Effect of soy protein products and gum inclusion in feed on fecal particle size profile of rainbow trout

Authors: Welker, Thomas; Liu, Keshun; Overturf, Kenneth - Ken; Abernathy, Jason; Barrows, Frederic

Submitted to: Aquaculture Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/18/2021
Publication Date: 9/6/2021
Citation: Welker, T.L., Liu, K., Overturf, K.E., Abernathy, J.W., Barrows, F. 2021. Effect of soy protein products and gum inclusion in feed on fecal particle size profile of rainbow trout. Aquaculture. 1:14-25. https://doi.org/10.3390/aquacj1010003.
DOI: https://doi.org/10.3390/aquacj1010003

Interpretive Summary: Replacement of fishmeal (FM) with soybean meal (SBM) can cause a diarrhea-like condition in rainbow trout (RBT) characterized by very fine fecal particles. These fine particles do not settle out of rearing water and avoid collection from raceway effluent. Fecal fines can have a negative impact on the environment by increasing suspended and dissolved solids loads and levels of other regulated nutrients, such as phosphorus. Despite these potentially negative effects, no research has studied the effects of individual ingredients (protein source) on fecal quality. We conducted two experiments to evaluate the influence of different protein sources on fecal quality (fecal particle size formation). In the first study, common plant and animal protein sources were evaluated. Building on Experiment 1, addition of feed binders (guar and xanthan gums) to diets known to cause diarrhea (soy protein concentrate as the primary protein source) in rainbow trout were compared to fishmeal in practical feeds (Experiment 2). Fishmeal based diets produced the highest quality feces – high levels of large particles and low fines. When rainbow trout were fed diets high in soybean meal or soy protein concentrate, fecal quality was poor and characterized by high levels of fines and low levels of large particles. Guar gum but not xanthan gum reduced fecal fines with the percentage of mid and large size particles dependent upon the level of soy protein source used. Feeding a diet containing 20 or 30% SPC with guar gum resulted in the most manageable waste production: the highest percentage of large particles and lowest percentage of fines.

Technical Abstract: Replacement of fishmeal (FM) with alternative plant protein sources, especially soybean meal (SBM), can cause a diarrhea-like condition in rainbow trout (RBT) characterized by very fine fecal particles. These fines do not settle out in raceway effluent for collection and can contribute to pollution of receiving waters by adding dissolved solids and other nutrients. In this study, we conducted two experiments. Experiment 1 examined effects of nine protein sources [sardine meal, menhaden meal, three types of soy protein concentrates (SPC), regular SBM, high protein SBM, corn protein concentrate (CPC), and poultry by-product meal (PBM)] on fecal particle size. Results show that all five soy-based diets produced feces in RBT, having 75.7-89.3% fines and only about 1.0% large particles. while the remaining four diets gave feces having a balanced particle size distribution profile. High crude fiber content in soy-based diets was found to be the likely causative factor for unbalanced fecal particle distribution in RBT. Experiment 2 examined if improvements in formulation could reduce the negative effect of soy-based ingredients. Ten practical diets were formulated to contain 40% protein and 20% lipid (FM, SPC, SPC + 0.3% guar gum, SPC + 0.3% xanthan gum, PBM + CPC, PBM + CPC + 10, 20 or 30% SPC, PBM + CPC + + 20 or 30% SPC + 0.3% guar gum). Results show that diets containing PBM, CPC, and 20 or 30% SPC with guar gum produced trout feces with the highest percentage of large particles and lowest of fines, while the diet containing SPC and guar gum resulted in trout feces having the highest content of mid-size particles. It was concluded that crude fiber in SBM and SPC caused undesirable fecal particle profiles in RBT, and addition of guar gum but not xanthan gum could significantly alleviate this negative effect.