Weed scientists now know what products do the best job of controlling glyphosate-resistant horseweed. But the best advice they can give growers is: Don't get caught short on time.

Resistant horseweed was first observed in the Mid-South in a three-county area of west Tennessee in 1999. It continued to spread and now, according to University of Tennessee Weed Scientist Robert Hayes, it's estimated to infest a half-million acres in west Tennessee and is a threat to other areas.

“The problem surfaced because repeated use of a single site-of-action herbicide (selection pressure) led to escape of the resistant biotype,” Hayes explained at the National Conservation Tillage Cotton and Rice Conference at Robinsonville, Miss.

He was one of several dozen experts to speak at the event attended by several hundred farmers, researchers, and agribusiness representatives from across the south. The conference is held annually by Mid-America Farm Publications, with Farm Press as media co-sponsor.

Adding to the horseweed problem, Hayes says, is its prolific seed production (50,000 to 250,000 seeds per plant) “coupled with the ease of dissemination by wind. 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 infestation from windblown seed of horseweed.”

The weed has proven tough to pin down for researchers, too. “We can get all types of emergence patterns and sizes,” Hayes says. “In the fall of 2002, we had very little emergence — virtually none in December, January, February, and through March 20. But on April 1, we had horseweed everywhere. So, you can have tremendous flushes that come at unpredictable times.”

Besides being extremely unsightly in a cotton field, uncontrolled horseweed infestations can lead to higher trash at harvest and the weeds can harbor plant bugs, Hayes says.

The susceptible horseweed biotype “is fairly easily killed at fairly low rates of glyphosate, but the resistant type will remain relatively green even at the highest glyphosate rates.

Here's more on Hayes' research:

For burndown, the most effective treatment for resistant horseweed has been a tank mixture of Clarity at 8 ounces per acre with glyphosate. Combinations of 2,4-D at labeled rates with glyphosate have been somewhat less effective.

Valor at 2 ounces per acre provides preemergent control of horseweed, but has not proven effective as a postemergence spray on emerged horseweed, thus the need for a combination with 2,4-D.

Inclusion of 2,4-D, Clarity and Valor with glyphosate improves the activity on several other weed species, including cutleaf eveningprimrose. However, care must be exercised with all these materials to prevent contact with non-target species, especially, 2,4-D and Clarity.

Clarity, 2,4-D, and Valor require waiting periods of 21, 30, and 30 days before planting cotton. To manage horseweed emerging during this period may require Gramoxone Max plus a residual herbicide, Cotoran or Karmex/Direx.

To manage escaped or newly-emerged horseweed after cotton emerges, MSMA over-top has been the most effective treatment until cotton can be post-directed, provided the temperature at application is above 85 degrees. Cotoran plus MSMA may be post-directed in small cotton.

Ignite has provided very good burndown control of 6-inch or smaller horseweed, and any cotton can be planted immediately. Ignite in Liberty Link cotton also showed effective control of 6-inch or smaller horseweed. “Keep in mind that you have a maximum of 80 ounces of Ignite per season,” Hayes says.

Envoke postemergence, at 0.1 to 0.15 ounces per acre to cotton after the fifth-leaf stage, has recently been labeled for partial control of horseweed. It may also be applied post-directed. “In our research, we killed the terminal meristem of glyphosate-resistant horseweed, but released the lateral buds for regrowth,” he notes.

Some fields were successfully burned down with Roundup and Clarity, yet were reinfested with horseweed during the season.

Once cotton reaches six-inches in height, the most effective glyphosate-resistant horseweed control has been with post-directed /lay-by Karmex/Direx plus MSMA, according to Hayes. At higher rates, Karmex/Direx will provide some residual control, but may carry over to fall-seeded small grains and cover crops.

Because it emerges throughout the growing season, it is extremely important to scout fields, especially Round-up Ready fields, for newly-emerged horseweed, Hayes says. Timeliness is the key to successful management. Smaller horseweeds are consistently easier to control.

“It's going to take a systems approach to manage glyphosate-resistant horseweed,” Hayes says. “We've had too many people who get blindsided, then they get into that 21/30-day window prior to planting and begin to lose some of their burndown options. We're going to need to have a plan and have a backup plan.

“Be on top of it,” Hayes says. “This is a windblown seed that can come in from a neighbor's field. No one really knows how far it can travel, so you need to be ready to take action so you don't come up short on time.”

E-mail: erobinson@primediabusiness.com