APPLICABILITY OF MICROBIAL PHYTASE TO INCREASE PHOSPHORUS AVAILABILITY IN SOY-PROTEIN-BASED DIETS FOR LARGEMOUTH BASS Micropterus salmoides

Over 50%  of the phosphorus (P) in feeds containing high inclusion levels of soy-protein feedstuffs can occur as phytic acid (PA), which is not digestible by fish. Supplementation of microbial phytase (PTS) into soy-based formulations to degrade PA can increase P availability to fish and minimize P excretion to receiving waters.

An eleven- week feeding trial was carried out to evaluate  the effects of supplementing PTS to  soy-protein-based diets on the growth performance and mineral composition of largemouth bass (LMB). A P-deficient (~ 0.25% total non-PA-P), PTS unsupplemented  basal diet was formulated to contain 42% crude protein and 12%  lipid and was used as the negative control (D1; -  Control).  The basal formulation was then supplemented with PTS to obtain five additional diets (D2 to D6) with PTS activities ranging from 250 to 2000 units (U)/kg. Finally, a positive control diet (D7, + Control) was formulated by supplementing Ca-P mono basic to the basal formulation.   Each diet was fed  twice daily (8:00 and 16:00) and to apparent satiation to triplicate groups of 20 LMB (initial weight ~10.25g) stocked in 110-L aquaria operating as a recirculating aquaculture system. Statistically significant LMB responses to dietary treatments were considered at P<0.05.  

Linear regression analysis on resulting data revealed no effects of PTS supplementation on LMB survival, feed intake, weight gain, and feed conversion efficiency; but positive trends were observed (Table). Orthogonal contrast analysis on selected treatments indicated that: i) + Control-fed groups consumed more feed, grew faster, and converted feed more efficiently than  -  Control- fed groups; ii) the supplementation of 2040 PTS U/kg of soy-protein-based feeds can improve LMB production performance and reduce the need for inorganic P supplementation. Results for P  retention and concentration in whole fish and skeleton mineralization will be presented.