An advanced Escherichia coli phytase improves performance and retention of phosphorus and nitrogen in rainbow trout (Oncorhynchus mykiss) fed low phosphorus plant-based diets, at 11 °C and 15 °C

Abstract

Two studies were conducted to evaluate the effect of phytase supplementation on the overall performance and nutrient retention efficiency of rainbow trout (Oncorhynchus mykiss) fed low-phosphorus (P) plant-based diets at water temperatures of 11 °C and 15 °C. In the first experiment at 11 °C, trout (average weight 56 g) were allocated to one of 4 dietary treatments, with 3 replicate 200L tanks per diet and 35 fish per tank. In the second experiment (15 °C), trout (average weight 42 g) were allocated to one of 3 dietary treatments, with 3 replicate 200L tanks per diet and 34 fish per tank. Diets were fed to satiation over a 75 or 83 day period for experiment 1 and 2, respectively. Treatments included a low-P diet (NC), with no added monocalcium phosphate (MCP), supplemented with 0, 500 or 2500 FTU/kg phytase (Quantum Blue). Experiment 1 also included a diet adequate in P (PC) with 1.8% MCP inclusion. In both experiments, feeding phytase linearly increased (P < 0.01) weight gain of fish fed NC diets. Feeding phytase at 2500 FTU/kg improved weight gain of fish exposed to water temperatures of 11 °C and 15 °C by 36% and 45%, respectively, compared to the respective NC. Feed conversion ratio (FCR) was also improved (linear, P < 0.001) by 14% with phytase supplementation at 15 °C. At water temperatures of 11 °C, supplementing phytase to the NC diet improved N retention (P < 0.05) beyond that of the non-supplemented NC and PC diets. Furthermore, 2500 FTU/kg phytase increased P digestibility by 56 and 27 percentage points, and P retention by around 29 and 17 percentage points, compared to the NC and PC, respectively. Consequently, supplementing 2500 FTU/kg phytase reduced (linear, P ≤ 0.001) N and P excretion by 13% and 55%, respectively, compared to the PC. At 15 °C, addition of phytase to the low-P diet increased (quadratic, P < 0.017) N and P retention in trout by approximately 4 and 24 percentage points, respectively. Supplementing phytase to the low-P diet reduced N (linear, P ≤ 0.001) and P (Quadratic, P < 0.01) excretion in trout, with addition of 2500 FTU/kg lowering excretion of these nutrients by 19% and 42%, respectively. These findings demonstrate the efficacy of an enhanced Escherichia coli phytase, even at low water temperatures, to support performance of trout fed low-P diets, while enhancing P and protein utilization and lessening environmental pollution.