Effect of gamma and microwave irradiation on ruminal degradability of dry matter,crude protein and metabolizable protein of Safflower meal

Introduction: Safflower meal (SF) is utilized as a protein source in ruminant diet, however the protein of is highly degradable by rumen microorganisms. One of the most important factors in determining the protein value of feed for growing or lactating ruminants is the amounts of dietary proteins escape from rumen degradation. Several processing methods such as chemical and physical treatments have been well applied to protect crude protein from microbial degradation. Chemical processing causes environmental pollution which may adversely affect protein digestibility in the small intestine and in some cases may appear in animal products (NRC 2001; Schwab et al 2007). Ionizing radiation such as gamma ray has so much energy it can knock electrons out of atoms and causes fragmentation, cross-linking, aggregation, and oxidation by oxygen radicals generated in the radiolysis of water in proteins. Advantages of microwave irradiation compared to conventional heating are faster heating rates, shorter processing times and energy efficiency. Studies have shown that GR irradiation of sunflower meal, cottonseed meal, and canola meal were successful in reducing degradation of CP by rumen microorganisms (Ghanbari et al 2015; Taghinejad-Roudbaneh 2016). However, there is lack of information on effects of heating and irradiation with gamma ray and microwave irradiation on SF protein degradability. Therefore, the purpose of present study was to evaluate and compare the effects of gamma irradiation at doses of 20 and 40 kGy and microwave irradiation for 3 and 5 minutes on ruminal DM and CP degradation kinetics as well as digestibility of SF. Material and Methods: This research was carried out in Karaj Animal Science Research Institute. First, safflower seeds of Golmehr cultivar were prepared from the institute approved by the General Department of Oilseed of the Ministry of Jihad-e-Agriculture. The DM of SF was determined and then, sufficient water was added to sample to increase the moisture content SF to 250 g/kg. Then, two samples (500 g each) were subjected to microwave irradiation at a power of 800 W for 3 and 5 min. Another two samples (500 g each) were subjected to gamma irradiation at doses of 20 and 40 kGy. The samples were ground to pass a 2 mm screen for the ruminal in situ study. Degradation kinetics of DM and CP were determined according to in situ procedure (Orskov and McDonald 1979). Six g of untreated or irradiated feed samples were incubated in the rumen of three ruminally fistulated bulls for periods of 0, 2, 4, 8, 16, 24 and 48 h. Then, bags were removed from rumen and washed with cold water and dried in an oven at 65 C⸰ for 48 hours and weighed. To determine the degradability of the DM and protein, residuals of bags were weighed. In order to determine the digestibility of SF, two-stage digestion was used method (Tilly and Terry 1963). DM and CP degradability parameters were estimated by using equation of P=a+b(1-e-ct). Chemical composition data were statistically analyzed in a completely randomized design and degradability data were statistically analyzed in a completely randomized block design with GLM procedures of SAS software. Differences among the means were determined by LSD test, at a significant level of p<0.05. Results and Discussion: Different treatments had not significant effect on the chemical composition of SF (p>0.05). The DM and organic matter digestibility of SF was reduced by irradiation (p<0.05). The mechanism for reducing digestibility in heat-processed feeds is very complex. During heat treatment, chemical reactions (such as the Millard reaction), protein structure changes, and cross-linking between protein and carbohydrates may occur. These reactions convert feed proteins into resistant digestible compounds and are responsible for reducing the digestibility of DM and organic matter after microwave processing (Van Soest 1994). Microwave irradiation for 3 and 5 minutes increased the rapidly degradable fraction of DM and CP and degradation rate of CP. Microwave irradiation for 3 and 5 minutes decreased the potentially degradable fraction of DM and CP (p<0.05), also irradiation by microwave for 3 and 5 minutes decreased slowly degradable fraction of DM and CP (p<0.05). Gamma irradiation at doses of 20 and 40 KGy reduced the rapidly degradable fraction of DM and CP (p<0.05). Gamma irradiation at doses 20 and 40 KGy reduced the effective degradability of CP and DM at ruminal outflow rate of 2, 5 and 8% (p<0.05). Reduction of CP degradability as a result of irradiation is due to the occurrence of cross-linking of polypeptide chains, denaturation and protein aggregation )Ciesla et al 2000). Under the effect of microwave irradiation for 3 and 5 minutes, the effective degradability of DM and CP increased compared to gamma irradiation (p<0.05). Gamma irradiation increased the potentially degradable fraction of CP (p<0.05). Gamma irradiation increased the slowly degradable fraction of CP (p<0.05). Gamma irradiation at dose of 20 kGy increased undegradable protein at ruminal outflow rate of 2, 5 and 8 %. Also, microwave irradiation for 3 and 5 minutes reduced undegradable protein compared to the control (p<0.05). Microwave irradiation for 3 and 5 minutes increased the rumen degradable protein at ruminal outflow rate of 5 and 8% (p<0.05). The highest amount of rumen degradable protein was related to microwave irradiation for 5 minutes (p<0.05). metabolizable protein (MP) of microwave irradiated-SF at ruminal outflow rate of 5 and 8% was not significantly different compared to control (p>0.05), however microwave radiation for 3 minutes significantly reduced MP of SF compared to control (p<0.05). Microwave irradiation for 3 minutes at ruminal outflow rate of 0.02, 0.05 and 0.08% decreased MP of SF by 6.23, 7.58 and 39.24% compared to untreated samples, respectively. Gamma radiation at doses of 20 and 40 kGy decreased MP at ruminal outflow rate of 2 and 5 %, but at ruminal outflow rate of 8% was not significantly different compared to control. Final Conclusion: The results of this experiment showed that however, gamma irradiation of SF decreased MP of SF, but gamma irradiation compared to other treatments decreased rumen degradable protein and increased rumen undegradable protein. Subsequently, in vivo studies are required to investigate effect of feeding irradiated-SF on performance of ruminants.