Phytase improves growth performance and reduces environmental costs of high-density culture of bullfrogs (Aquarana catesbeiana)

High-density culture is becoming popular in aquaculture to achieve higher profits, but at the cost of higher environmental pollution risks of the farm wastewater, especially nitrogen and phosphorus emissioms. Phytase, as a widely used feed additive, can hydrolyze the phytic acid in plant-based feeds and facilitate the following utilisation of the released phosphorus (P) nutrients by animals. As for the growing high-density bullfrog aquaculture, it is still unclear whether adding phytase can replace the diet minerals, improve the feed efficiency and reduce nitrogen and phosphorus emissions. Thus, this study aims to investigate the application potential of phytase in bullfrog feeding under high-density culture conditions. Bullfrogs were fed with a normal diet (ND), a low phosphorus diet (LD), and a low phosphorus and low calcium diet (LLD), with three levels of phytase (0 FTU/kg, 500 FTU/kg, and 1000 FTU/kg), respectively. The growth performance and physiological indexes of bullfrogs, and nitrogen and phosphorus emissions into farm wastewater were determined at the end of the experiment. When the addition level of phytase was 500 FTU/kg, bullfrogs exhibited the best carcass weight and feed efficiency. Low Ca level significantly increased Viscerosomatic index (VSI) and crude protein. Low P level significantly reduced crude protein, crude ash, muscle fiber density, serum alkaline phosphatase (ALP), and fecal P, while significantly increasing muscle fiber diameter and fluff width. The reduction in crude protein and crude ash could be partially compensated by the addition of phytase. Increasing phytase levels significantly reduced ALP and fecal P. Besides, the addition of phytase reduced the emissions of nitrogen and phosphorus from aquaculture wastewater, in which LD incorporating 500 FTU/kg phytase decreased the phosphorus emission by 32.58 %, and ND with 500 FTU/kg phytase yielded the reduction nitrogen emissions of 18.24 %. Collectively, the incorporation of 500 FTU/kg of phytase into low-phosphorus diets is recommended for bullfrog high-density culture to yield more favourable outcomes with lower environmental costs.