PARLIER, Calif. (AP) – Horseweed was once merely a nuisance to farmers – hard to pull out, quick to sprout back after cutting and capable of towering over tractors.

Now it’s becoming a full-blown nightmare worthy of an agricultural horror flick. Scientists in California have found clusters of the weed that are resistant to scores of herbicides, leaving farmers to fight an increasingly formidable and costly foe.

The weed, also known as mare’s tail, has always been around, but it wasn’t until last month that University of California researchers confirmed that some strains had become resistant to herbicides, posing a threat to the nation’s most productive farmland.

For decades, growers, gardeners and anyone looking for an easy way to beat back weeds have relied on the chemical glyphosate.

Farmers planting crops such as corn, soybeans or cotton that have been genetically engineered to survive the chemical could spray it liberally over their entire field, killing all the weeds and leaving only their crops standing.

However, glyphosate-resistant horseweed was found in Delaware in 2000, and has since been discovered in 10 other states.

The herbicide’s popularity may be partly to blame for breeding the resistant weeds, researchers said. By killing nonresistant weeds, it allows only the survivors – those few naturally resistant plants – to thrive.

“They’ve created a problem by relying on one solution to solve all problems,” said weed ecologist Anil Shrestha of the University of California’s Kearney Agricultural Center.

Developing resistance to a chemical isn’t unusual among plants and animals, scientists said. What makes the horseweed adaptation such a nuisance is how fast it reproduces and how big it grows, stretching 10 or 12 feet tall, sucking up scarce water and nutrients.

Bob Prys, a manager for the 13,000-acre Borba Farms in the San Joaquin Valley, said the weed became a problem just three or four years after his farm started growing herbicide-resistant cotton.

At first, workers sprayed the fields, killing everything but the cotton plants, and the farm saved money by having to till the fields less frequently. Now Prys said the farm is relying on weeding again and adding other chemicals to its herbicide mix – adding unexpected costs to the higher price they pay for the genetically modified seed.

Pete Christensen also watched his costs soar as herbicides became powerless to stop the weeds from choking grapes on his vineyard near Selma. Two years ago, he tripled the concentration of the herbicide and doubled the applications, but the weeds were growing thicker than ever.

“It was dominant in the landscape,” he said.

Biotechnology firm Monsanto Co., which develops crops resistant to the herbicides, recommends mixing chemicals to avoid the weeds also becoming resistant, said David Heering, a technical manager for the company.

“At the end of the day, they’ll still have fewer passes through the fields, and fewer weed-control problems,” Heering said.

The UC scientists recommend weeding, rotating crops, cultivating the land with farm equipment and the use of herbicides that kill horseweed seeds in the soil before they germinate.

Those measures will increase costs for farmers, but will prevent a more serious and costly problem later on, scientists said.

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