Phosphorus and nitrogen utilization efficiency in rainbow trout (Oncorhynchus mykiss) fed diets with lupin (Lupinus albus) or soybean (Glycine max) meals as partial replacements to fish meal

The study was conducted to compare two plant ingredients as dietary protein sources for rainbow trout on the basis of feed acceptability, survival, growth, feed conversion, nitrogen and phosphorus utilization efficiency and loading. Two extruded diets were formulated with the inclusion of a soybean meal (DS) and a lupin meal (DL). The control diet was a fish meal (FM) based diet. All diets were isonitrogenous and isolipidic. Triplicate groups of 65 fish (5.10 ± 0.10 g) were assigned to each diet. At the end of the experiment (66 days), all groups of fish fed diets had a similar final growth and feed utilization efficiency (P > 0.05). Nitrogen retention rate was higher for the group fed the control diet and in consequence the calculated loading amount of this nutrient resulted lower when compared with the test diets. However, phosphorus retention was higher in the groups of fish fed the experimental diets (DL 26.58 ± 0.22 and DS 27.67 ± 3.05) when compared to the control diet (22.08 ± 1.12) (P < 0.05). This represents a phosphorus loading of 8.33 ± 0.23, 8.96 ± 1.02, and 11.55 ± 0.67 kg/t production for the diets DL, DS, and control, respectively. Therefore, the results indicate that lupin meal can be used as a possible plant protein source for the formulation of low-phosphorus loading diets for rainbow trout without affecting feed acceptability and growth performance. This legume represents a clear opportunity to supply the high demand for plant protein sources for aquaculture. Further studies are needed to evaluate and compare different lupin species and varieties.