MOULTRIE — If anyone ever wanted to make a farm-themed horror movie, a hardy variety of pigweed would be a prime candidate for the villain’s role.

The mutant weed is immune to many herbicides in farmers’ arsenals, produces up to half a million seeds that can be spread by wind or by hitching a ride on farm equipment — thus passing the herbicide resistance to pigweed in other fields — and if not controlled can grow to as thick as a baseball bat and taller than an NBA center.

And it crowds out cotton plants, devastating a whole field if it can’t be stopped.

In the third year Colquitt County farmers have had to deal with it, many are forced to resort to pulling the weed by hand, a practice that is both time-consuming and expensive.

At this time there is no way to eliminate the Palmer amaranth that has become resistant to glyphosate, the generic name of Roundup, one of the most popular herbicides used in cotton production. But growers are trying to minimize its spread by hand-pulling and thorough cleaning of equipment.

The glyphosate-resistant Palmer amaranth was first confirmed in 2005 in Macon County, and has since been confirmed in 19 more, including Colquitt the very next year.

For years glyphosate was the equivalent to farmers and lawn enthusiasts of what penicillin was in combating harmful bacteria — something that worked with the added benefit of minimal environmental impact.

But like the overprescription of antibiotics has caused resistance to develop among bacteria, overuse of the herbicide led to a decline in its effectiveness, Colquitt County extension agent Scott Brown said.

Glyphosate-based herbicides work by inhibiting a specific enzyme that plants need in order to grow, according to the Web site howstuffworks.com. Without the enzyme plants cannot produce other proteins necessary for growth. Since most plants use the enzyme it works on nearly anything, making it a favorite of farmers, foresters, gardeners and biologists controlling invasive exotic plants.

A farmer killing 99.999 percent of pigweed each year for seven or eight years eventually has among the survivors a weed that is resistant to the chemical, Brown said. That produces a process of natural selection in which the glphosate-resistant strain predominates.

“The problem is now there’s so much out there (that) at some point you’re going to have it,” Brown said.

He estimated that 70 percent of cotton farmers likely have some presence of the Roundup-resistant plant.

“Next year it will be very difficult to find a field in Colquitt County that does not have glyphosate-resistant pigweed,” he said. “I think we’re getting phenomenal performance if you only have a dozen here and a dozen there in a field. They’ve done a lot of things to reduce the pigweeds down. That’s important because it keeps the seed level down.”

Farmer Brian Robinson, who had his first confirmed glphosate-resistant pigweed in cotton this year, said he is hand-pulling weeds in fields where pigweed survived two applications of Roundup. Some of the weeds have reached a height of five feet.

“It’s just real scattered,” he said. “You’ll go and you’ll spray a field and you’ll have little spots. It might be one spot, it might be two spots, it might be 10 spots.”