Does Vitreous Starch Content of Corn Hybrids Matter in Livestock?
Some producers claim that corn hybrids with higher vitreous endosperm will produce starch that is less digestible in the rumen. However, when hybrids are harvested as silage – before full kernel maturity – there is only very minimal difference between vitreous (hard or flinty corn) hybrids and floury (soft or dent corn) hybrids.
Vitreous versus Floury Starch
Vitreous endosperm is the higher density, yellow-colored starch granules on the outer edges of the kernel. They are tightly bound in a starch:zein protein (prolamin) matrix. This matrix becomes more prominent as the kernel approaches combining maturities (post black layer). Popcorn is an example of primarily vitreous starch.
Floury endosperm has whitish starch granules in the center of the kernel that are more loosely bound in a starch:protein matrix. Dent corn derives its name because this softer starch “dents in” at the top of the kernel as it matures.
Floury hybrids typically have lower kernel density and test weight. Seed company data on vitreousness, density or test weight is determined at dry-grain harvest maturity. This does not reflect kernel characteristics when harvested at silage or high-moisture kernel maturities.
|Circled area represents typical corn silage harvest maturities.
How Do Prolamins Impact Starch Digestion?
Prolamins are starch-encapsulating storage proteins (e.g. zeins). In some studies, these are shown to interfere with rumen bacterial access to starch granules. Concentrations of corn prolamins tend to be higher in vitreous endosperm than in floury endosperm. However, minimal prolamin differences exist in commercial hybrids before the black layer stage of kernel maturity typical of kernels at silage or high-moisture corn harvest. It’s best to rely on direct lab analysis for starch digestibility rather than predict it from prolamin measurements.
Reviewing Studies on Starch Digestibility of Higher Vitreous Hybrids
Recent studies have furthered the understanding of the mechanisms of starch digestion, but be careful when extrapolating these research results to “real world” feeding situations. Some of these studies investigated starch digestibility using extremes in vitreousness, ranging from 3% to 66% or from 25 to 66%. High yielding commercial hybrids have a much narrower range in vitreousness (typically 55% to 65%). Studies with hybrids containing more typical ranges in vitreousness (55%, 61%, 63% and 65%) show no significant impact in ruminal starch disappearance even when fed as dry rolled corn.
Finally, most studies ignored looking at starch digestion and only focused on the rumen when the small intestines are also capable of starch digestion, and with less gaseous losses than occur in the rumen. It is possible to evaluate total tract starch digestion by analyzing fecal starch. If fecal starch content of lactating cows is less than 4-5%, excellent starch digestion occurred in the animal in the small intestines. Total tract starch digestibility typically exceeds 96% for adequately fermented silage or high-moisture corn.
Effects of early harvest
As kernels mature from half milk line to blacklayer, weight can increase 24% and starch by 27%, showing that premature corn silage harvest drastically reduces starch content.
Implications for Dairies Feeding Vitreous Versus Floury Hybrids
It is recommended that dairies focus attention on kernel moisture/maturity, particle size and the increase in starch digestibility associated with duration of ensiled storage. The University of Wisconsin Grain Evaluation System (V2.0) lists the most important drivers of ruminal starch digestion as the following:
- Moisture (for example, length and extent of fermentation)
- Particle size (especially important to monitor processing in corn silage)
- Prolamin content, but only for corn fed as dry, combine-maturity corn. Research from Europe indicates fine-grinding of dry corn (common in dairy diets) will remove most of the negative impact of high prolamin, vitreous starch.
Also remember using past measurements of starch availability to select a silage hybrid is of questionable value because growing environment has a major impact on ruminal starch digestion, just like it does on fiber digestibility.
The foregoing is provided for informational purposes only. Please consult with your nutritionist or veterinarian for suggestions specific to your operation. Product performance is variable and subject to a variety of environmental, disease, and pest pressures. Individual results may vary.