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Rumen fermentation of starch: Where the money is

Steve Massie Published on 06 April 2011

The starch content of a dairy ration, how fermentable this starch is in the rumen and the total tract starch digestibility have a huge influence on many facets of a dairy operation.

Feed cost, income-over-feed cost, feed efficiency, milk production, microbial protein production, peak milk, lactation persistency, ketosis, fatty livers, displaced abomasums, colostrum quantity, butter fat, milk protein, weight loss in fresh cows, weight gain in late-lactation cows and heifer growth are all partially influenced by the level of starch in the diet and its level of digestibility.

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With the recent rapid increase in cost of starch sources, it is a good time to review where we are as an industry and how we can use available technologies to increase our bottom line.

Total starch is the number that most rations and forage tests report, but it is a partial answer at best. For example, a ration containing 30 percent total starch should initially be viewed as a high-starch diet. But what if whole shelled corn was the only starch source in this ration?

A second thought might then be that the ration will never support much milk because most of the corn will pass straight through the cows, undigested, and end up on the floor. Not all corn is alike, and not all starch sources are created equal.

The physical (type and extent of processing, moisture content, etc.) and chemical (degree of vitreousness) characteristics of a starch source have a profound impact on its rumen fermentability and total tract digestibility.

Rumen fermentation of the starch is where the money is. Safe levels of fermented starch will drive microbial protein production, facilitating lower protein diets with less need for expensive rumen undegradable proteins.

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We want to optimize the fermentable starch because it is typically the primary ration ingredient used to produce propionate, which the cow utilizes as a primary driver for milk production. Because of this need to know how much of the ration starch is rumenally fermentable, several lab tests have been developed to help nutritionists maximize fermentable starch without upsetting the health of the rumen.

For years nutritionists have been monitoring corn moisture and particle size to help determine how much of and where the starch was going to be digested. We know that wetter and smaller particles are more prone to rumen fermentation, but how fast and to what extent were still educated guesses.

Bushel weight has been used as a proxy for starch digestion in some grains. Higher bushel weight in corn is somewhat related to the level of floury (easily digested) or vitreous (hard to digest) corn. Bushel weight is also monitored in steam-flaked grains as a digestibility proxy.

Energy required for grinding gives an idea of digestibility (harder to grind, more electricity required, is associated with lower digestion). Another monitor has been the height of ground grain in a test tube (Stenvert Hardness). Softer/easier to ferment grain is “fluffier” and rises to a higher level in the tube.

Rumen fermentation amounts and rates are based on particle size, moisture and chemical properties. The amount fermented in the rumen is also controlled by the rate of passage out of the rumen. Starch is held by a prolamin protein matrix.

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It’s like eggs in a carton; the eggs are the starch and the carton is the protein matrix. Different corn varieties demonstrate different rates and extents of fermentation based on this protein-to-starch matrix even when they have the same particle size and moisture content.

Chemical methods for determining the prolamin protein content have been developed. This approach is used in the Wisconsin Grain Evaluation system.

The Degree of Starch Access test (DSA) developed at the University of Wisconsin is available commercially. This procedure works for grain corn only, not corn silage. The ability to differentiate between similarly degradable starch sources is limited. It has not been widely adopted.

Enzymatic tests for starch degradability are commercially available. These assays report the amount of starch disappearing in an enzyme mixture after a specific time period. The similarity to rumen conditions has been questioned.

Some labs offer an in vivo starch analysis. The ground grain is fermented in bags in the rumen and then subjected to acid/enzyme treatment to mimic post-rumenal digestion. In theory this should give a useful result. The analysis takes several days to complete in the lab.

Several labs have recently introduced the 7-hour in vitro starch digestion (IVSD7Hr) test commercially. This has been used in research for some time. This looks at how much of the starch is digested in a rumen fluid-filled test tube in 7 hours.

Equations have been developed to estimate how fast the starch is fermented in the rumen based on those results. Grinding the samples to a size that attempts to mimic chewing is important and a point of ongoing discussion among labs.

Gas release from in-vitro fermentation has been investigated in research labs for a number of years. Just recently it has become more commercially available. The starch source is immersed in rumen fluid and the amount of fermentation gas released is recorded every few seconds over a 24-hour period.

This analysis generates not only the extent of fermentation but also the rate that the gas is released. These results provide an estimate of how fast the starch will be digested and how quickly it starts (lag time). The challenge with this procedure is establishing uniform procedures across labs.

Also, the fermentation gas produced is a result of fermenting all material in the test tube (fiber, sugar, pectin, etc.), not just starch. Mathematical treatments and assumptions are used to help interpret the results.

All of the starch digestibility assay methods discussed attempt to provide the nutritionist with the information needed to optimize starch fermentation. An advanced ration-modeling program (CPM, CNCPS, AMTS, NDS, etc.) helps to fine-tune a balanced diet that maximizes microbial protein production, better starch utilization and higher feed efficiency.

Proper implementation of these considerations should help dairy producers feed more cost-effective rations. PD

Photo from thinkstock.com.

Steve Massie is a nutritionist withRenaissance Nutrition. Email Steve Massie.

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