Effect of adding protease to coextruded flax:pea or canola:pea products on nutrient digestibility and growth performance of rainbow trout (Oncorhynchus mykiss)

Limited supplies of fish meal and oil are a major constraint to the growth of the aquaculture feed industry. Coextruded flax:pea (F:P) and canola:pea (C:P) mixtures are commercially available products that have desirable fatty acid compositions but low protein digestibility. Three experiments were performed to determine if adding a protease to diets containing F:P or C:P would improve protein digestibility in rainbow trout. The addition of 250 g t−1 protease to C:P resulted in significant increases in coefficients of total tract apparent digestibility (CTTAD) for crude protein, lipid, energy and dry matter (P < 0.05). However, when the protease was added to F:P, there was no effect on the CTTAD of crude protein, lipid, energy or dry matter. In a second experiment, the effect of adding protease to diets containing either L:P or C:P on the growth performance of rainbow trout was assessed. The fish were fed one of five diets including: (1) fish meal control, (2) 240 kg t−1 L:P, (3) Diet 2 plus 250 g t−1 of protease, (4) 240 kg t−1 C:P, (5) Diet 4 plus 250 g t−1 of protease. The diets were balanced for digestible protein, lipid and energy and fed to the fish over an eight week period. The fish fed C:P or L:P had reduced weight gain compared to the controls but there were no significant differences in specific growth rates (P < 0.05). The addition of protease to the C:P diet resulted in a significant improvement in feed efficiency. However, the addition of protease to the F:P diet had no effect on growth performance. To assess whether the mucilage content of the flax in L:P was responsible for the lack of effect of protease, the digestibility of crude protein, lipid, energy and dry matter in dehulled flaxseed with and without 250 g t−1 of protease was measured. The addition of protease had no effect on the digestibility of dehulled flaxseed in rainbow trout suggesting that the mucilage content of flax does not inhibit the activity of this protease. The use of a commercial protease improved the nutrient digestibility of C:P but not F:P in rainbow trout. The benefits of protease addition should therefore be assessed for individual ingredients.