Effects of grinding, steam conditioning and extrusion of a practical diet on digestibility and weight gain of silver perch, Bidyanus bidyanus

As processing can affect the digestibility and utilisation of diets by fish , we examined the effects of grinding, steam conditioning and extrusion of a commercially available diet (SP35) on weight gain and performance of silv er perch. SP35 (35% protein, 18 MJ/kg gross energy) with approximately 80% of particles between 710 and 1000 mu m was either left (unground) or finely ground to 500 mu m (ground). Both unground and ground fractions were made into sinking pellets in a commercial steam pelleting mill with or without t he addition of steam (90 degrees C) A fifth diet was processed by pelleting finely ground material in a single-screw extruder (after the addition of a pproximately 5% fish oil) at a temperature of 120 degrees C. The extruded d iet floated or sank slowly. Each diet was fed to 50 juvenile silver perch ( mean initial weight 17.8 g) in each of three replicate 10000-1 tanks for 11 3 days, Fish gained between 55 and 71 g/fish during the experiment, and fee d conversion ratio (FCR) ranged from 1.5:1 to 2.0:1. Steam conditioning sig nificantly improved weight gain and FCRs while neither grinding nor the int eraction between grinding and steam conditioning had any effect. Fish were reluctant to consume the extruded diet and grew less on this diet than on t he steam-conditioned diets, although FCR was better than for all other diet s. Ground diets, uncooked and steamed, and the extruded diet were subsequen tly reground and 1% chromic oxide was added as an inert indicator. Each of these three diets was fed to juvenile silver perch (mean initial weight 2.5 g) in 170-1 cylindroconical tanks from which faeces were collected by sett lement to determine digestibility coefficients for dry matter, energy and n itrogen. Digestibility coefficients for dry matter and energy were higher f or the extruded diet but similar for the unsteamed and steamed diets. Prote in digestibility was unaffected by processing. These results indicate that for silver perch fed diets similar to SP35, diets should be steam-condition ed, but the additional expense associated with fine grinding is unwarranted with respect to gains in either fish performance or improvements in pellet stability. Extrusion significantly improved digestibility and FCR but cons umption of floating extruded pellets was reduced in our facility. Sinking, extruded diets deserve evaluation. (C) 2000 Elsevier Science B.V. All right s reserved.