Feed intake and nutrient digestibility and retention in Atlantic salmon fed diets with different physical pellet quality. A CREATE project

Three extruded salmon diets (9 mm) were produced with the same formulation but different physical pellet qualities. Pellet quality was measured in terms of bulk density, durability (Ligno and DORIS test), fat leakage, hardness and water stability. The largest difference among the feeds was found in hardness, which was measured to 201 N, 236 N and 86 N, respectively, for Diet 1, Diet 2 and Diet 3. The water stability, which in previous trials has seemed to affect feed intake, was 87.0 % for Diet 1 and 82.3 % for both Diet 2 and Diet 3. The three feeds were fed to Atlantic salmon with start weight 1.3 kg for three months. The relative feed intake (% of body weight per day) was significantly higher in salmon fed Diet 2 and Diet 3 than of those fed Diet 1. There were no significant differences in growth or feed conversion ratio. The apparent digestibility of energy, dry matter, nitrogen and fat was similar for all feeds, whereas the apparent digestibility of phosphorus and zinc was highest in Diet 1 and 2. No significant differences in retention of ingested nutrients were found, whereas salmon fed Diet 3 retained significantly more Zn than those fed Diet 2, with those fed Diet 1 in an intermediate position. In conclusion, feed intake in Atlantic salmon was different among diets with different pellet quality. The highest feed intake was found in salmon fed the diets with highest durabilities (Ligno and DORIS test), lowest water stabilities and lowest fat leakages, whereas pellet hardness did not affect feed intake. The digestibility of P and Zn, and the retention of digested Zn, was affected by the pellet quality. The measured parameters of physical pellet quality did not explain the significant differences that were found in feed intake and mineral utilisation in the salmon.