Digestion kinetics of corn grain as affected by processing and associative effects

Corn (Zea mays L.) grain is a good source of energy for both the ruminal microorganisms and the host animals. The extent and rate of digestion of corn grain depend on the chemical composition and architecture of the endosperm of corn grain. Information on digestible fractions and the digestion kinetics of corn grain is important to optimize ruminal fermentation and animal performance.;In the Cornell Net Carbohydrate and Protein System (CNCPS), carbohydrates are classified, based on the degradation rate, into A, B1, B2, and C fractions. Corn grain can be partitioned into ND soluble and ND insoluble fractions because of the small pool size and gas contribution from the ethanol soluble (A) fraction. The pool size, digestibility and gas yield of the digestible NDF fraction are relatively constant. It is not necessary to routinely analyze pool size and digestion kinetics of the digestible NDF fraction of dried corn. When dried corn was ground through a 1-mm screen, the average digestion rate of ND soluble and digestible NDF fractions is 16% h–1 and 11% h–1, respectively.;Thermo-mechanical treatments gelatinize starch granules and denature the storage protein, which enhance the digestion rate and digestibility of grains in the rumen. Pelleting did not alter the digestion kinetics of mixed concentrate. Extruding increased the digestion rate of the fast pool because more heat and shear forces are involved in the process. The changes in chemical composition due to extrusion (decreased soluble carbohydrate, increased lignin and ADIN) support the existence of Maillard reaction during extrusion. Formation of Maillard reaction products decreased the NDF digestibility, gas and VFA production in vitro.;The parameters of digestion are non-additive if there are associative effects between the ingredients in mixtures. The existence of associative effects influences the nutrient value of concentrate ingredients when they are mixed in the ration. Positive associative effects were evident for NDF digestibility when soyhulls was mixed with barley and corn. No associative effects were found when wheat shorts was associated with barley and corn. However, the gas production curves of fibrous by-products and starch-rich grain were additive. No associative effects were found in the in vitro gas production curves.