Effect of heat treatment on in situ rumen degradability and in vitro gas production of full-fat soyabeans and soyabean meal

The objective of this study was to determine the effect of the heat treatment of full-fat soyabean (FFSB) and solvent extracted soyabean meal (SBM) on the in situ dry matter (DM) and protein degradability, and in vitro gas production kinetics of the protein sources. Ruminal disappearance of DM and crude protein (CP), and in vitro gas production were determined after 0, 4, 8, 16, 24, 48 and 72 h incubation using the in situ ruminal degradation and in vitro gas production techniques, respectively. In situ DM and CP disappearances were fitted to the exponential equation p = a + b (1-e-ct), where a is the rapid degradable fraction and b is the slow degradable fraction. In vitro gas production data were fitted to the equation, y = A {1 – exp [- b (t-T) – c (√t - √T)]}. Where b and c are the initial gas production rate constant (h-1) and later gas production rate constant (h-1/2), respectively. The two protein sources were heat treated both with steam pressure in an autoclave at 120 °C and in an oven at 150 °C for 20 min. Heat treatment had a significant effect on effective DM degradability (EDMD), effective CP degradability (ECPD) and in vitro gas production. Although the heat treatments reduced the EDMD, ECPD and the amount of gas produced, the results were inconsistent between protein sources. The heat treatments applied in the autoclave and the oven reduced the ECPD0.02 of FFSB by 12.5% and 10.9%, respectively. On the other hand, heat treatment applied through the autoclave decreased the ECPD0.02 of SBM by 13.9%, but by 18.7% when heat was applied through the oven. Heat treatment of SBM using the oven seemed to be more effective than using autoclaving. Heat treatments in the autoclave and oven reduced the total gas production from FFSB by 7.25 and 7.32%, respectively, and from SBM by 12.69 and 7.91%, respectively. It was concluded that heat treatment is an effective method of altering the rumen degradation characteristics of DM and CP in SBM and FFSB. Both methods could be used to increase the proportion of the rumen non-degradable protein fraction in protein sources which would then reach the small intestines unaffected by ruminal fermentation. Keywords: Full-fat soyabean; soyabean meal; heat treatment; in situ protein degradation; in vitro gas production South African Journal of Animal Sciences Vol. 35 (3) 2005: pp.186-19