Effects of Gamma Irradiation on Ruminal Protein Degradability and Intestinal Digestibility of Rapeseed Meal

This study was carried out to determine the effects of gamma irradiation on dry matter (DM) and on crude protein (CP) degradation of rapeseed meal, using nylon bags and SDS-PAGE techniques. Ruminal nylon bags of either untreated or irradiated rapeseed meal at doses of 25, 50 and 75 kGy were suspended in the rumen of four non-lactating Holstein cows for up to 48 h. Intestinal crude protein digestibility was assessed using mobile nylon bag technique. Subunits of rapeseed meal proteins and disappearance patterns were traced and determined using sodium dodecyl sulphate polyacrylamid gel electrophoresis and densitometrical scanning. There were significant differences (P<0.05) observed (for DM and CP degradation characteristics and for effective degradability values) between untreated and gamma irradiated rapeseed meals. Gamma irradiation decreased wash-out fractions, degradation rate as well as effective degradability while increasing potentially degradable fractions of DM and CP. The effective DM and CP degradability of untreated and gamma irradiated rapeseed meal (at a rumen outflow rate of 0.05/h) were 67.8, 61.1, 54.1 and 50.5 % for DM, and 65.9, 53.4, 47.9 and 44.5 % for CP, respectively. According to slab gel analysis, rapeseed meal proteins were composed of two major components of napin and cruciferin, accounting for approximately 24 and 51 percent of the total meal proteins, respectively. Both proteins were multi-subunits. The molecular weights of 31.9, 26.0, 21.4, 19.5 kDa for cruciferin subunits and 8.0, 9.6 kDa for napin subunits were observed. Electrophoretic and densitometric analyses of untreated rapeseed meal protein residues revealed that napin subunits were degraded completely within 2 h, whereas the four subunits of cruciferin were not degraded even after 24 h of incubation. In gamma irradiated rapeseed meal, napin subunits were resistant until 12 h of incubation. The four subunits of cruciferin were not degraded even after 48 h of incubation. There were differences observed (P<0.05) between crude protein digestibility of untreated and gamma irradiated rapeseed meals. Crude protein digestibility of untreated, 25, 50 and 75 kGy gamma irradiated rapeseed meals were 72.3, 76.2, 78.5 and 80.1 %. The results finally indicate that gamma irradiation of rapeseed meal appears to be effective in decreasing ruminal protein degradability and increasing intestinal protein digestibility.