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The ups and downs of silage digestibility

David Weakley for Progressive Dairyman Published on 30 September 2016
Ups and downs

Depending upon how much silage inventory carryover your farm has on hand, you may be looking at making the switch to new-crop corn silage very soon, if you haven’t already. When making these transitions, it’s important to keep a close eye on starch content and digestibility.

During the course of corn silage storage, the total amount of starch in the pile will not change much (but can vary throughout the pile). What does change, however, is the digestibility of the starch. It will increase as the ensiled time increases.



Typically, starch digestibility increases over the next 90 to 180 days, and by 180 days, the digestibility will usually plateau. On average, starch digestibility can increase 15 percentage units during this time.

The upsurge in digestibility occurs because of the breakdown of prolamin proteins that protect the starch granules from microbial digestion. Think of prolamins as mortar and starch granules as bricks in a wall.

Proteolytic enzymes in the silage pile break down the prolamins holding the starch, or bricks, together during ensiling. This process allows for easier access to starch granules during microbial digestion in the rumen.

Know starch digestibility at every point

During the transition from fall to summer, the increase in starch digestibility can have a huge impact on ration balancing. Particularly in the summer, not having a handle on starch digestibility can lead to lower feed intakes and milkfat depression if there is an overload of starch digestion in the rumen. Measuring the changes in starch digestibility over time will help avoid these effects.

The best tool to help measure changes is a starch digestibility test. This test tells you how much starch will be available in the cow’s rumen from the corn silage before incorporating it into a ration. Knowing the starch digestibility is vital to avoid underfeeding or overfeeding starch.


There are multiple ways to test for starch digestibility. Regardless of the method you use, what is most important is that the results are accurate, quick and you can easily apply them in ration formulation.

You can take the sample anytime, but because of the effect ensiling time has on starch digestibility, taking the test over time once the storage unit is open becomes key. Once you take an initial sample, testing should continue every two weeks.

Economics for your herd

You can’t manage what you don’t measure. By knowing the starch digestibility of corn silage, adjustments to the ration can be made, while considering prevailing inventory and economics, to keep intake, milk production and profitability up.

For example, let’s say you were able to plant a sufficient amount of corn for a high-corn silage ration diet – over 22 pounds of corn silage dry matter (DM) per cow. Thanks to a good growing season, the resulting tonnage was enough to feed 25 pounds of silage DM per cow and begin feeding the new corn silage in the fall.

Let’s assume the average starch content of the current corn silage being fed is about 35 percent, with a ruminal starch digestibility of 75 percent. By the time summer rolls around, however, the starch digestibility is now 90 percent.

This will increase the ruminal digested starch amount by 1.3 pounds, likely creating an overload of available starch in the rumen. Your first thought would likely be to reduce the amount of corn silage in the ration. However, with so much inventory, this doesn’t seem like the most economical choice since it is a feedstuff that is difficult to sell and has already been paid for.


Assuming an average dry matter intake (DMI) throughout the year of 55 pounds per cow, this ration likely contains not only 25 pounds of corn silage DM, but 8 pounds of hay DM, 8 pounds of high-protein supplement DM and 6 pounds of ground corn DM, leaving 8 pounds of DM to fill out the ration.

In today’s economics, the least-cost solution could be more corn, but that isn’t an option in our example with so much available starch already in the corn silage. Adding the additional starch would only make the starch overload worse. The best option to fill the last 8 pounds of the cow’s ration is with a combination of high-fiber (low-starch) byproducts and available forages.

Incorporating available forages for the last 8 pounds of the diet can work – but only to an amount that will not exceed rumen fiber fill limits. After that, you will then need to incorporate byproducts with little to no starch content to fill those remaining pounds. These can include soyhulls, brewers grains or corn gluten feed. This, however, will increase purchased feed costs.

Current market conditions

Being mindful of dietary limits for both rumen-degraded starch and rumen-undigested fiber (related to rumen fill) and the prevailing costs for byproducts and grains, you can better design your planting decisions for the next season. Planning for a different forage program could give you more options to fill those additional 8 pounds as mentioned in the above example ration.

To help meet this goal, as you move into 2017, you could consider switching some of the acres assigned to corn silage to other highly digestible, lower-starch forages. Most of the time it is more economical to grow your own forages, in addition to using economical starch sources, rather than purchasing byproduct feeds.

Planning ahead can reap economic benefits. Do some calculations after harvest with your nutritionist to determine the starch and fiber digestibility limits on a wide variety of diet scenarios.

These calculations will help determine what makes the most sense with prevailing marketplace economics, ingredient inventory and crop land use. It could also help reduce the need to purchase as many byproducts on the go, which can be more expensive and weigh on the pocketbook when milk margins are tight.

To avoid limiting options when formulating the ration, keeping an eye on inventories and economics is key. Being able to measure the starch and fiber digestibility of ensiled corn silage in a ration will help reduce unwanted effects of a starch overload such as reduced intake and milk production, especially when summer comes around again. With these measurements, you’ll be more able to monitor and control their effects.  end mark

David Weakley has a Ph.D. in animal nutrition from Oklahoma State University and an M.Sc. in dairy science from the University of Wisconsin – Madison. He is employed by Forage Genetics International and has more than 30 years of experience in ruminant nutrition.

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David Weakley
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