Impact of dietary phytase on tilapia performance and biofloc water quality

Authors: Green, Bartholomew - Bart; Rawles, Steven - Steve; Schrader, Kevin; McEntire, Matthew - Matt; Abernathy, Jason; Ray, Candis; GAYLORD, T. - Us Fish And Wildlife Service; Lange, Miles; Webster, Carl

Submitted to: Aquaculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/27/2021
Publication Date: 4/29/2021
Citation: Green, B.W., Rawles, S.D., Schrader, K., Mcentire, M.E., Abernathy, J.W., Ray, C.L., Gaylord, T.G., Lange, M.D., Webster, C.D. 2021. Impact of dietary phytase on tilapia performance and biofloc water quality. Aquaculture. 541:1-11. https://doi.org/10.1016/j.aquaculture.2021.736845.
DOI: https://doi.org/10.1016/j.aquaculture.2021.736845

Interpretive Summary: Fish feed typically is supplemented with inorganic phosphorus to ensure adequate dietary phosphorus availability, especially in diets that contain high percentages of plant feedstuffs, which is the case for diets formulated for tilapia. Up to 80% of phosphorus present in plant feedstuffs is present as phytate, which is not bioavailable to fish and other monogastric vertebrates because their digestive system lacks enough phytase enzyme to free phosphorus from phytate. Thus, phytate is excreted unmetabolized and accumulates to high concentrations in the biofloc technology production system because of the high quantities of feed fed daily to fish. ARS researchers at Stuttgart, Arkansas, evaluated the fate of dietary phosphorus when hybrid tilapia were grown for 5 months to market size in outdoor biofloc production system tanks on three commercially extruded diets that included a diet with or without inorganic phosphorus supplementation and a diet without inorganic phosphorus but treated with the phytase enzyme. Supplementing the phosphorus-free diet with the phytase enzyme substituted for inclusion of inorganic phosphorus with no significant impact on tilapia growth and yield in the biofloc production system when compared to the diet supplemented with inorganic phosphorus. Furthermore, the environmental phosphorus loading from formulated feed was 38% lower when phytase substitutes for inorganic phosphorus. These results will aid feed manufacturers refine diet formulations for fish allowing greater inclusion of plant proteins and resulting in more environmentally sustainable formulated feeds.

Technical Abstract: We evaluated the fate of dietary phosphorus when hybrid tilapia (Oreochromis aureus x O. niloticus; 29.8 g initial weight) were stocked at 29 fish/m3 and grown for 5 months to market size (454 g/fish) in outdoor biofloc production system tanks (16.6 m3) on three commercially extruded diets that included a diet without inorganic phosphorus supplementation (P-Free), the P-Free diet treated with 6-phytase (3,000 FTU/kg; Quantum Blue™; P-Free + Phytase), and the P-Free diet supplemented with dicalcium phosphate (Control). The ideal protein diets were formulated to contain 27.7% digestible protein and 4% lipid from practical ingredients. Final weights (467-494 g), weight gains (1,636-1,723%), and average daily intake (2.10-2.18%) did not differ for fish fed the P-Free + Phytase or Control diets and were greater than for the P-Free diet (438 g, 1,528%, 2.27%). There were fewer market size fish (46.8-54.7%), feed conversion ratios were poorer (1.59-1.61), and intraperitoneal fat contents (6.96-7.40%) were higher for fish fed the phosphorus-free diets compared to the Control diet (65.3%, 1.46, 5.90%). Few differences in whole-body mineral retention efficiencies were detected although results suggest higher phosphorus retention in fish fed the P-Free + Phytase diet compared to the Control. There were no significant differences or discernible patterns in scale minerals. There was an indication (P = 0.05-0.10) that bone Mg, S, and Zn were lower in fish fed the P-Free diet. Histological scores of intestinal inflammation were numerically lower in fish at harvest compared to initial fish with few treatment differences but scores for inflammatory cell types overall were very low. Digestibility of DM and ash were higher in fish fed the P-Free + Phytase diet but protein and lipid digestibility were not different among diets. Hematocrit increased linearly over time in fish fed the Control diet and curvilinearly in fish fed the phosphorus-free diets, but no other differences in hematological variables were detected among diets. Geosmin, as opposed to 2-methylisoborneol, was considered the more relevant off-flavor compound and fillet concentrations were higher than those for 2-methylisoborneol, but concentrations found of either compound were unlikely to cause “earthy” or “musty” off-flavors. Rates of soluble reactive phosphorus, organic phosphorus, and total phosphorus accumulation in tanks fed the phosphorus-free were diets nearly half those observed for the Control diet while phosphorus loading from feed was 37.7% lower. Aspects of diet formulation with respect to target available phorphorus, Ca:P ratio, and ingredient phytate content are discussed.