by P. C. Hoffman and R. D. Shaver
The authors are in the Department of Dairy Science, University of Wisconsin-Madison.

As the old saying goes, corn is corn is corn, right? Let's take a closer look.

Traditionally, the variation in nutrient composition within dry or high-moisture corns was assumed to be minimal. It was common to use table values to define the nutritional composition of corn for ration- balancing purposes. But, many of you have noted variable milk yield and fat test responses when feeding high-moisture corn. This is a paradox, but new research can help explain what is occurring.

Different corn hybrids with varying endosperm properties can be ensiled between 20 to 40 percent moisture; with or without inoculants; at ambient temperatures of 10° to 70°F; ensiled whole or ground; treated with or without organic acids or microbial inoculants; stored in bags, bunkers, or oxygen-limiting silos; and allowed to ferment for one day up to a year or more, and still be classified as "high-moisture corn." So why would we expect all high-moisture corn to be of equal feeding value?

Dry versus high-moisture
The energy (NEL) value of high moisture is estimated to be 5 to 10 percent greater than dry corn of similar origin and particle size. The greater energy values for high-moisture corns primarily are due to greater total tract starch digestion for high-moisture corn. The greater digestibility of high-moisture corn starch often results in improved milk production.

Ensiling of corn also alters the site of starch digestion. More of the starch is digested (65 to 85 percent) in the rumen with high-moisture corn than with dry corn. Lactating cows digest about 50 percent of the starch in the rumen for dry corn with the rest of the starch that is digested being digested in the small and large intestines. Thus, the rate of starch digestion in the rumen is faster for high-moisture corn than dry corn. Sometimes, an excessively fast starch digestion rate may reduce rumen pH, milkfat test, dry matter intake, or a combination of these.

New research has demonstrated that the digestibility of high-moisture corn in storage can change dramatically over time. Also, our traditional forage-testing procedures do not work very well for evaluating the feeding value of high-moisture corn.

To understand the dynamics of assessing the feeding value of high-moisture corn we need a basic understanding of corn chemistry. The kernel is comprised of three basic parts . . . seed coat (pericarp), germ, and endosperm. The endosperm represents about 75 to 80 percent of the corn kernel by weight and contains the starch. However, the starch in the endosperm is not alone. It is in a matrix with storage proteins called prolamins or zein protein. These proteins are a natural plastic that are not soluble in water or rumen fluid. It is these proteins that encase the starch.

This mix of starch and protein often is referred to as the starch-protein matrix. More vitreous portions of the starch-protein matrix are easily recognized as the "little yellow chips" in ground dry corn. These proteins bind starch granules together, and the degree of binding alters the grinding efficiency of corn and surface area for bacterial attachment in the rumen.

In corn, these proteins (zein) comprise 50 to 60 percent of the total protein. Zein proteins are not inside the starch granule but are on the surface or the exterior of starch granules. We can see differences in the starch-protein matrix in dissected kernels of yellow dent corn. Starch-protein matrices appearing yellow, shiny, or glassy are classified as translucent or vitreous.

In a recent study, our research group monitored the fate of zein proteins in the starch-protein matrix in high-moisture corn over a long ensiling period (240 days). Fermentation reduced all zein proteins in the starch-protein matrix by between 10 and 40 percent. Because zein proteins are responsible for cross-linking starch granules together, the degradation of these proteins by silage bacteria (proteolysis) in high-moisture corn caused clusters of starch granules to disassociate (fall apart) as a result of fermentation. Thus, high-moisture corn appears to be different from dry corn as the result of a simple process.

When high-moisture corn is well fermented, starch granule clusters fall apart into individual starch granules because there is no functional hydrophobic protein left to hold them together. In dry corn or in poorly fermented high-moisture corn, starch granules are stuck together in clusters, and there is less surface area for rumen bacteria to digest starch.

A moving target
We likewise observed this effect. As starch granule clusters fell apart, the in vitro digestibility of high-moisture corn improved continually over the entire ensiling period. This new observation suggests that the digestibility of high-moisture corn is ever changing with ensiling time causing the digestibility of high-moisture corn to be a moving target rather than a fixed value.

Many of you and your nutrition consultants understand the role that fiber (NDF) and lignin play in forage digestibility, but there is no NDF or lignin in the endosperm of corn. The starch granules in corn are encased together by hydrophobic proteins, not fiber.

Highly vitreous corn hybrids (higher starch matrix protein content) may require ensiling at higher-moisture contents and (or) may require greater processing. The starch-protein matrix in high-moisture corn is altered by the fermentation process, but it must be good fermentation. Ensiling high-moisture corn too dry, too coarse, or at cold temperatures can delay the degradation of the starch-matrix proteins greatly causing the high-moisture corn to feed like dry corn for long periods of time.

What you can do
Prior to feeding, you should track and monitor the amount and form of protein in high-moisture corn. First, measure the crude protein content of corn, and take the value seriously. Numerous research projects have demonstrated a negative relationship between crude protein and starch degradability. The relationship is logical because the higher the protein content, the more likely hydrophobic proteins are encasing the starch.

The crude protein content of corn hybrids ranges from 7.5 to 12.5 percent (dry matter basis). Two other forms of protein probably should be monitored in high-moisture corn . . . soluble protein and ammonia.

Prior to ensiling, about 20 percent of the protein in corn is soluble in a buffer solution. In extensively fermented high-moisture corn, more than 70 percent of the protein may be soluble. The change in soluble protein is a marker of the degradation process of the starch matrix proteins. As silage bacteria degrade hydrophobic proteins, they become more soluble in buffer solutions.

Ammonia also may be an excellent marker of the status of starch-matrix proteins in high-moisture corn. At ensiling, corn virtually has no ammonia. The appearance of ammonia in high-moisture corn means that amino acids are being degraded by silage bacteria. In extensively fermented high-moisture corn, ammonia may represent more than 7 percent of the total protein. High-moisture corns with less than 2 percent of the total protein as ammonia indicate the degradation of starch-matrix proteins probably is minimal.

To sum up, we are learning a great deal about the chemistry of corn. We now know why all corn, especially high-moisture corn, is not of equal feeding value. Our traditional forage-testing procedures may not be well suited for feed grains. New procedures are needed to help better explain the different results often reported in the field.

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