Fighting Resistance
Residuals are Back in Vogue

By Henry Gantz
hgantz@meistermedia.com

Across the weed spectrum in the Southeast and Mid-South, there is no doubt that glyphosate- and ALS-resistant horseweed (a/k/a marestail) and palmer amaranth are causing the most anxiety.

"To say (resistance) has changed our thinking and our way of life is an understatement," says Dr. Alan York, North Carolina Extension weed specialist.
And because of that, Extension and University recommendations are calling for a return to residual herbicides that were, to use a sports metaphor, sent to the bench, replaced by glyphosate in Roundup Ready systems.

"We are at the point where we are going to have to fight this with residuals." says Dr. Trey Koger at the Delta Research and Extension Center in Stoneville, MS. "We have the residuals to help us combat the problem, and they are older chemistries. Yes, they are an added expense, but if we are committed to growing cotton, we cannot rely on glyphosate, glyphosate, glyphosate." Adds Tennessee Cotton and Small Grains Production Specialist Dr. Chris Main: "We pioneered the work on glyphosate resistance in Tennessee, particularly in cotton,"

"We've been using residuals for three years, so our guys are very familiar with residual-product use... and our guys did a good job of getting it under control."

Go Back in Time
Some of the residual products coming back into vogue are metolachlor, fluometuron, prometryn, diuron and the "yellow" herbicides.

One of the relatively new herbicides to hit the market is Envoke from Syngenta. Envoke is an ALS inhibitor in the sulfonylurea class of herbicides. Because it has a 90-day preplant restriction for cotton, it is recommended for fall/winter applications.

"The benefits of the fall applications of Envoke - and I think they are fairly obvious to a lot of cotton growers in many areas - are time-management issues, like planting," says Syngenta product manage Duane Martin. "Secondly - especially across the South - water conservation can be a huge benefit, in that weeds are growing through the wintertime and are taking moisture out of the soil profile."

Martin says Envoke has excellent activity on resistant marestail along with henbit, cutleaf evening primrose, chickweed and most of the mustard species.
In addition to glyphosate resistance, growers should be on the lookout for ALS resistance and take the appropriate steps where it is found. "As with a lot of the ALS herbicides, we are still seeing fantastic activity on many weeds," Martin says. "But growers need to be aware that we do have ALS resistance. They are going to have to plan for that and make sure they put the right herbicides in a tank mix to control resistant marestail."

In that case, paraquat- and dicamba-based contract herbicides should be added. If Envoke can be used alone, a non-ionic surfactant should be added.

Billy Beegle, whose consulting service includes west Tennessee, the Missouri Bootheel and northern Arkansas, began looking at Envoke in 2006 in two test plots - one in a tank mix with Clarity (a dicamba herbicide from BASF), and another in a tank mix with 2,4-D. "I was very pleased with both of them," he says. "We got 99% control of marestail - which is what we were going for - but it left poa anna out there, which is good. We need a little ground cover on these fields, either for wind in Missouri, or for erosion control in Tennessee."

Start Clean, Stay Clean
James Holloway, Syngenta's Senior R&D Scientist, says controlling weeds at emergence in the winter pays dividends in the spring. "If we don't start when those (weed) seedlings emerge, we're fighting a losing battle, and it will go into the following season. The quicker we get on them, the better off we are. If we have a clean seed bed as we go into the winter months, we have a better chance of having one going into the spring planting season."

Martin adds that, "simplicity is the key to success," and Envoke's rates are not based on soil types. "I think Envoke is one of those products that is pretty simple to use," he explains. The standard label rate for Envoke is 1/10th of an ounce per acre. "If grower's see big weeds out there at the time they want to apply Envoke, they may want to add another contact herbicide with it. They should get the burndown now and residual activity until spring."