Performance of post-juvenile rainbow trout Oncorhynchus mykiss fed diets manufactured by different processing techniques

Four different methods of pelleting-extruded (EXT), steam pelleted (STM), using a Universal Pellet Cooker (UPC), and expanded (EXP)-were performed on subsamples of a single batch of mash to determine the effects on feed pellet characteristics, in vivo feed digestibility, and performance of rainbow trout Oncorhynchus mykiss fed the diets for 16 wk. Results indicated that the STM feed was the most (851 g/L) and UPC feed the least (672 g/L) dense, and that UPC feed had the highest % float (8.3%) and, of the pellets that sank, the fastest sink rate (15.1 cm/sec). While some differences occurred among pelleted diets in loss of dry matter (% dry loss) after incubation in a water bath for up to 10 min, the overall % dry loss was only approximately 1-2%. The method of pelleting did not affect fish weight gain, feed conversion ratio, or specific growth rate. Measurement of the apparent digestibility coefficient of dry matter of the test diets using Cr2O3 as the marker ranged from 71-89%. While it appeared that the method of pelleting can affect feed characteristics and the amount of feed offered to the fish, fish performance was not affected.