Effects of gamma irradiation on protein degradation of soybean meal in the rumen

The aim of this study was to evaluate the effects of gamma irradiation (g-irradiation) at doses of 25, 50 and 75 kGy on ruminal dry matter (DM) and crude protein (CP) degradability and intestinal CP digestibility of soybean meal (SBM). Nylon bags of untreated or g-irradiated SBM were suspended in the rumen of four Holstein cows for up to 48 h, and resulting data were fitted to non-linear degradation model to calculate degradation parameters of DM and CP. From in sacco results, the water soluble fractions decreased linearly (P<0.001) and the potentially degradable fractions of DM and CP increased linearly (P<0.001) by g-irradiation. Effective degradability of DM and CP decreased linearly (P<0.001) with increases in irradiation dose. Gamma irradiation at doses of 25, 50 and 75 kGy decreased effective degradability of CP at rumen outflow rate of 0.05 h−1 by 17, 30 and 42%, respectively. Gamma irradiation increased linearly (P<0.001) intestinal mobile bag digestibility of ruminally undegraded CP. At doses of 25, 50 and 75 kGy, CP digestibility increased by 4, 15 and 23%, respectively. Electrophoretic analyses of untreated, 25, 50 and 75 kGy g-irradiated SBM protein residues revealed that two of the subunits of β-conglycinin (ά and α) were degraded completely after 2, 4, 8 and 12 h, respectively, whereas the β subunit of this protein was more resistant to degradation. In untreated SBM, the two subunits of glycinin (acidic and basic polypeptides) were degraded in the middle of the incubation period, but in 25, 50 and 75 kGy g-irradiated SBM, the acidic subunit of glycinin were degraded after 8, 12 and 24 h of incubation, respectively, and the basic subunit of glycinin were not degraded even after 48 h of incubation. In conclusion, SBM proteins appeared to be effectively protected from ruminal degradation by g-irradiation at doses higher than 25 kGy.