Starch digestibility: Can we make a difference?

By Pat Hoffman, Vita Plus dairy technical specialist
Given this year’s difficult growing and harvest conditions, starch is at the top of everyone’s minds.  Many producers are short on carry-over inventory, which means starch digestibility will be lower than ideal in many situations this year.  As they now move to high moisture corn (HMC) harvest, many producers are wondering what they can do to compensate for lower starch digestibility levels.

Regardless if it’s corn silage or HMC, to understand starch digestibility, we need to understand how starch is protected in the kernel.

On the inside
Within the seed, starch is encapsulated in waterproof proteins called prolamins.  To access the starch, rumen bacteria must first digest these prolamins, which is a slow process.  During fermentation, the prolamin proteins slowly degrade.  The relationship between fermentation and starch digestibility comes with good and bad news.

The good news is starch digestibility will improve dramatically with advanced fermentation time.  Total tract starch digestibility may increase 10% within a four-month storage period.

The bad news is research has clearly defined that the process is slower than originally hypothesized.  Even worse, if corn silage or HMC is ensiled too dry, it may take more than a year to improve starch digestibility.

With that said, if you read your normal forage and grain testing reports carefully, we can almost predict starch digestibility using the markers for soluble protein and ammonia nitrogen.  If the proteins in corn or corn silage are soluble, it means they were degraded during fermentation and are no longer capable of encasing the starch.  As soluble protein or ammonia increase, starch digestibility increases.

You can also follow these rules of thumb.  Prior to ensiling, 20% to 25% 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%, respectively, this means most of the proteins encasing the starch have degraded and starch digestibility potential is likely high.

On the outside
Every seed has an outer seed coating with properties that neither rumen bacteria nor enzymes can degrade.  Thus, a whole, unprocessed kernel of corn can pass all the way through a dairy cow without being digested at all.

Grinding or processing corn reduces the effects of the seed coating.  Corn used to be classified as either coarse-, medium- or fine-ground.  Today, mills and nutritionists routinely measure mean particle size to customize starch digestion needs for specific dairy cow groups.

When setting your grind size, think about what you need on your farm.  For example, one dairy may need to feed corn ground to 400 microns because a diet requires low dietary starch levels.  Another dairy may require corn ground to 800 microns because the remaining inventory of starch-containing feeds consists of rapidly degradable starch sources.

Using the information available
Feed and forage testing laboratories have great tools, such as a 7-hour in vitro starch digestibility measurement or fecal starch analysis, to help tailor rations for our lactating dairy cow diets.  In the short term, if starch digestibility is not ideal, increasing starch in the ration may be necessary.  In the long term, planning for sufficient carry-over inventory will provide a lot more flexibility.