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Glyphosate resistance a real ‘eye-opener’ for weed scientists

Stanley Culpepper wasn’t too concerned at first by a grower complaint about not being able to control Palmer amaranth with glyphosate.

“This is not uncommon,” says Culpepper, Extension agronomist with the University of Georgia. “We get 20 to 25 of these a year — someone didn’t use a residual herbicide, or they didn’t make the application properly. We didn’t think much about it.”

Culpepper collected some seeds from the grower’s field and took them back to the laboratory at the University of Georgia’s Rural Development Center in Tifton for testing. What he found “scared us quite a bit,” he noted.

That was in the fall of 2004. He went back to the same field in southwest Georgia last year and applied 33 ounces of Roundup WeatherMax or 1.5 times the normal rate on Palmer amaranth that was 4 inches in height, he told participants at Cotton Incorporated’s Crop Management Seminar in Memphis.

He displayed photos taken in the field 21 days after the application. “If you look closely, you can see the cotton,” he said, pointing to the slide of nearly solid green weeds. “But everything else is Palmer amaranth.”

Culpepper then increased the rate to 66 ounces or triple the normal amount, doubled that to 132 ounces, went to 198 ounces and finally to 264 ounces of WeatherMax or about 12 times the recommended rate.

“That was 24 to 30 times what I would normally use to control that species, and, as you can see, I still don’t have complete control. And if we had not immediately come back with a hooded or directed application in five or six weeks, we would not have been able to harvest that crop.”

Everyone has a bad day, said Culpepper, and he hoped that was his. So he and his fellow Extension workers conducted another trial in the field in Macon County in southwest Georgia.

“There was no confirmation of this in the world,” he noted. “We kept trying to figure out what we had done wrong. So we went out and applied 22 ounces of Roundup WeatherMax on Palmer amaranth that was 0.5 inch tall.”

Culpepper repeated the application when the Palmer amaranth was 3 inches tall and again when it was 10 inches tall, giving him three sprayings on the same plant. In the photo Culpepper displayed, you could see a few more Palmer amaranth seedheads in the untreated check than in the treated area.

“With 44 ounces and 88 ounces, we got a few less seedheads, but that was all,” he said. “The bottom line was that we were not going to be able to pick that crop. And we have lost the most effective and the most economical herbicide in the world.”

When the testing was completed in the fall of 2005, the weed scientists thought the Macon County field was the only location that had glyphosate-resistant Palmer amaranth. They spent considerable time and money trying to eradicate the resistant pigweed from the 500-acre field.

But random sampling of fields in a three-county area turned up evidence of resistance to glyphosate in Palmer amaranth in 47 percent of the samples. “This represented about 100,000 acres of cotton in Taylor, Macon and Dooly counties,” he said. “Some of the samples were not as resistant as the ones I just showed you, but some were more resistant.”

Last summer, Extension specialists found more evidence of glyphosate resistance in an area south and west of the three counties and have confirmed more cases in the same region this fall.

Culpepper showed some approaches for managing the resistant pigweed that have been less successful than the Extension specialists expected. “It’s very frustrating to me, and, hopefully, it will open your eyes to our future and where we’re going with this.”

This year, weed scientists working on the resistance problem applied 88 ounces of WeatherMax to 1-inch Palmer amaranth, followed by 88 ounces to 4-inch to 5-inch weeds. They post-directed another 88 ounces when the cotton was 10 to 12 inches tall.

“There is cotton in there, I promise you,” he said, pointing to a field filled with Palmer amaranth seedheads. “Cotton was planted and it received three applications of WeatherMax.

“You guys make recommendations just like we do,” he said, addressing the crop consultants in the audience. “And I want you to be thinking about how complicated this is going to make our lives.”

Culpepper displayed a photo of another field that appeared to have been overtaken by Palmer pigweed. The field received 1.75 pints per acre of Prowl H2O pre-emergence followed by 22 ounces of WeatherMax plus 1 pint of Dual Magnum on 4-inch amaranth and a post-directed application of 22 ounces of WeatherMax and 1.5 pints of Direx.

“We received 4.5 inches of rain on this field a week after application and got it activated perfectly,” he said. “But what we didn’t realize was that the yellow herbicides break down more quickly in saturated soil conditions. So, in essence, we lost our herbicide in 10 days. In 10 days, we lost that crop.”

The weed specialists also applied Prowl and 1 quart of Cotoran pre-emergence, but it failed to provide adequate control.

The Georgia Department of Agriculture obtained a Section 18 for the application of Reflex herbicide pre-emergence for control of glyphosate-resistant Palmer amaranth in cotton in 2006. EPA has since issued full federal registration for the product.

“Reflex performed well for us, but it can also give you some problems if you don’t have experience using it,” he said. “One of our better treatments was one in which we applied Prowl and 1 pint of Reflex, so work with your Extension folks on using this combination.”

Culpepper showed photos of test plots that received Prowl plus 1 pint of Reflex pre-emergence followed by WeatherMax and Dual at either 18 days after planting and before the pigweed emerges or 24 days after planting. The plot with the 18-days-after-planting application looked clean, but the plot treated at 24 days after planting was grown up in pigweed.

“How important is treating before the pigweed emerges?” he said. “It was six days between the time the pigweed emerged and the other application, six days when you could have been out doing something else. But, in that six days, we lost that crop.”

Reflex can be highly variable, he said, noting that, in 2006, it appeared to last 18 days while in 2005 it was active for 41 days. “This is the kind of thing that you and I must consider, and it’s going to vary from one end of the field to the other.”

Prowl-Reflex followed by a Roundup-Staple tank mix on 3-inch amaranth and WeatherMax plus Direx post-directed is another option, but growers and consultants again must be knowledgeable of conditions in the field being treated.

“Most of us are aware that Staple is an ALS inhibitor,” Culpepper notes. “What happens when you have ALS resistance and you have Roundup resistance and it’s in the same field?”

In one field, the weed scientists applied the full rate of WeatherMax and the label rate of Staple when the Palmer amaranth was 0.5 inch tall.

“We took out about 70 percent of the population which left about 30 percent of the plants still with us,” he said. “And what you’ve got is resistance to the two chemistries you can apply over the top of Roundup Ready cotton.”

Ignite can be another option — if it is applied on small Palmer amaranth. “Some of our growers have shied away from the LibertyLink system because the varieties don’t seem to yield as well in the Southeast. Ignite is also expensive, but it may be more cost-effective than a 12X rate of Roundup.”


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