Submitted to: Ruminant Nutrition Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 3/29/2007
Publication Date: 3/29/2007
Citation: Mertens, D.R. 2007. Grass forages:Dynamics of digestion in the rumen. Ruminant Nutrition Conference Proceedings. p. 6.
Technical Abstract: Grasses have been the mainstay of the diet of grazing ruminants for thousands of years. Ruminants evolved a multi-compartment stomach to provide an environment for microbial fermentation of the complex polysaccharides that comprise the carbohydrates in plant cell walls (what we nutritionists call fiber). Selective retention of large particles is a key factor in ruminal function to allow adequate retention time for digestion of slowly digesting fiber. Ruminal contents consist of an upper large particle mass that is regurgitated and chewed to reduce particle size and a lower liquid layer containing the small particles that can escape the rumen. Maintaining the bi-phasic nature of the rumen depends on the intake of long, effective fiber that simulates chewing and the secretion of salivary buffers to maintain ruminal pH above 6.0. To accurately predict digestibility, rumen models must have at least two pools of particles to represent the selective retention of large particles of fiber and passage of small particles. Grasses have long slender leaves that may result in slower rates of passage in the rumen and these long slender particles may cause some particle size separation methods to underestimate the proportion of large particles entering the rumen. To adequately quantify the digestion kinetics of forages, the process must be separated into lag phase, indigestible NDF and potentially digestible NDF. The potentially digestible NDF fraction is the only portion of fiber that can be described by first-order kinetics with a constant fractional rate of digestion. Indigestible NDF will never be digested in an anaerobic environment even after infinite time and is distinctly different from undigested NDF measured by animals or in fermentations less than 4 days. Indigestible NDF must be measured by long fermentation times or estimated by nonlinear extrapolation and subtracted from total NDF to estimate the potentially digestible NDF. Indigestible NDF is correlated to lignin concentration, but the relationship may not be consistent. Digestibility at maintenance levels of intake can be estimated by a simple summative equation that contains digestible NDF and digestible neutral detergent solubles. Indigestible NDF and neutral detergent solubles are ideal nutritive entities with uniform availability across forages. Potentially digestible NDF and total NDF have variable digestibilities among feeds, but equations using potentially digestible NDF and indigestible NDF can predict most of the variation in total NDF. The dynamic computer model, Karoline, indicates that indigestible NDF and rate of digestion of the potentially digestible NDF are the most important feed characteristics affecting nutrient supply to the cow. Rumen residence time is also an important factor related to intake and digestibility that may be a function of both the animal's physiological status and the physical characteristics of the forage. The NDF concentration of grasses can be used to formulate rations that will maximize forage inclusion while allowing the animal to achieve its target milk production. The effects of changing the digestibility of NDF can be incorporated into the NDF system for formulating rations.