LITTLE ROCK (AP) -- Genetics and herbicide use are contributing to the rise of a strong strain of horseweed, troubling farmers who likely will have to spend millions of dollars to fight the plant that is immune to a common weed-killer.

A weed scientist who confirmed the horseweed's presence in an Arkansas cotton field said it could cost the state's farmers as much as $9 million to combat it next year. The weed is also present in fields from the Midwest to the East Coast.

Ken Smith, of the University of Arkansas Cooperative Extension Service, said the weed stalks in Mississippi County were eventually killed with a mixture of chemicals, but the wind-blown seeds likely have spread to other fields in the region.

"We're probably to the point where it's going to be too late to give them good control this year and they're going to suffer some yield loss,'' Smith said.

Farmers exclusively using glyphosate and glyphosate-infused seeds to fight weeds in their fields likely will need to take additional steps before next year's planting season. Smith said the weed could affect 600,000 acres by next year's planting season in February and March, costing farmers an additional $8 to $15 per acre.

Farmers have been using glyphosate since 1974, when it was developed as Roundup by Monsanto. The herbicide is now made by several different companies and it is widely used because it apparently doesn't harm the environment.

David Heering, a Roundup technical manager for St. Louis-based Monsanto, said horseweed, also known as marestail, is typically found in no-till areas because it can't grow in a tilled field. No-till areas help reduce soil erosion and also help farmers reduce planting costs.

Heering said products sold in areas where the resistant horseweed is confirmed will include instructions about mixing other products with Roundup to combat the weed.

"Another product could be developed to control it, but there are several products that can be used together to control it,'' Heering said. In 1996, Monsanto designed glyphosate-immune seeds for soybeans, cotton and other plants. That allowed the farmers to spray herbicide without damaging their crops, and tilling wasn't as necessary.

The resistant horseweed has been found in Arkansas, Iowa, Indiana, Missouri, Tennessee, Delaware and Maryland.

A weed doesn't build resistance to herbicides overnight. Bob Hartzler, a weed scientist at Iowa State, said the mutation that made horseweed resistant to glyphosate likely occurred 1,000 years ago.

More recently, exclusive use of the herbicide killed the horseweeds that weren't genetically resistant to glyphosate. Now, the horseweed containing the biotype that makes it resistant to Roundup is becoming dominant.

The samples Smith brought to his greenhouse at the University of Arkansas at Monticello have been difficult to kill. He said he has used eight times the normal use rate for herbicides and the weed is still flourishing. Two things about glyphosate's effectiveness and the horseweed's resistance worry weed scientists -- soil erosion becoming a problem once again and a long wait for an improved herbicide.

Smith estimates that 60 percent of Arkansas' cotton acreage is part of the soil conservation effort. If an herbicide solution can't be found before next spring, farmers may have to plow their land before planting.

Smith said Monsanto and other chemical companies are aware of the horseweed's growing resistance to the glyphosate, but it could take time to develop a new product. The infused seeds, Roundup and similar products have worked so well for the past 30 years, no one has been working on improvements.

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