On many farms, the most commonly used weed killer in the United States isn't cutting it any more.

Researchers say heavy reliance on glyphosate, commonly known by the brand name Roundup, has made the surviving weeds tougher, forcing farmers to shift tactics and rely on more costly methods to get rid of them.

"The areas infested with our main culprits — mare's tail, water hemp, giant ragweed — they increase pretty substantially every year, maybe 10 percent in terms of acres affected," said Bill Johnson, a weed science professor at Indiana's Purdue University.

The problem of herbicide resistance has been around almost as long as herbicides have. It's evolution 101: Weeds that can survive spraying will pass along their genes to their progeny, while their susceptible neighbors die off. And glyphosate, which has been around for 40 years, isn't necessarily more likely to produce resistant weeds than its predecessors, Johnson said.

The big change was the development of "Roundup-Ready" crops, introduced by Roundup manufacturer Monsanto in the mid-1990s, he said. Those genetically modified corn, soybeans and other plants were able to survive glyphosate exposure, so farmers could spray the herbicide on their fields — and many came to rely on that single chemical as a result.

"We didn't exercise really good judgement," Johnson said. "We over relied on the herbicide, and Mother Nature will not be defeated ... as a result, a number of weed species have increased in intensity."

Glyphosate made up 57 percent of all the herbicides used on corn and soybeans in 2013, according to an April report from the US Department of Agriculture (USDA). The agency has now identified 14 species of glyphosate-resistant weeds in the United States, and 32 have been documented worldwide, according to a government-industry-university coalition that tracks the issue globally.

In Indiana, growers are now grappling with glyphosate-resistant giant ragweed, water hemp, mare's tail and Palmer amaranth, which is an increasing threat to the soybean crop, Johnson said. Resistant mare's tail, or horseweed, has now been identified statewide, he said.

"It may not be in every field, but it's in every county," he said.

In Georgia, Palmer amaranth is a costly nuisance for cotton growers, University of Georgia weed scientist Stanley Culpepper said. In the last decade, farmers have spent more than $1 billion trying to keep it in check, he said.

Palmer amaranth is already an aggressive weed, growing up to two inches a day in the summer and producing a large volume of seeds. Glyphosate became the weapon of choice against it, largely because of its low cost, Culpepper said — until resistant weeds emerged.

"There's no new monster out there," he said. "This monster has lived for many, many years. We just lost of one of the most effective tools we had to control that monster."

Many farmers are going back to some low-tech methods to beat back that monster. Culpepper and Johnson said growers are relying more on tilling fields, which tears up weeds at the root, and planting more cover crops that hog the sunlight. The US government subsidizes the planting of cover crops in order to support that practice, Johnson said — but tilling takes time, labor, and puts additional wear on equipment, while the cost of adding other herbicides to the mix of chemicals can range between $5 to $35 an acre, he said.

Charla Lord, a Monsanto spokeswoman, said the company's collaborations with university weed scientists found that Palmer amaranth could be controlled by combining dicamba, another popular herbicide, and glyphosate.

"[O]f course, successful management requires using diversified tactics, including herbicides with multiple mechanisms of action," she said.

Monsanto and Bayer, another leading glyphosate producer, promote a variety of strategies to head off the threat. The industry's major players also support a research initiative, the Herbicide Resistance Action Committee, to address the issue.

The USDA recommends using different herbicides to reduce the chances of resistant weeds emerging. And in 2014, the Environmental Protection Agency approved a Dow AgroSciences product called Enlist Duo, which combines glyphosate with another herbicide, 2,4-D, that was designed to attack weeds in genetically modified corn and soybean fields — but the agency limited its approval to six years, allowing it to re-examine the resistance issue.

Johnson said that combination will give farmers "some very powerful tools" in the short run. But if they just use the new product like they applied glyphosate, "We're going to end up in the same place."

The safety of glyphosate for humans has also been under renewed scrutiny internationally. In August, the World Heath Organization's International Agency for Research on Cancer (IARC) declared it "probably carcinogenic" to humans — a dramatic departure from the US Environmental Protection Agency and other agencies, which have found it safe to use.

Monsanto is conducting its own review of the IARC decision.