Woodland, Calif. - The worst U.S. drought in half a century is withering the nation's corn crop, but it's a fertile opportunity for makers of genetically modified crops.

Agricultural biotechnology companies have been pouring hundreds of millions of dollars into developing plants that can withstand the effects of a prolonged dry spell. Monsanto Co., based in St. Louis, has received regulatory approval for DroughtGard, a corn variety that contains the first genetically modified trait for drought resistance.

Seed makers, such as Pioneer Hi-Bred International Inc. of Johnston, Iowa, and Swiss company Syngenta, are already selling drought-tolerant corn varieties, conceived through conventional breeding.

At stake: a $12 billion U.S. seed market, with corn comprising the bulk of sales. The grain is used in such things as animal feed, ethanol and food. The push is also on to develop soybean, cotton and wheat that can thrive in a world that's getting hotter and drier.

"Drought is definitely going to be one of the biggest challenges for our growers," said Jeff Schussler, senior research manager for Pioneer, the agribusiness arm of DuPont. "We are trying to create products for farmers to be prepared for that."

Their efforts come amid concerns about genetically modified organisms, or GMOs, and the unforeseen consequences of this genetic tinkering. Californians in November will vote on Proposition 37, which would require foods to carry labels if they were genetically modified. The majority of corn seed sold is modified to resist pests and reap higher yields.

Opponents say the label would unnecessarily dampen further development that is intended to feed a growing global population dependent on the U.S., the largest exporter of corn and soybean.

"Trying to create drought-tolerant crops is not going to be easy to do," said Kent Bradford, director of the Seed Biotechnology Center at the University of California-Davis. "We certainly need all the tools (available) to do that, and that includes conventional breeding and adding transgenic traits. We don't need to stigmatize these approaches."

To that end, Monsanto and DuPont, among others, have donated millions of dollars to the "No on 37" group, which has raised about $25 million to combat the labeling effort.

Those in support of labeling say the law is merely intended to give consumers more information about the food they eat - and to draw attention to GMO ingredients.

"I find it really funny that (opponents) are so scared of labeling," said Ignacio Chapela, a University of California-Berkeley professor of microbial ecology. "I'm not saying that every GMO is deadly, but I'm also recognizing that we shouldn't be so glib about it and look the other way and hope for the best."

Despite objections from anti-GMO activists, biotech companies are going full steam ahead on developing and patenting drought-tolerant plants they can sell at a premium.

"We as a research group are focusing on this 100 percent of the time," Pioneer's Schussler said. "A year like this, where you have this really widespread drought over a large portion of the Midwest, is very unusual."

The new lines of drought-tolerant crops are only the beginning. Scientists have no plans to let up on research to improve these first-generation seeds and develop a corn plant to protect corn growers' bottom lines.

U.S. farmers took advantage of an early planting season because of favorable weather conditions and planted 96.4 million acres of corn this spring, the most acreage since 1937.

That led to huge profit gains for corn-seed sellers: Monsanto saw its second-quarter corn seed business grow to $2.82 billion, up almost 18 percent from the year-earlier period. DuPont reported a 13 percent rise in second-quarter sales for its agriculture division to $3.4 billion, led by its corn sales.

But the intense heat of this summer's drought destroyed more than half the corn crop. The last time the harvest was expected to be this bad was in 1995.

Corn prices have set records ahead of an expected shortage that will ripple down to consumers, who will pay more for their food this fall and into next year.