Effects of feed processing method (extrusion and expansion-compression pelleting) on water quality and growth of rainbow trout in a commercial setting.

To determine the effects of feed processing method (extrusion or expansion followed by compression pelleting) on feed physicochemical characteristics, fecal stability, water quality, and growth performance in rainbow trout, three types of feed pellets (expanded sinking [EpS], extruded sinking [ExS], extruded floating [ExF]) were prepared, analyzed, and fed to juvenile rainbow trout (initial weight = 285.8 ± 15.2 g) at three feeding rates based on feeding rate curves developed at Snake River Farm (Buhl, Idaho, USA) (median rates [%BW] for feeding rate 1: 0.85; feeding rate 2: 0.95; feeding rate 3: 1.05) for 124 days. Results showed that all feeds had similar chemical composition, but the degree of starch gelatinization (as % of starch) was significantly higher in ExS (90.01 ± 0.26) and ExF (84.82 ± 0.63) than in the EpS (9.09 ± 0.41) feed, which led to extruded feeds having much higher water stability, fecal durability, and lower phosphorus discharge. The ExF feed produced better growth (% increase; 154.4 ± 18.5) and feed conversion (1.06 ± 0.06) compared to EpS (126.3 ± 12.4 and 1.26 ± 0.14) and ExS (121.7 ± 7.8 and 1.22 ± 0.04) in rainbow trout. Growth (% increase) improved as feeding rate increased (feeding rate 1: 125.6 ± 10.3; feeding rate 2: 140.8 ± 16.0), but further improvements were not observed at the highest feeding rate (feeding rate 3: 135.9 ± 22.8). The present study is the first to show that the extrusion process not only produces pellets having better quality than the expansion-compressed pelleting method but also improves fecal size and durability in water, and therefore, use of extruded feeds has the potential to improve waste collection and removal and reduce contribution to pollution in effluent. [ABSTRACT FROM AUTHOR]