Nutritional value of heat-treated soybean meal for channel catfish ( Ictalurus punctatus)

Abstract

A study was undertaken to evaluate the effect of heat treatment of defatted raw soybean meal (RSBM) on the growth performance, hematology, immune response and resistance of channel catfish to Edwardsiella ictaluri challenge. Six practical-type diets were formulated to be isocaloric (2.8 kcal DE/kg diet) and isonitrogenous (34% crude protein). A diet containing 45% of commercial soybean meal (CSBM) served as the control. RSBM, non-heat treated (RSBM0) and heated in an autoclave using the dry cycle at 130 °C and 22 psi for 5 (RSBM5), 10 (RSBM10), 20 (RSBM20), and 40 min (RSBM40), was used to isonitrogenously replace the CSBM in the control diet. Each diet was fed to juvenile catfish (4.98 g) in triplicate aquaria twice daily to apparent satiation for 10 weeks. Another batch of diets containing 1% of chromic oxide was used for measurement of apparent digestibility coefficients. Heating RSBM for 20 min or longer lowered trypsin inhibitor (TI) content and increased the apparent protein digestibility (APD). The protein dispersibility index (PDI) decreased with an increase in the duration of heat treatment. Fish fed CSBM and RSBM40 diets had similar weight gain, protein efficiency ratio and apparent protein utilization which were significantly higher than those of fish fed the other diets. Feed intake significantly increased when RSBM was heated for 20 min or longer. Fish fed the RSBM40 diet had significantly lower feed intake but higher feed efficiency than those fed the CSBM diet. Whole-body protein was highest for fish fed the RSBM40 diet but did not differ from that of fish fed the CSBM diet. Hepatosomatic and visceral indices (HSI and VI) of the groups fed CSBM and RSBM40 diets were significantly lower than those fed other diets. Total cell count, red blood cell count, hematocrit and hemoglobin were not significantly affected by dietary treatment. Plasma lysozyme activity and protein of fish fed the CSBM and RSBM40 diets were similar and significantly higher than those of fish fed the other diets. Cumulative mortality 14-day post challenge with E. ictaluri was significantly lower for fish fed the RSMB5 and RSBM10 diets but significantly increased when heating time was increased to 40 min. Macrophage chemotaxis in the presence of exoantigen and antibody titer against E. ictaluri was higher for the groups fed the RSBM containing diets than those fed the CSBM diet, although the differences were not always significant. Results of this study indicate that autoclaving the RSBM for 40 min lowered the TI and PDI, improved the nutritional value of RSBM and increased plasma lysozyme and protein. However, this level of heat treatment significantly decreased macrophage chemotaxis, antibody titer and resistance of fish to E. ictaluri challenge.