Now that a biotype of glyphosate-resistant horseweed has been released into Delta cotton fields, there's not much farmers — especially those downwind from a source of seed — can do to prevent its spread.

With resistant horseweed out of the barn, so to speak, focus shifts to control at burndown, planting and in-season, according to Robert Hayes, weed scientist at the University of Tennessee, Jackson, Tenn.

Hayes, speaking at Cotton Incorporated's Crop Management Seminar in Tunica, Miss., noted, “Glyphosate-resistant horseweed has grown from a problem that impacted producers only in the Delmarva (Delaware, Maryland and Virginia) area and a few counties in west Tennessee to a great concern for cotton producers throughout the Mid-South.”

Glyphosate-resistant horseweed has been confirmed in Kentucky, Ohio, Indiana, Arkansas, Mississippi, North Carolina, Alabama and Missouri. It thrives in a periodically undisturbed environment, which is common to the no-till or reduced tillage areas of west Tennessee. In the Mid-South, horseweed can emerge all during the year, according to Hayes.

Hayes says the problem surfaced due to repeated use of a single site of action herbicide (selection pressure) that led to escapes of the resistant biotype.

The prolific seed production of horseweed, amounting to 50,000 to 250,000 seed per plant, coupled with the ease of dissemination by wind, “lends to the widespread distribution of glyphosate-resistant horseweed,” Hayes said.

“Crop rotation and rotating herbicide site of action can, in general, help with weed resistance, but these strategies will likely be of little benefit in preventing infestations from windblown seed of horseweed,” Hayes said “There is little you can do to avoid it if you are downwind of a source of seed.”

Resistant weeds are easy to spot, according to the weed scientist. “When you see dead weeds and live weeds of the same species growing side by side, that should be a real clue that something is wrong. If you'll investigate, it's either an application problem or a resistance problem.”

Here is an update on treatment options, from studies conducted by Hayes, Tennessee Extension weed scientist Larry Steckel and Arkansas Extension weed scientist Ken Smith.

Hayes' most consistent preplant burndown treatment was a tank mix of Clarity at 8 ounces per acre with 22 to 32 ounces of glyphosate. The next most effective treatment was a tank mix of glyphosate at 22 to 32 ounces and 2,4-D at 16 ounces.

Valor (at 1 to 2 ounces) is also a good tool that can be used effectively in a preplant burndown program, according to Hayes, particularly in a fall application. “Valor will prevent emergence through the spring. Or you can add it as a tank mix partner for Roundup/Clarity to go out early.”

Hayes noted that Clarity, 2,4-D and Valor require minimum waiting periods of 21, 30 and 21 days, respectively, before planting cotton. To manage horseweed emerging close to or at planting may require Gramoxone WeatherMax plus a residual herbicide such as Cotoran/Meturon, Karmex/Direx or Caparol. Ignite at 32 to 40 ounces is also an option in this timeframe.

MSMA is an option to manage escaped or newly emerged horseweed after cotton emerges. Ignite can also be applied postemergence over-the-top of Liberty Link cotton, “which provides excellent horseweed control,” Hayes said. “Remember that there is a maximum use rate of 80 ounces of Ignite per season.”

Envoke postemergence after cotton reaches the fifth-leaf stage will suppress horseweed, according to Hayes. “In our research, we killed the terminal meristem of glyphosate-resistant horseweed with Envoke, but sometimes released the lateral buds for regrowth.”

Post-directed options include Cotoran-Meturon plus MSMA, Karmex-Direx plus MSMA, or Suprend. Suprend is a package mixture of the active ingredient in Envoke with prometryn, the active ingredient in Caparol.

Layby treatments effective against resistant horseweed include Karmex/Direx plus MSMA. At higher rates, Karmex/Direx will provide some residual control, but may carry over to fall-seeded small grains and cover crops.

The added chemical cost of controlling glyphosate-resistant horseweed at early burndown is $4 to $10; at planting, $9 to $10; postemergence, $6 to $10; and post-directed, $10 to $12.

“It is extremely important to scout fields, especially Roundup Ready fields, for newly emerged horseweed because it emerges throughout the growing season,” Hayes said. “Timeliness is the key to successful management. Smaller horseweeds are consistently easier to control.”