Particle size and digestibility

There are three main determinants of starch digestibility. In order of importance, they are particle size, kernel moisture and length of ensiling time for fermented corn. To help determine size, more and more labs are using corn silage kernel processing scoring.

“This makes sense given the variability in corn silage processing, which is influenced by equipment differences and varying kernel maturity over extended harvest windows,” Ramsey explains. Modification in roller-mill designs (e.g., shredlage) may reduce the need for this method in the future.

To help with adequate and consistent size, it’s good to raise questions about particle size of grain to everyone involved in the operation.

Some producers and nutritionists lead in regard to sensitivity to grain particle size distribution or establishing clear goals, such as having less than 5 percent whole kernels and 10 percent fines.

Like any silage, ensiling high-moisture corn requires good management during packing and storage. The degree of processing required differs for dairies and feedlots.


Dairies tend to process finer because of the faster rate of passage through a cow’s digestive system. The finer the feed is processed, the better it packs.

Particle size reduction facilitates air exclusion during packing and helps avoid air penetration into the exposed face during feeding.

Digestion site

“When discussing starch digestion, it is important to clarify if the lab method is addressing the rumen, the intestines or the total tract,” Ramsey states.

If starch is fermented in the rumen, microbes use the energy to synthesize protein for the animal to digest downstream.

When starch escapes the rumen and is digested in the small intestine, there is no energy loss that would come from ruminal fermentation or metabolism.

Therefore, responses in milk yield and in body energy reserves to site of starch digestion will vary with metabolizable protein status, intestinal digestibility of starch reaching the small intestine, value of an increased supply of metabolizable energy and the potential indirect effects of available energy and nutrients on energy partitioning by the lactating cow.

Ruminal starch digestion is typically lower for lactating cows than for feedlot steers presumably because dairy cows have a higher feed intake and consume more dietary forage – neutral-detergent fiber (NDF) – which increases ruminal outflow rates.

As a fraction of total starch digestion, the proportion of starch digestion that occurs post-ruminally from processed (flaking, high-moisture) corn grain is greater for lactating cows than for feedlot cattle.

Starch absorbed as glucose from the intestines can have more than 20 percent greater caloric value than starch fermented to volatile fatty acids within the rumen.

From an energy standpoint, total tract digestion of starch, not ruminal starch digestion, should be the primary focus.

Ruminal starch methods that increase ruminal degradation of starch generally increase the digestibility of residual starch that enters the intestines, although there will be a reduced supply due to more extensive ruminal digestion of starch.

Several reviews have suggested that intestinal starch digestibility decreases as starch flow to the intestines increases.

However, when calculated within a processing method, post-ruminal digestion does not decline as the passage of starch to the small intestine increases.

Neither dry-rolled nor whole corn is particularly well digested post-ruminally. Poor processing of corn kernels, which hydrate and escape the rumen in the liquid phase, may predispose cattle to jejunal hemorrhage syndrome if the reduced starch surface area limits pancreatic amylase activity, thereby allowing excessive amounts of starch to flow to the lower intestinal tract.

“This increases the substrate to support growth of toxin-producing organisms such as clostridia or aspergilli,” Ramsey explains.

“Assuming adequate particle size, it may be beneficial, in some feeding situations, to shift the site of starch digestion downstream to the intestines if the rumen microbial protein yield is adequate and acidosis is a concern.”

Changes during storage

Silage nutrients and digestibility are not static during storage. While most people may understand that microbial activity continues during storage, not everyone realizes all of the nutritional changes that may occur.

During storage, corn silage, snaplage and high-moisture corn will increase in starch digestibility. The higher the moisture level during storage, the more likely starch digestibility will rise over time.

Corn kernels are protected by a protein covering known as zein. These prolamin proteins cover the starch and protect it from being fermented by rumen microbes or digested by intestinal enzymes.

They are the reason growers must process kernels to expose starch for digestion. Zein protein becomes more soluble during storage, releasing more starch for potential rumen fermentation and intestinal digestion.

In addition, the breakdown of the zeins into soluble protein in the silage results in increased rumen-degradable protein for rumen microbial fermentation. The end result is that rumen microbes can access both the starch and protein more readily, reports Ramsey.

Accessing more starch

The result of silage storage is a “hotter” ration. This can lead to shifts in performance.

“As starch digestibility improves in the months after initial storage, producers may have the opportunity to back some of the expensive commodities out of the ration,” Ramsey says. “This can save money and allow them to fine-tune animal performance.”

Dairy producers may also have an opportunity to back off protein supplements since more microbial protein will be produced in the rumen.

For instance, if a ration developed in the fall calls for 2 pounds per cow per day of soybean meal, the operation may be able to cut back by a half-pound per cow per day in the spring.

Spotting the change

The first task is to identify when starch digestibility levels are rising.

“As previously mentioned, labs offer a seven-hour starch digestibility test that can show changes in starch digestibility in storage,” Ramsey says. “An operation can take advantage of this and conduct a test in the fall, and another on the silage in the spring, and adjust the ration.”

Another signal comes from a rise in soluble protein levels in the silage. “As storage time continues, rumen starch availability and rumen-degradable protein levels will increase,” Ramsey explains.

Snaplage undergoes similar changes as corn silage, gaining perhaps 2 percent starch digestibility per month during the first six months in storage.

High-moisture corn has a more pronounced increase in starch digestibility when ensiled at moisture levels above 25 percent.

At these moistures, the greatest increase in starch availability occurs during the first 30 to 45 days in storage. Digestibility will continue to drift higher more gradually for the remaining time in storage.

Digesting the digestibility

Knowing that starch digestibility will change over time in storage is crucial. Producers should take advantage of available tests to help determine starch digestibility throughout the storage period.

Also, monitoring processing in silage and particle size in high-moisture corn, along with analyzing a composite fecal sample from eight to 10 animals, may be the most economical way to begin troubleshooting where starch digestibility issues are originating.

Other lab methods offer an integrated approach to identifying the most important factors affecting starch digestibility; they also help quantify the increase in starch digestibility during storage and the affects of a fine particle size in unfermented corn.  end mark

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