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Understanding starch digestibility inside and out

Pat Hoffman Published on 06 May 2015

We feed lactating dairy cows starch sources that may be ground, flaked, processed, rolled or ensiled. In addition, we store feeds containing starch in bags, bunkers and bins at different moisture contents for different lengths of time.

With all of the possible combinations, trying to figure out starch digestibility in the feeds we put in front of dairy cows can seem like a daunting task.

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Despite years of science that have defined physical and chemical factors influencing starch utilization by dairy cows, dairy producers and nutritionists have been challenged to integrate physical and chemical composition of feed grains into diet formulations.

In the last five years, scientists from our land-grant universities have taken a bit of mystery out of the equation. It seems – regardless of the starch source – the key to figuring out digestibility is to understand how starch is protected inside and out … literally.

Outside protection of starch

Let’s tackle the outside first. Almost all the starch we feed to dairy cows comes from seeds. Seeds have seed coats, which have some fascinating properties that can greatly influence starch digestibility in dairy cows.

For example, the seed coat of corn is poorly digested because rumen bacteria cannot attach themselves to it. Furthermore, enzymes that can degrade starch (such as the amylase in human saliva) cannot degrade the seed coat of corn. Thus, a whole kernel of corn can pass through a dairy cow without being digested at all.

This is why we grind or process corn. The goal is to reduce the effects of outside protection of starch, breaking apart the seed coat. This isn’t a new concept as dairy producers have been grinding corn for years to enhance its starch digestibility.

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What is new is detailed quantification of corn-grinding efficiency as it relates to starch digestibility.

In the old days, we defined ground corn as either coarse, medium or fine-ground. Today, our mills and consultants routinely measure mean particle size or natural logarithm units of mean particle size. The results are used to customize starch digestion needs for specific dairy cow groups.

For example, one dairy may need to feed corn ground to 400 microns because a diet low in dietary starch is required. Another dairy may require corn ground to 800 microns because the remaining inventory of starch-containing feeds consists of rapidly degradable starch sources.

We also reduce outside starch protection via kernel processing in corn silage. The new kernel processing rolls are fitted to corn silage chopping equipment. Early research data from these new roll types suggests better kernel processing while increasing forage particle length.

In addition, the measurement of corn silage-processing efficiency has increased, and new methods on the horizon will show how well our corn silage choppers are processing kernels.

Inside protection of starch

Attacking the outside doesn’t do the full job of exposing the seed’s starch to rumen bugs. In all seeds, starch is encapsulated in waterproof proteins called prolamins. Rumen bacteria must first digest these prolamins, and that’s a slow process. Thus, starch is protected from the inside, too.

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The proteins that encase starch are slowly degraded during fermentation. However, the relationship between fermentation and starch digestibility comes with good and bad news.

First, the good news: Starch digestibility of feeds like corn silage and high-moisture corn (HMC) will improve dramatically with advancing fermentation time. Total tract starch digestibility may increase 10 percentage units within a four-month storage period.

The bad news: Research has clearly defined that the process is slower than originally hypothesized. And – even worse – if corn silage or HMC is ensiled too dry, it may take more than a year to improve starch digestibility to a desired level.

That said, if we read our normal forage and grain testing reports carefully, we can almost predict starch digestibility. The key is to understand the shape and form of the protein in corn silage or HMC.

Here’s how it works: The protein markers we use are soluble protein and ammonia nitrogen. If the proteins in corn or corn silage are soluble, it means they were degraded by fermentation and are no longer capable of encasing the starch. In this way, soluble protein can be used as a marker of starch digestibility in HMC and corn silage.

A number of studies published in the last five years have observed that as soluble protein in corn silage and HMC increases, starch digestibility also increases. Scientists have even taken electron microscopes and shown that when these proteins become soluble, the starch granules fall apart.

The rules of thumb are pretty easy to follow, too. Prior to ensiling, only 20 to 25 percent of the protein in corn is soluble, and the starch is encased in waterproof proteins. When soluble protein in HMC or corn silage is greater than 50 or 60 percent, respectively, most of the proteins encasing starch are degraded, and starch digestibility potential is likely quite good.

The other protein marker in HMC and corn silage is ammonia nitrogen, which has some real advantages over soluble protein as a marker for starch digestibility.

Mainly, it is easier and faster to measure. Unfermented corn kernels do not have any ammonia. This means if we find ammonia nitrogen in HMC, it has to come from the degradation of proteins that encase starch. Using ammonia as a marker for starch digestibility in corn silage is not as straightforward, but new information is sure to come in this regard.

Again, the rules of thumb are not hard to remember. HMC with less than 1 percent of the total nitrogen (or protein) as ammonia nitrogen indicates the degradation of starch-matrix proteins is minimal. Ammonia nitrogen levels greater than 5 to 7 percent of total nitrogen in HMC or corn silage indicate extensive protein degradation has occurred, and starch digestibility is likely quite good.

Using the information available

Our feed and forage testing laboratories have great tools such as seven-hour in-vitro starch digestibility or fecal starch analysis to help tailor rations for our lactating dairy cow diets.

However, new university research shows that we should pay careful attention to particle size and to the types of proteins in our starch-containing feeds. If we know how to carefully interpret the markers of outside and inside protection of starch, then starch digestibility of feeds becomes much less mysterious. PD

pat hoffman

Pat Hoffman
Dairy Technical Specialist
Vita Plus

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