Should Mid-South farmers be adding a residual herbicide to their glyphosate applications on Roundup Ready and Roundup Ready Flex cotton sooner rather than later?

Some growers have been applying residual herbicides at lay-by to try to prevent late flushes of problem weeds, such as Palmer amaranth, from germinating and producing more seed.

But studies by weed scientists indicate that may not be the best approach for forestalling glyphosate resistance in Palmer amaranth, a problem that is turning up in an increasing number of areas across Arkansas and other Mid-South states.

“When you apply Valor, which is very effective on pigweed, at lay-by, the computer models show we have about a 75 percent chance of Palmer amaranth developing resistance (to glyphosate) over time,” says Ken Smith, Extension weed scientist with the University of Arkansas.

“If you add a residual herbicide to the burndown treatment with glyphosate (Roundup, Touchdown, others), it skews the resistance curve to the right (delays the onset of resistance) and reduces the probability to 35 percent.”

Speaking at the Mid-South BMP Workshop sponsored by the National Cotton Council and The Cotton Foundation in Tunica, Miss., March 18, Smith said there's something to be said for trying to reduce the amount of weed seed going into the soil at the end of the growing season.

“We've been talking a lot about sustainability,” he said. “Weed scientists think that one of a farmer's goals should be to have less weed seed in the soil at the end of the season. If you have more, then the system is not sustainable.”

Smith and the University of Tennessee's Larry Steckel conducted the weed management portion of the BMP Workshop, which is built around the “First 40 Days” and “Fruiting to Finish” publications developed by Extension, university and USDA researchers and consultants with the assistance of the National Cotton Council and Bayer CropScience.

Steckel, Extension weed scientist with the West Tennessee Experiment Station in Jackson, Tenn., said the better name for the series from the weed science standpoint might be the “First 70 Days.”

“The clock actually started in February,” said Steckel. “Our experience shows that you've got to jump on weeds, particularly marestail or horseweed, when they are small if you want to get good control.”

Research has shown that late February may offer the best window for applying herbicides on horseweed. For farmers in west Tennessee, where glyphosate resistance showed up in horseweed several years ago, that means a tank mix of dicamba (Clarity, Oracle, Banvel) with the burndown application of Roundup.

Although horseweed can emerge 10 or 11 months of the year, the two peak events are in February and March and in August and September. “With these dry springs, we've been getting less control of horseweed with dicamba,” said Steckel, who acknowledged the problem might not be as severe with the rains that have been inundating the Mid-South in recent weeks.

“One of our problems is that we are seeing much larger populations of horseweed than in the past. Where we used to see 10 plants in a small area, we're seeing 20 to 25 and 85 percent control of 25 plants is not good enough. If you miss them with the dicamba, the game's over. You have to live with them the rest of the season.”

Those and other weeds that emerge in early spring are more competitive in cotton than in other crops that may be better adapted to the cooler, wetter soils than cotton may be, according to weed scientists.

“If you control those weeds when they're small, they're not only easier to control, but you also eliminate the competition with cotton,” says Jennifer Ralston, technology development manager with Monsanto.

Ralston, a member of Monsanto's weed resistance team, acknowledged that many farmers prefer to wait until later in the season to apply a residual herbicide so they only have to make one application.

“If you put a residual down early, it will provide better control and give you more flexibility,” she said. “If you wait, you may have weather problems and not be able to get in the fields in a timely manner.”

The choice of a residual herbicide can vary from one region to the other. Monsanto has been offering a new herbicide formulation called Parrlay that contains metolachlor, an older residual labeled for the control of more than 20 grasses and small-seeded broadleaf weeds.

Several Southern states have also requested and received registrations for fomesafen or Reflex, a soybean herbicide, for use on glyphosate-resistant Palmer amaranth in cotton. Reflex can be rainfall dependent in a region where rains can be hit and miss.

“There can be a difference in performance on sandy and silt loam and clay soils,” says Ralston, “And, in the Mid-South, farmers have experienced splash-back problems — water can hit the soil surface and splash the herbicide on the cotton, causing injury.”

Smith recommends that farmers use a residual herbicide such as Cotoran, Direx, Prowl or Caparol with their burndown herbicide. If no burndown application is made, they should use a residual herbicide at planting.

“The choice doesn't matter as long as you use a residual in one of those applications,” he said. “We use a lot of Direx in Arkansas because we're cheap.”

For over-the-top postemergence applications, Steckel says, growers should tank mix another herbicide with the Roundup or glyphosate spray; herbicides such as Dual (metolachlor) or Staple.

“Products such as Cotoran or Caparol can be tank-mixed with Roundup or hooded sprayer applications,” he said. “Dual, especially, will provide good control of pigweed or Palmer amaranth.”