Glyphosate-resistant marestail infested over 200,000 acres of cotton in west Tennessee this year, a troubling 36 percent of all cotton acreage in the state, according to University of Tennessee weed scientists.

The resistant weed was also a problem on around 200,000 acres of soybeans, according to Neil Rhodes, Extension weed control specialist at UT. Rhodes and UT weed scientist Bob Hayes spoke on the subject at the recent Milan No-Till Field Day.

There were a number of reasons for the severity of the outbreak, according to Rhodes, including no-till systems, marestail's prolific seed production, the ease at which the seed migrates and weather.

Because of the infestations, UT weed scientists and Monsanto, which manufacturers glyphosate products, are recommending changes in weed control for west Tennessee cotton growers next year.

The first incidence of the glyphosate-resistant marestail occurred in 1999, according to Rhodes. “We began to get reports of non-performance in Lauderdale County. The complaints have increased over the years.”

Last year, UT weed scientist Bob Hayes officially confirmed glyphosate-resistant marestail in west Tennessee. “This year, we have gotten reports just about everywhere we have cotton,” Rhodes said. “Unfortunately, it has spread quite rapidly.”

Rhodes says that changing production systems are part of the problem.

“As Roundup Ready cotton has become prevalent, we've changed our burndown systems. Before, we would come in with an early burndown application of Roundup or another glyphosate product mainly for marestail and then we would follow that up with an application of Gramoxone and Cotoran at planting. You have three different modes of action and can do a lot better job.

“With Roundup Ready cotton, we spray Roundup, stick the seed in the ground and go. We think this is a big factor as to why we've seen the problem increase.”

In addition, strange weather may have tricked marestail this spring, according to Rhodes. “Marestail usually behaves as a winter annual or a biennial (lives for two years). The winter annual germinates in the fall and grows vegetatively through the winter. In spring, it bolts — produces a stalk — flowers, produces seed and then dies.”

In 2002, however, “we saw a substantial amount of spring germination of marestail. That was unfortunate because we had a number of producers use Clarity and Roundup early on resistant ground. They did a good job of cleaning those fields up, but they wound up with some re-infested fields.

“The weather fooled the marestail,” said Rhodes. “We had a warm up early, then it cooled down in May and made the plant think it was fall again and it started germinating. Hopefully, that's something we don't run into very often, but it did complicate matters this year.”

Marestail is also a prolific seed-producer, producing 25,000 to 250,000 seed per plant, noted Rhodes. “The seeds are small and can move on the wind and on cotton pickers and stalk mowers.”

Here are several options for control of resistant marestail beginning with early preplant burndown:

  • Fall applications of Valor provided good control. Controls marestail and leaves poa annua, providing a natural cover.
  • Gramoxone Max or Boa plus Direx provided good control. Good on small weeds and works better in warm weather. Do not expect a long period of residual control, so marestail may reappear.
  • Clarity plus glyphosate worked well. Keep in mind there is a minimum waiting period of 21 days between the day of application and planting cotton. Also within that 21 period, at least 1 inch of rainfall is needed. Excellent control, but may see some new germination.
  • 2,4-D provided excellent control and is inexpensive. There is a 30-day waiting period required on some labels for cotton. Watch for drift to tomatoes, roses and greenhouses. Consider dedicating a sprayer to 2,4-D applications.
  • At-planting options should focus on working a different mode of action into a Roundup Ready system. Consider Gramoxone or Boa plus Cotoran or Meturon.
  • Early post-direct applications include MSMA plus Direx or Karmex. When making the application early, hold the Direx application to 1 pint per acre. At layby, the rate can be increased to 2 pints per acre.
  • For hooded sprayer applications, consider Gramoxone or Boa and 2 pints of Direx.
  • If all else fails, tillage is an option. However, it will destroy erosion-controlling residue. Check your conservation plan and with NRCS to see if it allows tillage.
  • Hoeing may reduce the spread of small patches.

Hayes says the best thing that growers can do is be aware of the potential for an outbreak of glyphosate-resistant marestail. “Get it when it's small. Once it gets some size, it's very hard to control.”

Even if a grower cleans up a field prior to planting, “that doesn't mean he's not going to have some problems in the season. Get out and look at those fields. If you see it coming, get on it with a post-directed application.”

Monsanto is recommending an application of Clarity and Roundup 21 to 35 days prior to planting for control of resistant marestail, according to Bob Montgomery, market development specialist for the company.

“Part of that is to address the issue of spring emergence. If we go in there during that timeframe, get the weeds burned down, get crop emergence and normal weed control practices under way, we'll keep the weed from re-infesting the land before you plant the cotton.

“We believe that if the grower hits the window for the burndown application, there won't be an issue with spring emergence.”

However, if there is a spring emergence of the weed, Monsanto's in-crop recommendation is a post-directed application of MSMA and Direx.

Montgomery notes that if the company's recommendations are followed, an adequate size differential for making post-directed applications should exist at that time. Applications should be directed at marestail 6 inches and smaller.

The recommendation will be on all no-till cotton in west Tennessee, according to Montgomery.

David Heering, product technical manager for Monsanto, noted the incidence of resistant marestail in conventional tillage is evidence that it's not necessarily a no-till or Roundup Ready problem.

“Based on our research, we believe that the resistant marestail was a biotype that has always existed that we have selected for,” he said.

“We believe the Roundup Ready cropping system still represents a good value to the grower and we recommend that he continue to grow Roundup Ready crops.

“Our recommendations, as always, are to start clean,” Heering stressed. “Do a good job at burndown.”


e-mail: erobinson@primediabusiness.com.