With the proliferation of herbicide-resistant weeds in crop fields, Larry Steckel has some not-so-good news for Mid-South farmers: “Nothing about weed control is going to be as easy as it once was. Even with the new technologies in the pipeline, weed control is going to be much more complex.”

“The good old days of weed control with two or three shots of Roundup over the top are gone and are never coming back,” he said at the annual conference of the Mississippi Agricultural Consultants Association at Mississippi State University. “We’re just running herbicides into the ground one after another.”

The Mid-South “has the distinction of having the most glyphosate-resistant weed species anywhere,” says Steckel, who is associate professor/row crop weed specialist, University of Tennessee Department of Plant Sciences, Jackson. “Mostly, the problems are with marestail and Palmer pigweed, but there are others.

Giant ragweed, in Tennessee and Arkansas, is becoming a huge problem. It’s very competitive, and once it’s growing in a field, it’s hard to deal with. Up and down the Mississippi River, there’s a lot of concern about common water hemp. Italian ryegrass is increasingly a problem.

“We don’t have glyphosate-resistant johnsongrass in Tennessee yet, but it’s a problem in Louisiana, has been documented in Mississippi, and there are some spots in Arkansas. At the Southern Weed Science Society meeting, Louisiana researchers confirmed they have johnsongrass that is resistant to both Select and Fusilade.“I don’t think glyphosate-resistant johnsongrass will spread like marestail or Palmer pigweed have, but we need to keep an eye out for it and do all we can to limit its spread.”

An increasing problem in Tennessee, Steckel says, is two glyphosate-resistant weeds in the same field — pigweed and marestail. “Generally, we’re seeing marestail following pigweed, and sometimes those are followed by giant ragweed, making a three-way problem. This makes control much more complex.

“I had a lot of calls last year asking what to do with fields that have both Palmer pigweed and marestail. I told them to forget the marestail and go after the pigweed, because it’s a lot more competitive. It’s the one we’ve got to try and control. It’s going to be the main weed we’ve got to deal with going forward. Just one pigweed per 3 meters of row can reduce soybean yield by 17 percent. If you have a greater number of pigweeds, yield losses can mount significantly.”

Pigweed, a native of the desert Southwest, “is an unbelievably competitive plant,” Steckel says. “From an agronomic standpoint, you can’t help but admire it — its taproot can go 5 feet deep, it can tolerate very high temperataures, and it’s amazingly drought- and stress-tolerant. Work at the University of California-Davis back in the 1980s showed it had the highest photosynthetic rate of any plant they had measured.