Eastern Tennessee and northern Georgia got the worst of the drought this year, but the misery also extended to areas like southern Kentucky where Robey Farms is located. “We may have gotten one more rain shower than the others did,” but not much more, explains farm partner Adam Robey.

The Robeys tested their new-crop corn silage prior to putting it into silage bags. “We always test for nutrient value,” Robey says, but this year it was even more important. The Robeys will continue to keep a close eye on it as they feed it to their cows. There are certain potential problems that can occur with drought-stressed corn during ensiling.

Once you understand the physiology of corn plants and how drought affects them, you will see why these precautions are in order.  

Physiology 101

Perhaps the best analogy is that of a clogged drain. The corn plant continues to produce sugars through photosynthesis, but without a fully developed ear on the plant for those sugars to go and form starch, they back up into the stalk.

A few things have to happen for this to occur. First, the drought has to occur during or after pollination — and be severe enough to result in aborted kernels or poor kernel fill. And, the plant attained its growth prior to pollination and is now operating as a fully developed photosynthetic machine. 

As long as the leaves are green, and photosynthesis occurs, the plant will produce photosynthate in the form of sugars and organic acids, points out Joe Lauer, corn agronomist at the University of Wisconsin.

Normally, these sugars would move down the stalk to the ear. But, in the absence of an ear or viable kernels, they translocate down the stalk and sequester themselves in the stover material instead. (See illustration on page 21.)

A different situation occurs if the drought occurs prior to pollination. The plant may not grow as large, but still end up with a viable ear, depending on the success of pollination. In that case, the sugars move to the kernels and form starch, as they normally would.   

Implications for feeding

“Yield is dramatically affected by drought, but quality isn’t necessarily affected by drought because of increased concentrations of sugars and organic acids in the stalk.” Lauer says.

Think of it as re-partitioning: More of the energy gets stored in the stover rather than in the grain.

With normal corn silage, starch from grain contributes about 65 percent to 70 percent of the energy value, says Randy Shaver, dairy nutritionist at the University of Wisconsin. But drought-stressed corn, because of re-partitioning, may still end up with about 85 percent to 90 percent of the energy value of normal corn silage, even thought it is missing much of the grain, he adds.

University of Wisconsin extension spreadsheet estimates have shown that pounds of milk produced per ton (of silage fed) may only drop 3 percent to 8 percent when feeding silage from drought-damaged corn compared to normal corn. 

But that assumes that things went well in the bunker silo or silage bag after the drought-damaged corn was stored. 

There is some question whether silage from drought-damaged corn is automatically more susceptible to spoilage during ensiling than normal corn. Just having extra sugars in the stalk won’t necessarily result in more spoilage, points out Richard Muck, agricultural engineer with the U.S. Dairy Forage Research Center in Madison, Wis.. But if you get extra sugars left over after fermentation — when the silage reaches a pH where the lactic-acid bacteria stop growing — those sugars could provide a substrate for spoilage organisms when the silo is opened again and oxygen comes back into the system, he says. And, even before the silo or bag is opened, certain yeast organisms could take that extra left-over sugar and produce ethanol. That, in turn, can change the feeding value of the silage.

Keep an eye on the silage as it comes out of a bunker silo or silage bag.

Sugar in Drought-Stressed Corn

The tassel on this plant isn’t fully developed, which means the drought may have occurred prior to pollination. Drought stress before pollination affects cell elongation and growth, so the undeveloped tassel is a tipoff. Short plants are another sign of early drought-stress. The lack of a fully developed ear is a sign that the drought persisted past pollination. Without kernels, the sugars produced through photosynthesis move down the stalk and sequester themselves in the stover rather than the ear.