Virginia resistant pigweed herbicide rotation

RAY DAVIS, center, brother Jeff, left, and father Raymond, Sr. are all active in keeping resistant weeds at bay on their Courtland, Va., farm.

Virginia's Ray Davis keeping resistant weeds at bay

• Virginia’s Ray Davis says resistant weeds likely will continue to increase in his area and that the rotation of herbicide chemistries is vital to his success.

Ray Davis has seen first-hand the damage glyphosate-resistant ragweed and marestail (horseweed) weeds can cause to crops, and he’s heard all the horror stories about resistant Palmer pigweed, noting he wants no part of fighting that battle, as long as he can avoid it.

Davis farms approximately 1,800 acres in Southampton County, in southeast Virginia. His primary crop is cotton, but he also plants wheat, soybeans, corn and peanuts.

The Virginia grower says resistant weeds likely will continue to increase in his area and that the rotation of herbicide chemistries is vital to his success.

For the past two years, he has planted Stonevilles’s ST 4145LLB2 cotton for the LibertyLink technology. Last year he took his battle against resistant weeds a step farther by planting FiberMax  FM 1944GLB2 with GlyTol LibertyLink for greater options in weed control.

Davis’ first experience with herbicide resistant weeds came about 10 years ago, when he began seeing ragweed that shouldn’t be there, based on the herbicides he applied.

“We would see spots of it here and there, but we were able to rotate with different crops and use a different mix of herbicides to keep in under control,” he says.

Giant ragweed used to be the ugly duckling, standing tall along fence and ditch rows, but a heavy dose of herbicides used to manage cocklebur, sicklepod and a host of other common weeds found in cotton and grain fields in the Upper Southeast kept ragweed under control and of little economic importance to growers.

The onset of glyphosate technology allowed growers to spray one time and be done with a wide spectrum of weeds, including ragweed.

Over the heyday of glyphosate-based weed programs, little else was done to manage ragweed and other commonly occurring weed pests.

When weeds began developing resistance to glyphosate, several of these weeds, like ragweed, began to show up in increasing numbers in soybean fields in the Midwest and later extending down to cotton and grain fields in Virginia.

Now, growers have a real problem with ragweed and horsenettle, two of those weeds that were once killed as a side benefit to glyphosate herbicides.

Grows rapidly

Giant ragweed grows rapidly, has an extended period of emergence, and is able to thrive in many environments. These features make it a major competitor with field crops and make it especially difficult to deal with without the benefit of glyphosate.

In recent tests at the University of Tennessee, 12 herbicide treatments were tested for control of ragweed.

Of these materials, glufosinate alone, glufosinate plus glyphosate, glyphosate plus pyrithiobac, and glufosinate plus fluometuron were the most effective.

However, the only combination that showed 90 percent control of giant ragweed without reducing crop yield was glufosinate followed by another treatment of glufosinate.

Davis contends this kind of success in controlling ragweed may encourage growers to rely too heavily on Liberty herbicide and the new Glytol technology.

Unfortunately careful stewardship of farmland isn’t always enough to thwart weed resistance. Davis says.

“In the past few years, we’ve been able to pick up more land and some of it comes from growers who planted soybeans. It seems like they didn’t pay such close attention to rotating chemistry, so we are fighting that battle with ragweed again.

“We are getting more dependent on new technologies to keep these kinds of problems under control.

“One thing we have to do is use this new technology wisely. If we over-use this technology, like we did Roundup, we will probably be facing the same problem, and there’s not any new technology coming along to replace the Liberty Link technology,” he adds.


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“Over the past decade or so, we feel like we’ve gotten a pretty good handle on managing ragweed. The biggest problem we face now is with glyphosate resistant marestail. You can get rid of these weeds, using plenty of products to do so, but a month later, they seem to come back heavier than ever,” Davis says.

“I’m not a scientist, so I don’t know exactly what’s going on with these marestail weeds, but I know the amount of herbicide I applied should have killed them.

“We don’t want to continue spraying glyphosate after glyphosate to control these weeds, because we know what that can lead to,” Davis says.

Last year he planted FiberMax 1944 cotton that includes genetic-driven tolerance to both glyphosate and glufosinate, and says it worked well to get him started on a program to manage resistant marestail.

Helps with several weed problems

“The FiberMax 1944 seems to be the right product at the right time for our farming situation — for help with several weed problems,” he adds.

He isn’t alone in his fight against glyphosate-resistant marestail, also called horseweed in some areas. It has been reported in cotton and grain fields in Virginia for the past several years.

Marestail emergence occurs both in the fall and the spring. Best control in Virginia appears to come by controlling the weed during the rosette stage, which usually occurs in March.

 A frequently used management system includes a burndown treatment applied in early March of glyphosate mixed with either 1.5-2.0 pints of 2,4-D or 0.5 pint of Clarity to control glyphosate-resistant horseweed.

Effective use of these materials requires some precise timing, which is not always easy at this time of year. Applications of 2,4-D must occur 30 days before planting.

Clarity should be applied 21 days before planting and one inch of rainfall must accumulate prior to planting.

To reduce spring emergence, the pre-emergence residual herbicide Valor can be applied with the burndown treatment in early spring.

Careful stewardship of Valor is also important, because the ppo-inhibiting family of herbicides, including Valor, has already been linked to resistance to waterhemp in the Midwest, a close cousin of Palmer amaranth.

Gramoxone and Direx with crop oil concentrate can provide fair to good control of glyphosate-resistant horseweed if applied under warm temperatures (greater than 70 degrees). This mixture should be applied at least 45 days before planting.

Using the new FiberMax variety seed with both glufosinate and glyphosate tolerance overcomes numerous timing and application restrictions in dealing with marestail, Davis says.

“We started out using straight Liberty in combination with Cotoran and other older herbicide chemistries, and we were doing a good job with most weeds.

“However, if we get rains in August or late summer, fall panicum just goes crazy. We needed something to control these late flushes of fall panicum and 1944 gives us the options we need,” he adds.

Though the Virginia grower battles herbicide resistance problems with ragweed and marestail, the big bully on the resistance block is Palmer amaranth, and that’s a battle Davis says he hopes to avoid.

Resistant Palmer pigweed has been reported in southeast Virginia, mostly along the North Carolina border and is not widespread.

“We don’t take any chances with Palmer pigweed, when anybody in our general area sees what looks like a pigweed that shouldn’t be there, we call each other and we pull those weeds and get rid of the seed,” Davis says.

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