Glyphosate-resistant pigweed found in Georgia

A “probable” case of glyphosate-resistant pigweed (Palmer amaranth) has been found in central Georgia. Suspected to be confined to only several locations, the weed’s discovery isn’t a surprise for many weed scientists who have long predicted glyphosate resistance in pigweeds.

The “probable” label has been attached to the case until further tests are conducted. According to a Monsanto press release, “Additional work regarding heritability — whether the lack of control is passed on to the next generation — is needed to confirm resistance.”

“Monsanto was notified of this in June,” said David Heering, Roundup Technical Lead. But suspicions were raised last fall when Stanley Culpepper and Steve Brown, both Georgia Extension weed scientists, were alerted to a problem field of Palmer amaranth.

“They saw very large Palmer amaranth and felt it might have been a (herbicide) performance issue,” said Heering. “(They thought) perhaps the Palmer amaranth was too large when it was sprayed. But they went ahead and collected seed.”

This spring, the scientists initiated greenhouse trials with the seed and currently have field research under way. Last month, after Monsanto was told of the pair’s resistance findings, the company began investigating the situation.

“We also want to provide recommendations to the grower for controlling the Palmer amaranth on his farm. The initial recommendation was to put a banded application of Staple out. The grower did that. He came in with his Roundup — in addition to the Palmer amaranth he had morningglory, sicklepod, goosegrass and a couple more weeds. The Roundup controlled all the weeds except for some Palmer amaranth that was left. He then came back under the hood with Gramoxone to burn down the Palmer amaranth. That was followed by a lay-by residual of diuron.”

At this time, said Heering, the producer’s fields are “very clean.”

Monsanto will continue working in Georgia to find a solution for managing the weed.

“One of the things we’ll drive home is the use of a preplant residual herbicide to help control it. There are quite a few choices available: Prowl, Treflan, Staple (and others).”

Heering said he views the resistant weed as manageable.

“Monsanto (would rather growers not) use the Roundup Ready system in a manner where we get to a point of resistance. So we’re strongly recommending pre-plant residuals to help manage not only Palmer amaranth but other species that have very high seed counts and multiple germinations during the year. We’ll be promoting the use of pre-plant residuals in the Roundup Ready and Roundup Ready Flex systems.”

Does news of the resistant pigweed surprise Heering?

“It’s difficult to predict which weed will become resistant, when it will become resistant and where it’ll be resistant. Now that we know there’s resistant Palmer amaranth, we’ll be proactive with recommendations like (pre-plant residuals).

“Monsanto takes product stewardship seriously. When growers are using the Roundup Ready technology we want to make sure growers make applications using the labeled rate at the right time. Don’t let the weeds get too large. Put together a system that provides a high level of control…across the board.”

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