Energy is arguably the most valuable aspect of corn silage, and 65 percent of the total energy supplied in corn silage is found in the grain. Seventy percent of this energy originates from starch, with the remaining divided equally between the fat and protein content of the grain.
Starch is produced when six carbon sugar molecules combine, and this process continues until the corn plant reaches physiological maturity, more commonly known as black layer.
It is common for the corn plant to deposit 0.5 to 1 point of starch per day, so the longer we can keep our corn plants in the field, the more starch will be produced. It is now recommended to delay harvest of healthy plants until the kernels are closer to 3/4 milkline to maximize starch deposition.
Modern hybrids have significantly improved their late-season plant health, which helps in increasing starch deposition while only minimally affecting fibre digestibility.
With the vastly improved late-season plant health, coupled with technologies such as foliar fungicides, corn plants can maintain fibre digestibility much later into the growing season. Late-season plant health is also advantageous with growing seasons lagging in heat units.
Numerous research studies clearly show fibre digestibility declines only minimally in healthy corn plants as they dry down from 30 percent dry matter to 38 percent dry matter (70 percent to 62 percent moisture), but starch production increases significantly.
The combination of wanting more energy in our silage, processing capabilities, having healthier plants in the fall and the ability to achieve higher compaction densities in bunkers and piles has allowed growers to harvest corn silage at 3/4 milkline rather than 1/3 to 1/2 milkline, which was common in the past.
Starch production significantly contributes to not only increasing the tonnage and energy density of silage but also in reducing the moisture in the whole plant. The continued health of the overall plant allows for continuation of photosynthesis and the deposition of sugar through the vascular system of the plant.
The transformation of sugar to starch is dependent on the pathway from the kernel to the cob to remain not only open but steadily fed – in effect, a two-way valve. In essence, it is the production of starch in healthy plants that reduces the water content of the kernel.
Thus, healthy plants are quicker at lowering their kernel moisture by depositing starch than those impaired by drought, disease, etc. We have seen this effect in areas that were drought-stricken; the corn leaves were brown while whole-plant moisture was still quite high.
As corn matures, the dry matter of the entire plant, being composed of stover and grain, increases. The major impact for increasing dry matter is starch (grain) production.
The ear is always drier than the stover, so an increase in the ear-to-stover ratio increases not only the total plant dry weight but also the percentage of dry matter in the total plant. This helps explain why healthy plants can remain green while kernel maturity can be at 3/4 milkline.
Moving harvest timing from 1/3 or 1/2 milkline to 3/4 milkline will result in the kernels being “harder” at harvest. This higher dry matter silage allows for the harvest of more starch but gives rise to the need to ensure aggressive kernel damage so the cows can completely utilize the starch in these drier kernels.
However, producers who lack the ability to process (roll) kernels on the chopper may have to harvest at earlier kernel maturities or shorten the chop length to ensure adequate processing.
While it is helpful to have a post-harvest, laboratory measurement of kernel damage (ideal to have 70 percent or higher processing score), it is equally important to have an easily implemented field method to make processing adjustments as the crop is being harvested.
Producers are encouraged to take a 32-ounce cup sample of several loads each hour, then spread the sample out and pick out every whole and half kernel (Figure 2).
If that number exceeds two to three kernels, it is important to discuss ways to improve kernel processing with the chopper operator.
There are several factors that chopper operators can check to improve processing: length of chop (longer typically makes it more difficult to damage kernels adequately), roller mill wear (life of 400 to 1,000 hours depending upon the mill), roller mill gap (1 to 3 millimetres depending upon chop length and kernel maturity), aggressiveness of the rolls and, perhaps the most important, roller mill differential (typically 20 to 40 percent).
If left unattended, the result will be a loss in energy since unprocessed kernels may escape both the rumen and intestine undigested.
Validation of the degree of kernel damage can be further accomplished by collecting fecal samples from 10 to 12 cows and submitting to a lab for fecal starch analysis. The goal is to have less than 3 to 5 percent starch in the manure.
Levels higher than this could indicate poor processing of corn silage (or other starch sources) resulting in inefficient use of starch in the diet, possibly predisposing cows to metabolic disorders and failing to meet the milk production potential of the designed ration.
The starch in corn silage is often considered the “villain” when cows are fed high levels of silage, do not respond as expected, experience low butterfat tests or display inconsistency in manure scores.
The villain image has lessened as laboratory starch, starch digestibility and fecal starch values have become commonly available and nutritionists have learned to focus on reducing supplemental grain to complement the starch delivered from modern corn genetics.
Starch levels in the entire dairy diet typically range from 30 percent for new-crop corn silage to as low as 22 to 24 percent when feeding long-fermented corn silage. Corn silage drifts up in ruminal starch digestibility for about six months before plateauing.
Failure to account for starch digestibility changes may explain some of the “spring acidosis” and milkfat depression seen on dairies feeding high levels of corn silage in conjunction with high-rumen fermentable high-moisture corn.
It is not possible to have a corn silage that is “too high in starch”; it all comes down to balancing the diet accordingly.
Getting the most out of corn silage is a priority for many of us. With the advancements in corn plant genetics, kernel processing and bunk compaction capabilities, delaying silage harvest to 3/4 milkline can help us achieve just that. 
-
Martina Pfister
- Dairy Specialist – Central/Eastern
- Ontario and Maritimes
- Dupont Pioneer – Canada
- Email Martina Pfister
