Phosphorus is a required nutrient for rainbow trout and is supplemented as a mineral phosphate to meet fish’s dietary needs. As plant-derived ingredients are increasingly utilized in trout feeds, increased P supplementation is often needed. This is due to the inability of trout to digest phytate bound P commonly found in plants. Increasing the efficiency of digestion and retention of P from the plant-based feed for rainbow trout will have both economic and environmental benefits for producers. Phytase has been shown to be effective in releasing phytate-P in aquafeeds and in other animal industries supra-dosing phytase has shown benefits other than improving P digestibility.
To test the efficacy of phytase to release phytate-P in trout feeds to meet fish requirements and reduce P discharge, a 2 x 3 factorial experiment was conducted with rainbow trout to test phytase (Quantum Blue, AB Vista) at 0, 2500 and 7500 FTU/kg in a fishmeal-based and plant-based feed. These formulations were also compared to positive control formulations with monocalcium phosphate supplying dietary P requirements. Fish were reared in 15 °C in triplicate tanks of 40 fish weighing on average 31.9 g each for 12 weeks. Feeds were extruded, floating 4mm pellets with phytase top-coated prior to final oil top-coating.
Rainbow trout grew less when no P was supplemented as monocalcium phosphate and no phytase was added. Interactive effects were observed for growth, feed conversion ratios and feed intake with phytase addition between fishmeal-based and plant-based diets. In plant-based feeds, final fish weight increased form 153 g when no P or phytase was supplemented to 208 g with supplemental P and an average of 213g when phytase was utilized. Phytase improved FCR from 1.14 to 0.90 in plant-based feeds with no effect on fishmeal-based feeds. Phytase addition reduced water-soluble P waste loads by 43% from the fishmeal-based feeds and 56% from the plant-based feeds.