The aim of this research was to determine the effect of using phytase, carbohydrases (NSPases: β-mannanase, xylanase, β-glucanase), and protease in a diet for juvenile rainbow trout Oncorhynchus mykiss on the nitrogen (N) and phosphorus (P) retention efficiency, and solid and soluble discharges.
Seven isoproteic (42% CP) and isocaloric (20% EE) experimental diets were formulated to meet the nutritional requirements of rainbow trout and evaluated under a completely randomized design. Soy-derived protein ingredients made up 30% of the final formulation, with 25% corn gluten and only 10% fishmeal. The diets were identified as follows: C, Control: No exogenous commercial enzyme addition; M: Diet C + β-mannanase (100,000 BMU kg-1); F: Diet C + Phytase (4,000 FTU kg-1); FM: Diet F + β-mannanase (100,000 BMU kg-1); FMX: Diet FM + Xylanase (4,000 XU kg-1); FMXG: Diet FMX + β-glucanase (500 BGU kg-1); FMXAP: Diet FMX + Amylase (400 AU kg-1) + Protease (8,000 PU kg-1). The experiment involved 25 fish tank-1. It was carried out in 21 tanks of 300 L under a RAS with continuous removal and filtration of feces. All procedures were performed in accordance with the reference guidelines of the Guide for the Care and Use of Laboratory Animals. The Nutrient Retention Efficiency (RE) for N and P was calculated according to the following formula: 100 x (BWf x Nutrientf ─ BWi x Nutrienti) x (FCR x (BWf ─ BWi) x Nutrientdiet)-1; Solid (particulate) N and P discharges were calculated in grams per kg of body weight gain as nutrient intake x (100 ─ ADC) x 100-1; Meanwhile, for dissolved nutrient discharge, the following formula was used: nutrient intake x ADC x 100-1 ─ retained nutrient. Once the assumptions of normality and homoscedasticity were verified, the data underwent an analysis of variance.
Results are shown in Figure 1. The FMXAP diet showed the best N mass balance profile, whereas phytase-included diets yielded the best P mass balance profile.
Dietary protein digestibility from FMXAP diet enhanced N retention and reduced solid N discharge. Phytase lowered soluble N discharge, correlating with improved N retention, a relevant finding for recirculating aquaculture systems (RAS). Phytase substantially decreased solid P discharge and increased P retention (from ≈35% to ≈50%). However, soluble P discharge increased, likely due to dietary P levels (≈1.2%) exceeding requirements and subsequent urinary excretion, precluding achievement of reported minimum P discharges. No productive or environmental benefit of NSPases over individual phytase use was established, suggesting the absence of insoluble NSP-mediated "cage effect" and negative effects of soluble NSP at the tested concentrations.