Over the years, farmers have faced a host of new weed problems. It seems like when new herbicides appear on the market, weeds go back to the drawing board and return to the field of battle with a new defensive strategy.

Farmers have seen it with resistance to herbicides in barnyardgrass, pigweed, horseweed and now ragweed.

Weed scientist Ken Smith with the University of Arkansas Cooperative Extension Service says three or four locations across Arkansas appear to have Roundup-resistant ragweed. “The Jefferson County location seems to have one of the more resistant biotypes. It hasn't responded to anything we've tried on it,” Smith said.

He said the plant looks like regular ragweed, but it's extremely resistant to Roundup (glyphosate), a commonly-used herbicide.

The problem showed up after a farmer sprayed a 50-acre soybean field with glyphosate for several years. After a while, he wasn't getting control over the ragweed, Smith said. A spot with weeds in a field got bigger and bigger.

“Glyphosate has done a good job for farmers many years,” Smith said, “but we're seeing more and more resistance beginning to develop.

“We know that we have resistant horseweed (or marestail) in Jefferson County. So we have at least two kinds of weeds resistant to glyphosate in the county. We're afraid that these will spread.”

Smith said the problem was bound to happen because many farmers have continually sprayed Roundup in the same crop over several years. It has been effective, but now resistant plants are popping up in different places.

“We've rotated our chemistry more in cotton. In cotton, we can't use glyphosate past the four-leaf stage. So we switch to other products as the cotton develops.”

What can farmers do if resistant ragweed shows up in their crops? In soybeans, farmers have additional chemical products besides Roundup that can do a fair job of controlling resistant ragweed, Smith said. He said FirstRate may be the product of choice in soybeans. “It won't give 100 percent control, but it makes the problem manageable.

“The problem isn't going away,” Smith said. “Once you have it, you have a seed bank in the soil. At this point on, you're in a management strategy. These management strategies will be adequate enough to allow you to farm.”

The only good thing about ragweed is that it doesn't spread as easily as horseweed, Smith said. It needs to be distributed by contact with farm equipment. Horseweed, on the other hand, is windblown.

“But a bigger fear than that is that pigweed will become resistant to glyphosate. If it happens in cotton, we won't have anything to go over the top of the cotton to take pigweeds out. We don't have good management strategies for pigweed. We know there is resistance in Georgia and North Carolina, and they suspect they have it in Tennessee.

We have a couple of suspect fields in Arkansas.

“It's just a matter of time,” he said.

What you do on your farm may affect how soon you get resistant pigweed, according to Smith. Farmers have some control over their destiny with pigweed, but they don't with horseweed.

He said a brochure will be available soon to farmers through county Extension offices to provide best management guidelines for managing resistance in each major Arkansas crop. The publication is called Herbicide Resistance, a Growing Issue in Arkansas.


Lamar James is an Extension communications specialist with the University of Arkansas.