Screening of nutrient digestibilities and intestinal pathologies in Atlantic salmon, Salmo salar, fed diets with legumes, oilseeds, or cereals

Ten different plant protein and/or starch sources were studied in a 5-week experiment with triplicate groups of 0.7-kg Atlantic salmon in seawater, pre-adapted to a diet with fish meal, faba beans, sunflower cake, and wheat gluten as sources of protein and starch. The experimental ingredients were corn gluten, defatted soybean, defatted sunflower, dehulled lupin, defatted double-low rapeseed, whole field pea, whole and dehulled faba bean, whole wheat and naked oat, tested one at a time (14-24% inclusion). The diets were balanced by addition of pure wheat starch and/or pure cellulose, to obtain equal macro nutrient compositions. The control diet consisted of fish meal, wheat starch, cellulose and fish oil. The results showed reduced faecal dry matter content in fish fed the soybean diet and to a lesser extent in those fed the sunflower, lupin and rapeseed diets. Diets containing lupin and rapeseed resulted in a moderate increase in the viscosity of the digesta, while diets with wheat and oat increased viscosity of digesta more. Apparent digestibility of lipid decreased linearly with increasing dietary cellulose level. A significant reduction in the digestibility of crude protein was seen for the soybean, sunflower, rapeseed and oat diets, reflecting reduced digestibilities of most amino acids. The salmon fed the corn gluten, lupin, pea, bean and wheat diets had protein digestibilities comparable to the control group. The digestibility of phosphorus was highest for salmon fed the rapeseed diet and lowest for fish fed the oat diet. Faecal excretion of sodium was highly elevated for salmon fed the soybean diet, and moderately elevated for fish fed the corn gluten and sunflower diets. Faecal excretion of zinc was elevated in the fish fed the oat diet. None of these observations were significantly related to the dietary concentration of phytic acid. A histological examination of the stomach, mid- and distal intestine of all groups showed no other irregularities than enteritis in the distal intestine of salmon fed soybean meal. The present study demonstrated a potential for several plant ingredients, such as field pea and faba bean, partly replacing high-quality fish meal in diets for Atlantic salmon, based on nutrient digestibilities and absence of pathologies in the stomach and intestine.