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The silk road to better drought tolerance

Those theories about modern corn hybrids being more drought tolerant were put to the test in 2012.

None of them can make grain on no moisture, but some hybrids show incredible resilience with limited moisture. And they get better every year.

Tom Hoegemeyer has spent most of his life studying corn in a part of the Corn Belt that is often short on moisture — Nebraska. He's the grandson of the founder of a longtime family seed business, Hoegemeyer Hybrids, which sells corn and soybean seed in that region. With a doctoral degree in plant breeding, he's now a professor of the subject at the University of Nebraska.

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It's the silks

He says part of the secret to breeding corn that can withstand prolonged drought is in diverting the limited moisture to the right parts of the plant. While you might think the tassel needs most of the moisture to ensure good pollen shed and seed set when it is hot and dry, you'd be wrong. To make corn, it's the ears that need the most moisture, particularly the silks, even if the plant is stressed.

“We have plenty of pollen,” says Hoegemeyer. “It doesn't take pollen from many plants to pollinate an entire acre, if it's well distributed.”

Rather, he says, drought-tolerant corn must divert water from the tassel, stalks, and leaves to the ear, particularly to the silks that capture pollen to make a kernel of corn.

“Silks are 98% water,” says Hoegemeyer. “If you are short on moisture at pollination, some of the silks don't elongate and don't capture pollen to carry to the kernel site. No kernel embryo forms.”

In this case, you get ears with partial kernel fill. The visible sign of that is drought-induced tipping back from the end of an ear, or the side-of-ear zippering effect on small ears.

“Drought-tolerant corn hybrids are selected for multiple genes that allow for better silking,” he says. “They partition more water and sugar flow to the silks and developing kernels.”

The misconception is that it's the tassel and pollen shed that suffer the most in a drought. Hoegemeyer says most pollination in corn takes place before 10 a.m. — before the hottest part of the day.

The critical stages

Corn has several vulnerable stages when severe stress (such as drought) can significantly impact yield.

The first is early, during the six- to eight-leaf stage. “That's when ear diameter is determined — whether it will have 18 to 20 rows around it or 12 to 14 rows,” Hoegemeyer says. This happens at a point in the season when there is good soil moisture and there are cooler temperatures.

The second critical stress time is 10 days before to 10 days after pollination. If the plant has good moisture and good sugar availability to the kernels, you will get longer ears and better fill to the tip.

The final critical period is the grain-fill period after kernels are set. Selecting corn hybrids that are superior in grain fill is hard work; you have to measure it as it progresses following kernel set.

“Anything you can measure, you can select and breed for it,” Hoegemeyer says. “We're just beginning to look at grain fill, and I think there's good potential to improve it in the face of drought.”

The root of the matter

Corn rooting patterns also play a role in drought-tolerance, says Hoegemeyer. In sorghum varieties with superior drought-tolerance, roots tend to grow more vertically to access deeper water.

“It's not more root mass in total, because that would divert energy from the plant, but rather, the roots grow more vertically,” he says. “I suspect that is the case in drought-tolerant corn, too.

Another thing that Hoegemeyer thinks may lead to improved drought tolerance is an agent in plants called chaperonins. In the face of stress (such as drought) on a plant, protein molecules start to degrade, lose shape, and stop functioning normally. Chaperonins are another class of proteins that help those molecules hold their shape and continue functioning, even under drought stress. While this is an emerging technology, Hoegemeyer suspects plant breeders will find ways to transgenically impact these key metabolic functions when plants are under stress. 

Aflatoxins Surface in 2012

The 2012 drought year had some weird fallout. Who would have thought mold would develop in corn and result in some loads being discounted or rejected by end users?

Aflatoxin infections typically are limited to the South, notes Pat Steiner, corn portfolio head for Syngenta. This year was an exception, though. “This year, we saw it in Kansas, Missouri, and even into Iowa,” Steiner says.

Penalties can be severe, as loads can be docked $1 per bushel or more, he says. Aflatoxin-infected corn can be toxic to livestock even at low levels. Storage can also be an issue.

Even corn with low levels of aflatoxin in a humid environment can impact the whole barge, says Chuck Lee, Syngenta North America corn head. Ditto for infected corn in a bin. Hybrids infected with aflatoxins can also adversely impact corn destined for ethanol markets, he says.

So how did it start? “Aspergillus fungi entered corn ears when ears did not fill out,” explains Bruce Kettler, public relations manager for Beck's Hybrids.

Aflatoxin infections can be hybrid related, says Kettler.

“In some cases, genetics may tighten the husk as it responds to the stress of drought,” he says. “If the husk is tight, disease pathogens can enter and survive due to no air movement.”

Unfortunately, this is something you cannot check on prior to the season other than just noting general disease-tolerance levels.

Sound insect management also can slice entry points of Aspergillus fungi into the corn plant. “Insect control is crucial,” says Steiner.

In areas where aflatoxin occurs frequently, Syngenta's Alfa-Guard GR can be broadcast to corn between growth stages V10-V12 and R1 (14 days before tasseling to onset of active silking). Alfa-Guard is a biological control product that can curb aflatoxin infestations.

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