اثر روش‌های عمل‌آوری فیزیکی و بیولوژیکی دانه ذرت بر تولید گاز و فراسنجه‌های تخمیری در شرایط برون‌تنی

Introduction: Cereal grains are fed to livestock to supply energy, and the major energy source of the most of  cereal grains is starch. Therefore, starch fermentation potential in the rumen is an important indicator of nutritional value of cereal grain in ruminants' nutrition. Due to lower loss of methane and heat, available energy supply for animal is greater for starch digestion in the small intestine than for starch fermented in either the rumen or large intestine. In order to obtain maximum starch digestion, corn and sorghum grain must be processed before using in livestock nutrition. There are several different methods for processing of cereal grain, which in turn have different impacts on the extend and place of digestion and fermentation of starch, based on the shape and structure of starch granules. Briefly, the aim of all processing methods is to optimize  the rumen fermentation, equilibrate degradability and digestion of starch in different parts of the digestive tract, minimize the rumen acidosis risk, and increase the amount of undegraded digestible starch delivery to the lower parts of the digestive tract. Corn grain is one of the most important feedstuffs in nutrition of high producing animals because it can provide high amounts of metabolizable enrgy to support high levels of milk or meat production. Several physical and chemical methods have been used and studied frequently. However, despite their effectiveness, using the chemical methods is doubtful because of their poisonous effects in animals and human. Most recently new biological processing methods have been proposed for optimizing  of corn grain starch degradability in rumen. So, it can affect the rumen environment and synthesis of microbial protein and determine the place and amount of starch digestion in the different parts of the digestive system. Therefore, the aim of this study was to investigate the effects of different physical or biological methods or methods containing combination of two or three methods of processing on chemical composition and rumen degradability of corn grain.
Materials and methods: This experiment was conducted in a completely randomized design with six treatments and three replicates. Experimental treatments were including: 1) unprocessing corn grain (control), 2) steam-flaked corn grain (3 to 5 minutes), 3) yeast (Saccharomyces cerevisiae) treated (4% with the ratio of 1:2( and steam-flaked corn grain, 4) microwaved (850 W for 3 minutes) corn grain, 5) steam-flaked (3 to 5 minutes) and microwaved (850 W for 3 minutes) corn grain 6) yeast (Saccharomyces cerevisiae) treated, steam-flaked and microwaved (850 W for 3 minutes) corn grain. In order to treatwith yeast, corn grains were mixed with a solution of 4%fermenter (Saccharomyces cerevisiae) in a 2:1 ratio and then, incubated in 35°C for 24 hours. For gas production determination, the ruminal fluid was collected from rumen of three canolated mature Dalagh sheeps with an average weight of 45±5.2 kg before the morning feeding. Experimental animals were fed based on their maintenance requirements and had a free access to water during experimental period. Rumen fluid was immediately transferred to lab and purified using 4 leyer cotton textures and then mixed with artificial saliva with a 1:2 ratio in an anaerobic condition and finally, poured into glass vials along with 200 mg samples of each treatment. Glass vials incubated for 96 hours inside a benmarie bath with about 39°C. The gas production of samples was recorded at 2, 4, 6, 8, 12, 24, 36, 48, 72 and 96 hours of incubation. Experimental data were processed based on Ørskov and Mc Donalds non-linear equations and analyzed using SAS 9.1 statistical software.
Results and discussion: In general, the results of this experiment showed that the different processing methods had a significant effect on the corn grain potential gas production and gas production rate. The lowest gas production potential amounts were related to steam flaked and steam-flaked+ yeast   treated corn grain and the steam-flaked + yeast treated + microwave treatment had the highest amounts of gas production potential (481.5±3.48 and 479.1±3.87 vs. 523.3±6.69 ml/200 mg DM, respectively). Gas production rates of all treatments except for combination of three methods also were significantly higher than control and the highest amount was related with flaked corn grain (0.0771±0.0017 ml/h). Various processing methods had a significant effect on digestibility of organic matter of corn grain, the estimated metabolizable energy content and short chain fatty acids production of different treatments had no significant difference, however, the partitioning factor, final pH and ammonia concentration among different treatments were not significantly different. The digestibility of organic matter of combination method (yeast  treated +steam-flaking+ microwave irradiation) method and steam-flaked + microwave treatment and steam flaked treatments were significantly higher than control, however the organic matter digestibility in rest of treatments were not significantly different comparing with the control. Among the different processing methods, the treatment processed with combination method (yeast  treated +steam-flaking+ microwave irradiation) had the highest amounts of metabolizable energy, digestibility of organic matter and short chain fatty acids production (respectively 13.27 MJ/ Kg DM, 88.10 g/kg DM and 1.81 mmol/200 mg DM). Treatments containing microwave processing significantly reduced in vitro digestibility of dry matter and organic matter in comparison with the control treatment. The highest amount of estimated microbial protein production among the treatments were related to Steam-flaked+ yeast, Steam-flaked and microwave irradiation treatments (respectively 171.73, 170.23 and 170.11 mg/g DM).
Conclusion: The results of this study showed a significant difference in gas production parameters and in vitro digestibility of corn grain among processing methods used in this experiment and the best response in influencing the nutritional value of corn grain for improving rumen fermentation and dry matter degradability can be attributed to microwave irradiation as well as the combination method.