Vitreous Starch in Corn Hybrids
What is Vitreous versus Floury Starch?
- Vitreous (also called hard, or flinty) endosperm are the higher density, yellow-colored starch granules on the outer edges of the kernel which are tightly bound in a starch:zein protein (prolamin) matrix. This matrix becomes more prominent as the kernel approaches grain harvest maturities (post black layer). Popcorn would be considered nearly 100% vitreous starch.
- Floury (also called dent or soft) endosperm has whitish starch granules in the center of the kernel 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.
- More floury hybrids typically have lower kernel density (more air space between starch granules) and test weight; however, only about 40% of the variation in grain density can be attributed to test weight due to influences of kernel size, shape, maturity, germ content and pericarp slickness on test weight measurements.
- Seed company data on vitreousness, density or test weight is typically determined at dry-grain harvest maturity and is not necessarily related to hybrid kernel characteristics when harvested at silage or high-moisture corn maturities.
What are Prolamins and How Do They Impact Starch Digestion?
- Prolamins are the starch encapsulating storage proteins like zeins, and other proteins (albumins, globulins,
glutelins) shown in some studies to interfere with rumen bacterial access to starch granules. Corn prolamins tend to be in higher concentrations in the vitreous endosperm than in floury endosperm. Minimal differences exist for prolamin content in commercial hybrids harvested pre-black layer.
- The difficulty in interpreting starch digestibility studies is that floury hybrids tend to produce more fines during rolling or grinding and these fines readily slip through pores of in situ digestion bags thus making it appear like floury endosperm is more ruminally available.
- Most studies to date lack detailed chemical and physical analysis of grain samples before processing (starch
content, kernel size, absolute density) and after processing (geometric mean particle size and distribution,
exposed surface area), thus it is difficult to draw valid conclusions from the current published literature.
What About Recent University Studies Showing Reduced Starch Digestibility with Higher Vitreous Hybrids?
- Recent studies have furthered the understanding of the mechanisms of starch digestion but caution is needed when interpreting results based on the kernel handling and vitreous ranges of hybrids being tested.
- For example, extrapolating results from well-designed and executed studies on kernel maturity (Correa et al., 2002; Ngonyamo-Majee et al., 2008) to the feeding of fermented high-moisture corn or corn silage is open to question when kernels were assayed as unfermented grain and not exposed to the modifying effects that the fermentation process can have on the starch:protein matrix.
- Other studies have investigated starch digestibility using extremes in vitreousness ranging from 3% to 66% (Taylor and Allen, 2005a,b,c) or from 25 to 66% (Allen et al., 2008) of the starch being vitreous. Although such wide extremes in vitreousness (and prolamin content) may aid in the understanding of how one specific mechanism can limit starch digestion, caution should be exercised when applying these findings (or milk production expectations) to field situations where rations are built around commercial hybrids with a much narrower range in vitreousness (typically 55-65%).
- Studies (Corona et al.,2006) with hybrids exhibiting more typical ranges in vitreousness (55, 61, 63, and 65%) showed that increasing vitreousness of dry rolled corn failed to significantly impact ruminal disappearance of starch.
- Finally, starch digestion in the small intestines is ignored in most studies that would contribute to total tract starch disappearance, especially with dry (unfermented) corn grain.
What is the Impact of Fermentation on Starch Digestibility?
- Vitreousness has little, if any, impact on the digestibility of starch from corn fermented as silage or high-moisture grain.
- Microbial activity during fermentation and the chemical action of various fermentation end-products (acids, yeast-generated alcohol) alter kernel storage proteins removing most of the negative effects of zeins (prolamins) on starch digestibility. Consequently, total tract starch digestibility typically exceeds 96% for adequately fermented silage or high-moisture corn.
What Are the Dairy Feeding Implications of Vitreous versus Floury Hybrids?
- Until there is a better understanding of the variability and magnitude of kernel characteristics needed to influence actual animal performance, most dairies should focus on quantifying and managing the variability routinely experienced in corn grain and corn silage related to kernel moisture/maturity, starch content, particle size/distribution and the increase in starch digestibility associated with duration of ensiled storage. The UW Grain Evaluation System is a good approach to quantifying the most important drivers of ruminal starch digestion; that being moisture (e.g. extent of fermentation), particle size followed by prolamin content especially for corn fed as dry, rolled (not ground).
- Excessive rumen starch digestion may result in acidosis events; therefore starch needs to be fed to the cow to produce a proper balance of rumen fermentation and small intestine digestion. Increasing ruminal starch digestibility may or may not be of benefit depending upon other dietary ingredients, the need to drive microbial protein production and the possible desire to shift starch digestion to the intestines to reduce the potential for acidosis.
- For corn fed as dry, ground or rolled grain, the starch from very finely ground (700-1000 microns), more floury endosperm hybrids (with a thin or loose seed coat) will generally be more extensively degraded in the rumen compared to starch from more vitreous hybrids.
- For corn fed as fermented corn silage or high-moisture corn, more vitreous hybrids with large kernels (reduced pericarp:endosperm ratio) appear preferable because the process of fermentation (protein matrix solubilization and acid hydrolysis) will minimize most adverse effects of vitreousness. Total tract starch digestion in these feeds, if processed correctly, typically exceeds 97%.
|All files in the following list are .pdf format.|
- Corn Biochemistry: Factors Related to Starch Digestibility in Ruminants. Hoffman et al, University of Wisconsin, 2010 (PDF 188 KB)
- Relationship Between Corn Vitreousness and Ruminal In Situ Starch Degradability; Correa et al 2002. J. Dairy Science, p 3008-3012 (PDF 107 KB).
- Relationships Between Kernel Vitreousness and Dry Matter Degradability for Diverse Corn Germplasm Ngonyamo-Majee et al 2008. Animal Feed Sc. And Tech. P 247-259 (PDF 327 KB).
- Endosperm type of Dry Ground Corn Grain Affects Ruminal and Total Tract Digestion of Starch in Lactating Dairy Cows; Allen et al 2008; J. Dairy Sc. V. 91, Supp-1; p 529. (PDF 339 KB)
- Corn Grain Endosperm Type and Brown Midrib 3 Corn Silage: Site of Digestion and Ruminal Kinetics in Lactating Dairy Cows, Taylor-Allen 2008. J Dairy Sc 88. P. 1413-1424. (PDF 179 KB)
- Impact of Corn Vitreousness and Processing on Site and Extent of digestion by Feedlot Cattle, Corona et al 2006. J An. Sc., V 84, pa 3020-3031 (PDF 275 KB)