed utilization of rainbow trout. Replacing 36% fishmeal with a combination of soybean meal and pea protein concentrate negatively influenced the nutrient digestibility and feed conversion ratio, but did not affect the growth rate of rainbow trout.