Physical and chemical properties, gas fermentation parameters, starch digestibility and starch granules structure in Iranian corn grain Single cross 702 in comparison with different kind of imported corn grain

Introduction[1] Corn grain represents the most important energy source in ruminant diets. In high-producing dairy cows, the diets contain high levels of corn in order to meet their energy requirements. Corn has a complex structure where a range of nutrients interact with each other or physically associations. Ultimately, the quantity and availability of these nutrients determines the nutritional value of this grain. In corn nutrients and energy utilization is influenced by both intrinsic (e.g. starch granules and protein matrix structure) and extrinsic (e.g. growing and storage conditions, climate and agronomy conditions and grain drying process) factors. Corn endosperm represents more than 80% of total grain and is composed of starch granules that are imbedded in a protein matrix and surrounded by plant cell walls. Starch granules size, starch composition (amylose/amylopectin ratio), starch encapsulation (by endosperm cell walls and protein bodies) are among the factors with the most influence on digestion of starch. The structure and distribution of starch and protein matrix network of grains vary in different corn varieties. The Objectives of this study were to evaluate physical and chemical properties, gas fermentation parameters, starch digestibility and starch granules structure in Iranian corn grain Single cross 702 in comparison with different kind of imported corn grains (Russia, Ukraine and Brazil).   Materials and methods Corn grain samples (Single cross 702, Russia, Ukraine and Brazil) were obtained from Khorasan Razavi Agricultural and Natural Resources Research Center, Mashhad, Iran. The apparent density was measured and the samples were analyzed for DM, OM, CP, EE, NDF, ADF, starch. Gas production was conducted in a 125 ml amber flask with three series of incubation. Gas production parameters were calculated. Also, rumen, intestine and total tract digestibility of DM, starch and CP were determined by using the in situ mobile bag procedure. Different parameters of damaged starch (the absorption of iodine, Ai%; damaged starch content in UCD, Chopin units; UCDc, Chopin units on protein basis matter) were determined using the amperometry method (Chopin, ZI Val de Sein, 92390 VLG, France). Starch gelatinization was determined according to the enzymatic procedure (AACC Method 76-31.01; K-SDAM, 09/2018). Scanning electron microscope (LEO 1450 VP, USA), at an accelerating voltage of 25 kV, and under 2500x magnification to study the grain structure was done. Data were analyzed by GLM procedure of SAS with a completely randomized design. Results and discussion Apparent shape of Single cross 702 was smaller than other corn varieties. Apparent density was higher in Brazil corn than other corn varieties. DM, OM and EE were not shown significantly difference between corn varieties; however, CP, ADF, NDF, starch, NFC, TDN, NEg and NEl were significantly affected by different corn varieties. Starch in Single cross 702 corn (69.03%) was significantly lower than Russia (71.04%), Ukraine (70.36%) and Brazil (71.49%) corns. Asymptote gas production (A) was not influenced by different corn varieties; however, the real gas production in time 24 and 48 h incubation in Brazil, Russia and Single cross 702 corns was greater than Ukraine corn. The instant rate of gas production until 8 h incubation in Single cross 702 and Russia corns was greater than Ukraine and Brazil corns. The time for fermentation of 25 and 75% of substrate in Single cross 702 and Russia corns were significantly reduced than other corn varieties. The PH, NH3-N and total VFA of bath culture in 24 h didn’t influence by corn varieties, however acetate to propionate ratio in Brazil corn was greater than other corns. In spite of that the rumen starch digestibility of Single cross 702 (61.59%) and Russia (59.51%) was increased than Ukraine (45.31%) and Brazil (40.51%) corns; however, Ukraine (97/.06%) and Brazil (98.39%) corns showed the intestine starch digestibility greater than Single cross 702 (93.79%) and Russia (93.87 %) corns. The starch gelatinization in both of Single cross 702 (4.24%) and Russia (4.17%) was greater than Ukraine (3.78%) and Brazil (3.32%) corns. The scanning electron microscopy showed that the starch granules size was not uniform in the Single cross 702 corn and the number of small starch granules was greater than other corn varieties. Also, the thin protein matrix was observed in the Single cross 702 corn. In contrast, the starch granules size in the Russia and Ukraine corns were larger and uniform. In Brazil corn, the starch granules were arranged with greater density and a non-smooth surface was observed on the granules. Many studies were done on nutrient value of different corn varieties. Numerous factors can affect the grain chemical composition, physical properties and starch availability on corn grain that include cell wall structure, type of endosperm (floury or vitreous), starch granules and protein matrix, genetic and environment. Corns contain higher floury to horny starch ratio showed greater starch gelatinization and greater starch digestibility in the rumen and total tract. Findings of this study represent the Single cross 702 and Russia corns showed higher gas production, rate of gas production, starch gelatinization and rumen digestibility of starch than Brazil corn. Conclusion It is concluded that the Single cross 702 corn in terms of flurry endosperm, gas production, gas production rate, starch gelatinization and rumen digestibility of starch was similar to Russia corn, although the structure of starch granules was different. Brazil corn had a horny endosperm and showed lower rate of gas production in initial hours of incubation, lower starch gelatinization and higher intestine digestibility of starch than other corns. It generally seems that the result of this study and similar studies can offer useful information about corn grain for farmers and the animal feed manufactures for processing of corn.