A fast-spreading plague of "super weeds" taking over U.S. farmland will not be stopped easily, and farmers and government officials need to change existing practices if food production is to be protected, industry experts said on T hursday.

"This is a complex problem," said weed scientist David Shaw in remarks to a national "summit" of weed experts in Washington to come up with a plan to battle weeds that have developed resistance to herbicides.

Weed resistance has spread to more than 12 million U.S. acres and primarily afflicts key agricultural areas in the U.S. Southeast and the corn and soybean growing areas of the Midwest.

Many of the worst weeds, some of which grow more than six feet and can sharply reduce crop yields, have become resistant to the popular glyphosate-based weed-killer Roundup, as well as other common herbicides.

Monsanto's Roundup worked well for many years. It became prevalent with the commercialization of "Roundup Ready" crops Monsanto developed to tolerate the weedkiller, making it easy for farmers to treat their fields.

But now super weeds have developed a resistance to Roundup, and farmers are scrambling to figure out how to combat their weeds.

"We don't have that next technology. We have to get back to the fundamentals," said Shaw, who chairs a task force that is working with the U.S. Department of Agriculture on how to tackle weed resistance problems.

Several farmers spoke out about their struggles at the summit, as did experts from the USDA and crop consultants.

"This is our number one issue," said Arkansas crop consultant Chuck Farr. "It is a challenge every day, every field."

Harold Coble, an agromist and weed scientist with the USDA, called the problem of weed resistance a "game changer" and said farmers must become more versatile. Too many have simply been relying on the chemicals for too long, he said.

A joint report from the USDA and the Weed Science Society of America said "a significant proportion of growers are not practicing adequate proactive herbicide resistance management." Such "indiscriminate" use of herbicides is effectively making the problem worse, year after year.

It will be at least 20 years before any new chemical modes of action are available in the market for farmers to fight weeds with, said Coble.

Many weed experts recommended at least a partial return to limited tillage, which is largely frowned upon because it encourages soil erosion. Some experts recommended use of "cover" crops, planted to cover a field after harvest to stymie weed development while adding nutrients to the soil.

The industry is also looking at the use of multiple herbicide mechanisms with newer and more specific labeling to combat varying weed densitites. Experts discussed using equipment that can collect weeds and weed seed at harvest along with grains, so weed seed can be removed and destroyed.

Because short-term strategies can be costly for farmers, many industry players would like to see government or industry incentives to help producers.

"Why would I want to do something that is going to cost me more and make me do more work," said Steve Smith, a corn and soybean farmer. "This is what growers are saying."

Smith is also a member of the Save Our Crops coalition that is fighting a new Dow Chemical proposed herbicide that he and other critics say will be harmful and exacerbate weed resistance over the long term.

Dow is seeking regulatory approval of a newly formulated herbicide built on traditional 2,4-D chemical herbicide that would be marketed in conjunction with genetically altered 2,4-D resistant crops.

Critics say the Dow products can do more harm than help, but the company and supporters say it is at least a short-term answer.

"We need the technology now," said John Davis, an Ohio corn grower who is helping Dow promote its new 2,4-D products.