A “probable” case of glyphosate-resistant pigweed (Palmer amaranth) has been found in central Georgia. The weed is suspected to be confined to several locations. The 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 pending further tests. “Additional work regarding heritability — whether the lack of control is passed on to the next generation — is needed to confirm resistance,” Monsanto said in a press release.

“Monsanto was notified of this in June,” said David Heering, Roundup Technical Lead. 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. After Monsanto was told of the pair's findings, the company began investigating.

“We want to provide recommendations to the grower for controlling the Palmer amaranth on his farm. The initial recommendation was to put out a banded application of Staple. The grower did that. He went in with Roundup — in addition to the Palmer amaranth he had morningglory, sicklepod, goosegrass and a couple more weeds. The Roundup controlled all the weeds except some Palmer amaranth that was left. He went 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. Quite a few choices are 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 preplant residuals to help manage Palmer amaranth and other species that have very high seed counts and multiple germinations during the year. We'll be promoting the use of preplant 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 (preplant residuals).

“Monsanto takes product stewardship seriously. When growers are using the Roundup Ready technology, we want to make sure they make applications at 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.”


e-mail: dbennett@primediabusiness.com