عنوان مقاله [English]
Introduction: Dietary protein is the most important factor determining milk N efficiency, urinary N losses, and consequently, ammonia emissions from dairy cow manure. High crude protein of the diet will decrease N efficiency of the cow through increased N losses in manure. Ration for high producing dairy cows can safely balance with low CP content if a suitable source of RUP selected for the diet. Therefore, it is essential to use feed stuff, having more intestinal digestibility than rumen degradability. Thermal processed soybean can play an effective role in fulfilling protein requirements, not only to deliver essential amino acids but also to have the most efficiency. OBJECTIVE: The aim of this study was to determine the effect of different heat-processing methods on rumen degradability of protein and dry matter of soybean using in situ and gas production methods. Material and Methods: Heat treated soybean samples were requested from two commercial animal feed producer companies. Experimental treatments were included 1) control: whole soybean with no processing (WS) 2) Roasted: roasting for 20 min at 156 C° (RS) 3) Extruded soybean: Extruded for one minute at 130 C° with 25 percent moisture (ES). The samples were dried in 60° oven for 48 hours. Chemical composition of samples was determined according to prescribed procedures of AOAC (2003). Also dry matter fermentation and disappearance of each sample were determined using in vitro gas production and in sacco techniuqes respectively. The collected data were analyzed in completely randomized design using SAS 9/1 software. In gas production method 200 mg of samples were incubated in 2,4,6,8,12,16,24,36,48 and 72 hours. Kinetics of digestion were estimated using the following model: Gp = A (1-e-ct). Ruminal degradability of feed samples was measured using two fistulated Ghezel lamb and in situ technique and the dry matter and protein degradability were determined. 500 mg of the sample was incubated in 2 replications (two samples in each lamb) for 0,2,4,6,8,12,16,24,36,48 and 72 hours. Degradation kinetics of dry matter and crude protein were calculated using the model of y = a + b (1-e-ct). Results and Discussion: The result indicated that heat processing, reduced dry matter digestibility of soybean (P < 0.05). Cumulative gas produced during fermentation was 309.8, 299.4 and 297.0 for the raw, roasted and extruded soybean respectively. There was no significant difference between treatments during the initial hours of incubation, but spending 16 hours incubation resulted to significant differences among the treatments which continued until the end of incubation time (P <0.05). Estimated nutritive value parameters such as metabolizable energy (ME), net energy for lactation (NEl), short chain fatty acids (SCFA) and organic dry matter digestibility (OMD) showed significant reduction in both processing methods. Changes in the structure of soybean proteins by heat-processing methods and reduced ruminal digestibility of the protein by microorganisms, which reduces dry matter fermentation, can explain the lower amount of gas production in Roasted and Extruded soybean in present study. Lower fermentation resulted to reduction in estimated ME, NEL, DOM and SCFA of the feedstuffs. Results showed that roasting and extruding can reduce degradability rate of crude protein. It seems that heating of soybean during processing can cause changes in the structure of protein in which hydrogen and non-covalent bonds turn down and changes the conformation by altering amino acids locations. Degradation rate (c) of crude protein of raw, extruded and roasted soybean were, 0.071, 0.064, 0.041 percent hour-1, respectively. It seems that because of lower degradation rate, roasted soybean should has less ruminal degradation and consequently higher rumen undegradable protein (RUP) than extruded soybean. The great difference in degradation rate (c) between extruded and roasted soybean in the present experiment can be attributed to the lower temperature in the extrusion versus roasting of soybean. However no significant differences were found for C fractions of dry matter among the experimental feedstuffs. Conclusion: Extruding and roasting of soybean can reduce rate of gas production and thereby fermentation of soybean from 16 hours of incubation which contribute to more passage of this protein source to the small intestine of ruminants. Lower degradation rate of crude protein for the roasted soybean can result to higher amount of protected undegradable protein in the small intestine; however heat damage to protein when high temperature is used for processing of soybean should be considered.