Improved fecal particle size profile in Rainbow Trout fed feeds containing different ratios of animal meal and plant protein concentrates: Effect on nitrogen and phosphorus partitioning

Objective: Our objective in this study was to evaluate the effects of fish meal replacement with plant and animal protein sources accompanied by a feed binder on nitrogen (N) and phosphorus (P) partitioning in Rainbow Trout Oncorhynchus mykiss feces.Methods: Three diets in which fish meal (FM; reference diet) was completely replaced with a combination of poultry meal (PM), corn protein concentrate, soy protein concentrate, and guar gum (binder) were evaluated to determine the portioning of N and P in fecal particle size-classes (fines: <0.6 mm; mid-size particles: 0.6-1.2 mm; large particles: >1.2 mm) and compared with a commercial feed for Rainbow Trout. Feces were evaluated for particle size composition and the content of total N (TN) and total P (TP) in each size-class.Result: There was a significant difference in fecal particle size composition for trout fed the experimental diets compared to those given the commercial feed. Fish that received the four experimental diets produced feces with approximately 43% large particles and 24% fines compared to 6% and 67% for trout that were fed the commercial diet. The TN in feces was significantly higher for trout that were fed the FM diet (3.84%) than for trout fed the FM-free diets, and TN also declined linearly (2.68, 1.91, and 1.59%) as the content of PM was replaced with plant-based protein. A diet x particle size interaction effect on fecal N content was also observed in which a significant increase in TN from large particles compared with fines occurred for the experimental FM diet, but there was little difference between size-classes for the non-FM-based feeds. The feces of trout fed the FM diet contained more TP (100,007 mu g/g) than the feces of fish receiving the FM-free diets (46,751-72,659 mu g/g), likely due to higher dietary P content of FM. However, TP concentrations followed a similar trend as TN in the FM-free feeds, declining as the proportion of plant protein sources increased (and as PM decreased). The large fecal particles (72,235 mu g/g) contained more TP than the mid-size particles (68,926 mu g/g) or fines (61,974 mu g/g), which could be due to leaching in the latter two particle size-classes, as the potential for solubility likely increased as particle size decreased.Conclusion: These findings may have important implications for improving the fecal particle size profile for sedimentation and removal of regulated nutrients through the formulation of Rainbow Trout feeds in replacement of FM with plant protein.