One of the most important aspects in evaluating the effectiveness of alternative feed ingredients is the determination of digestibility. Several alternative feed ingredients have been tested in aquaculture feeds to replace fishmeal for sustainable aquaculture. Nevertheless, imbalanced amino acid profiles, poor digestibility and palatability, and presence of anti-nutritional factors (ANFs) limit their use in aquafeeds. Therefore, one of the strategies is to supplement enzymes for improving nutrient digestibility. Among enzymes, proteases have potential use in reducing ANFs, such as protease inhibitors, and breaking down macromolecular proteins. The efficacy of supplemental protease across a wide range of protein ingredients has not been previously investigated. Therefore, this study was conducted to evaluate the effects on apparent digestibility coefficients (ADCs) of dry matter, crude protein, amino acids, and gross energy when dietary protease was added to 17 different protein ingredients using rainbow trout as a model species.
In vivo digestibility was determined for 17 ingredients with and without protease supplementation (175 g kg-1, Jefo Nutrition Inc., Quebec, Canada) fed to rainbow trout. The ingredients consisted of two feather meals, two poultry by-product meals, two meat and bone meals, sardine meal, menhaden meal, black soldier fly larvae meal, Methanococcus maripaludis single cell protein, soybean meal, canola meal, distiller's dried grains with solubles (DDGS), cottonseed meal, peanut meal, sunflower meal, and algae (Spirulina sp.) meal. A batch of test diet containing 30% test ingredient and 70% reference diet mash (combined on a dry-matter basis) was prepared and analyzed. Trout (average weight, 250 g) was used in the digestibility trial. Each of the experimental diets (reference and 34 test diets) was fed to two replicate tanks of fish in a completely randomized design to apparent satiation. Feces were expelled from each fish using gentle pressure on the lower abdomen of fish. ADC of diets and ingredients, for dry matter, protein, amino acids and energy were calculated. Apparent digestibility was calculated using fecal material pooled from 30 fish/tank, and all data are expressed as the mean ± standard error of the mean (SE). Data were subjected to a Student's t-test to test for protease effect using SPSS Version 20.0 (SPSS Inc., Chicago, IL, USA).
ADC of dry matter for rainbow trout ranged from 51.0-86.6% for animal products and single cell protein and 33.1-70.1% for plant products without protease supplementation. ADC (without protease supplementation) of protein and energy ranged from 55.4-84.5% and 58.1-90.2%, respectively, for animal products and 70.0-83.8% and 32.9-76.0%, respectively, for plant products. Supplementation with the commercial protease (175 mg protease complex/kg of diet) resulted in ingredient-specific ADC increases for dry matter, energy, cysteine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tyrosine, alanine, aspartic acid and glutamic acid, with most ingredients having improved digestibility of at least one amino acid. Protease supplementation had the most profound improvement on ADCs for soybean meal, including dry matter and most individual amino acids.
In conclusion, supplementation with the protease complex resulted in ingredient-specific ADC increases for dry matter, energy, cysteine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tyrosine, alanine, aspartic acid, and glutamic acid, with most ingredients having improved digestibility of at least one amino acid. Protease supplementation had the most profound improvement on ADCs for soybean meal, including dry matter and most individual amino acids. Overall, this research demonstrates the benefit of the evaluated protease supplementation on the digestibility of feed ingredients commonly used in rainbow trout and other commercially cultured fish feeds, although the degree of improvement in digestibility varied among ingredients.