Effect of extrusion of wheat and corn on gilthead sea bream (Sparus aurata) growth, nutrient utilization efficiency, rates of gastric evacuation and digestive enzyme activities

The nutritional value of corn and wheat, raw or extruded (WR, WEx, CR and CEx), as carbohydrate sources at high inclusion levels (40%) in gilthead sea bream diets was evaluated by studying nutrient apparent digestibility coefficients (ADC), growth, nutrient retention efficiencies, body and liver composition, gastric evacuation time (GET) and trypsin and -amylase activities. A growth experiment was performed for 10 weeks during which gilthead sea bream of 13 g initial weight reached a weight of 39.1-46.4 g. Feed consumption was different among treatments and resulted in similar digestible energy and protein consumption. CR indicated the lowest performance parameters. Extrusion improved growth and feed and nutrient efficiencies, the effect being stronger in most of the cases for corn than for wheat due to the good nutritional characteristics of WR. Whole body lipid content increased with extrusion. CR exhibited the lowest nutrient and energy digestibilities. Extrusion improved significantly all ADC values, a larger effect being observed for the corn diets. The passage of food through the stomach was described by an exponential model for all diets, and GET computed was higher in groups fed diets containing the extruded starches, being double (513 min) in WEx diet compared to other diets. Amylase levels measured in the gut supernatant increased after feeding. Diet CR exhibited the lowest amylase values and WEx the highest. A reduction of amylase activity (AA) was apparent for all diets after 4 or 6 h after feeding except for the CR diet for which no decrease was observed. Trypsin levels remained relatively stable after a significant initial increase after feeding. Corn containing diets, especially CR, exhibited lower trypsin activities than wheat containing diets in accordance with higher trypsin inhibitor levels measured for corn. The results of this study indicate that extrusion improves the nutritional characteristics of both cereals. Wheat is however a good carbohydrate source even in the raw form. Gut transit times and enzyme activities indicate differences, which could contribute to the nutritional value of the carbohydrate sources evaluated.