By Dr. Charles J. Sniffen, Phd, Fencrest, LLC (Holderness, NH)
Starch degradability is a rapidly evolving story with interest stemming from work done by Dr. Allen at Michigan State in high producing cows. Additionally, work at Wisconsin on vitreousness and on prolamin proteins in corn has highlighted the differences among corn silage hybrids and different sources of corn grain. For years, we have focused on grain and silage yields per acre. Doing so, we have been able to achieve very high forage and grain yields but have ignored the nutrient characteristics of the corn we are growing. Many years ago Dr. Mike Allen (Michigan State) pointed out the importance of considering fiber digestibility, which in time has been accepted and in recent years have been incorporated into selecting and growing silage-only hybrids. However, until recently we have still largely ignored the availability of starch in various hybrids.
Most US hybrids have been developed for export with harder grains that needed to stand up to a lot of handling without breaking down. These hybrids tend to be more vitreous. Unfortunately, the starch availability from these grain sources is lower, mostly in the rumen but also whole tract.
The main prolamin protein in corn is zein, which is poor in quality hydrophobic protein. Prolamin proteins are a large family of plant proteins, some of which are poor quality proteins, only soluble in ethanol solutions. Because of this insolubility, they are poorly digested in the rumen and in the small intestine. As corn grain matures, zein protein develops and results in the starch granules becoming inaccessible by rumen bacteria. As the DM of corn silage goes much over 30%, starch degradability decreases. This again is due to the increase in the zein content of the kernel. However, over time in an acid medium, zein becomes soluble and the starches will become more available. The increased soluble protein as a % DM reflects the slow breakdown of the prolamin proteins, mainly zein, with time in the silo.
The genetic background, floury vs. vitreous, can greatly affect starch digestibility in both flint and dent corn. As vitreousness increases starch digestibility decreases which is shown clearly in the work done by Philippeau and Michalet-Doreau and the chart below (middle right).
Much of what we know on how endosperm type affects ruminal starch digestibility can be taken from work done by Dr. Allen at Michigan State. You can see in the chart below how corn endosperm type, floury or vitreous, effects ruminal starch digestibility in both dry and high moisture corn. An interesting note is that in both dry and high moisture corn apparent ruminal digestibility is higher with the floury grain.
The chart to the upper right represents an enzymatic assay that CVAS conducted (vitreous corn). The data was summarized from samples that came into their lab. What is interesting is that the samples in October and November that represent the new crop corn silage are lower in starch degradability, likely because the zein protein has not begun to breakdown through fermentation. Another interesting thing is that it takes until March to get to maximum degradability.
It was a significant move for us to adapt the concept that fiber in forages had different fermentabilities based on environment, maturity, and genetics. We now accept these ideas and are routinely formulating rations based on these concepts. It should be added that we are still evolving in this area.
Likewise we are now moving into the area of starch digestibility and how endosperm type can affect this. Michigan State’s 7-hour in vitro assay offers us a starting place, which allows us to see the differences in starch fermentability in endosperm type. Hopefully this too will evolve with time, as more come to realize the importance of starch fermentability in the rumen and the impact endosperm type has on it.