Apparent digestibility of protein, amino acids and gross energy in rainbow trout fed various feed ingredients with or without protease

Apparent digestibility coefficients (ADCs) of dry matter, crude protein, amino acids, and gross energy in various feed ingredients with or without protease supplementation (Jefo Nutrition Inc., Quebec, Canada) were determined for rainbow trout in a two-year study. The ingredients consisted of two feather meals, two poultry byproduct 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 reference diet with 0.1% yttrium oxide as inert marker was mixed with test ingredients in a 70:30 ratio to produce a series of test diets. Fish were fed their respective diets and fecal samples were collected by stripping. ADC of dry matter for rainbow trout ranged 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 the majority of individual amino acids. Overall, this research demonstrates the benefit of 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. Further studies with this protease are recommended to evaluate its effects at higher inclusion levels and in other species of cultured fishes.