The William H. Miner

Agricultural Research Institute

Miner Institute
Farm Report

FROM THE PRESIDENT'S DESK — UNDERSTANDING RUMEN FIBER DIGESTION

At the recent Cornell Nutrition Conference there was considerable discussion of our (in)ability to accurately predict rumen fiber digestion. In this era of high-priced grain and feeding higher-forage diets, our ability to predict forage digestibility and cow response is more crucial than ever.

An important, and often overlooked, component of fiber digestion is the interaction between larger and smaller particles within the rumen. Greater than 50% of NDF particles in the rumen are below the critical size for escape – in other words they are sequestered within the rumen digesta mat by larger particles. This allows for more complete fiber digestion and greater energy available to the cow. Our current nutrition models do not account for sequestration and selective retention of feed particles. They assume that all particles have the same probability of escape — which is simply not true.

A good example of sequestration of small fiber particles is the interaction between long forage particles and smaller particles of non-forage sources of fiber (i.e. fibrous byproducts such as soybean hulls, beet pulp, or distiller’s grains). A series of studies conducted at the University of Nebraska evaluated the effect of manipulating rumen digesta mat consistency with coarsely chopped dry hay on the passage rate of various non-forage fiber sources. In one study, increasing the rumen digesta mat with added chopped alfalfa hay resulted in a 35% reduction in the passage rate of the small particles of wet corn gluten feed and a 37% increase in ruminal NDF digestion which was associated with 11 lb/day greater milk yield. Similar responses were observed for soybean hulls. It is clear that smaller particles of highly digestible fiber need to be slowed and retained in the rumen in order to achieve the most efficient use of this fiber. Interactions of this magnitude between fiber sources of varying particle size need to be incorporated into nutrition models – and presently they are not.
What about forage particles of varying size? Earlier this year, Michigan State researchers compared the rumen dynamics of cows fed alfalfa and orchardgrass silage. They observed that for both silages over 55% of particles in the rumen were less than 2.36 mm, indicating that particle size itself was not the limiting factor for passage. Orchardgrass increased the amount of large NDF particles in the rumen compared with alfalfa, which likely entrapped smaller NDF particles resulting in a slower passage rate that was observed for grass. The bottom line is that sequestration of smaller particles was greater for orchardgrass than for alfalfa which resulted in greater rumen fill. This interaction is commonly observed in the field as well.

How does corn silage fit into this discussion? At Miner Institute we have been evaluating the effect of NDF digestibility on how quickly particles break down during chewing and consequently pass out of the rumen. BMR corn silage often doesn’t elicit the same chewing response as conventional corn silage. It also turns over in the rumen more quickly than conventional silage, resulting in greater dry matter intake. Understanding the dynamics of rumen passage and digestion of corn silage and other forages is a high priority for us. If we expect to successfully feed higher forage diets, then we must improve our nutrition models and our ability to predict rumen fiber digestibility.

— Rick Grant

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The Miner Institute Farm Report is written primarily for farmers and other agricultural professionals in the Northeastern U.S. and Eastern Canada. Most articles deal with dairy and crops topics, but also included are articles dealing with environmental issues and global agriculture as well as editorial commentary.

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