Digestibility and Physical Characteristics of Feed Have Important Influences on Both the Rate of Particle Passage from the Rumen and the Rate of Feed Intake
As indicated earlier, feed does not leave the rumen until it is broken down into small particles. Microbial action and remastication (as discussed later) are primarily responsible for particle size reduction in the rumen, and the rate of breakdown of fiber is chiefly a function of its digestibility.
Poorly digestible fiber takes longer to be broken down sufficiently to enter the zone of potential escape compared with fiber of greater digestibility. This means that poorly digestible fiber remains in the rumen longer than fiber of greater digestibility. Because there are fixed limits to the volume of the rumen, the rate of feed intake cannot exceed the rate of ingesta outflow, therefore, intake of poorly digestible feeds is always less than intake of highly digestible feeds.Feed preparation can influence this relationship. Chopping or grinding of poorly digestible forages increases their rate of passage from the rumen because less particle size reduction is necessary before pieces can pass into the omasum. Chopping or grinding also usually increases the amount of material that an animal can eat because the rumen throughput is increased. Often, however, digestibility is decreased by chopping or grinding of forages, because the duration of exposure to microbial action is reduced as a result of rapid passage of feed through the rumen. Thus, physical form (length) and digestibility each have an effect on rate of passage from the rumen and also on feed intake. In general, forage material of relatively high digestibility’ has a rumen half-life of approximately 30 hours, whereas poorly digestible material has a half-life of up to 50 hours.