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From VOA Learning English, this is the Agriculture Report.

Too much use of an important chemical can fuel the spread of new weeds. Some weeds are strong enough to resist weed killing products, also known as herbicides. But researchers are finding natural ways to deal with unwanted plants.

During the harvest at the United States Department of Agriculture's research station, just north of Washington D.C. Scientist Steven Mirsky walks through a field of dry corn in search of enemies. The sight of some of those enemies causes him to cry out - look at the size of this pigweed plants right here. The plants are more than two meters tall, they have taking water, nutrients and light from nearby crops.

Pigweed can quickly get out of control. Each plant can produce up to 500,000 seeds. Many farmers control pigweed and other problem plants by growing genetically modified crops. Such crops do not react to the effects of the weed-killing product called Roundup. Farmers can treat their field with Roundup and kill the weeds, but the chemical does not harm the crop.

Steven Mirsky, however, notes that some crops may be resisting the treatment.

"That system works and it works well. But the repeated application has the potential to cause resistance, and we're certainly seeing resistance on the rise across the country,” said Mirsky.

He says cotton growers in the southeast U.S. have pigweed that resists Roundup. Steven Mirsky and others are studying another possibility: controlling weeds with plants instead of chemical products.

In autumn, they cover the ground with what farmers call a "cover crop" like rye. Through the spring, they let it grow until it reaches a height of nearly two meters, but they do not harvest it. Instead, they roll it flat.

He separates several centimeters of plant material to get to the cool earth.

"And by keeping the ground cooler, it also inhibits the germination of weeds."

A machine cuts through the protective mat to plant the crop, it grows through the summer. Steven Mirsky says rolling cover crops do more than just fight weeds. Plants pushed down in the spring break up and become rich soil, that means better harvests in the fall. Farmers across the country like the idea.

Mr Mirsky says it could greatly increase production of organic crops grown without the use of herbicides. He says the method will not work everywhere, but where it succeeds, it offers farmers a new tool to crush an old enemy.

And that's the Agriculture Report from VOA learning English. I'm Milagros Ardin.