Monsanto: recommendations for glyphosate-resistant weeds

This spring, most of the calls Zach Shappley has taken involve burndown problems. Often, those are related to glyphosate-resistant marestail.

“That has a lot to do with a warmer winter,” says the Monsanto technology development representative working in Arkansas. “Lots of weeds had more growth than normal. That was coupled with the dry conditions — weeds weren't taking up herbicides…Those two things may have played into (producers) not getting as good of control as in the past.”

Producers' utilization of glyphosate/Clarity plus a residual has grown tremendously since 2005. Shappley thinks the trend will continue.

“Some are using Valor while others are using Direx, Caparol, Cotoran and other products. There's a wide range of what people are adding to the burndown…This helps deal with, first of all, marestail that might come up between burndown and planting or right after planting. Also, these chemistries are relatively effective on things like pigweed.”

Anything producers can do to prolong another herbicide application is good. With a residual “maybe they won't have any weeds when it's time for the first over-the-top. And another chemistry — something like metolachlor — will probably be added at the four-leaf stage. That will widen the window for things like pigweed.”

Obviously the threat of glyphosate-tolerant/resistant Palmer amaranth (pigweed) is of prime interest to Monsanto. In areas where escapes are noted, “we're working up a program for (quick) reporting to county Extension agents.

“We're also working with (Arkansas Extension weed scientists) Ken Smith and Bob Scott. This program will allow county agents to identify those fields and take samples from those escapes and (place them in) a large-scale screening program.”

At a minimum, the screening will nail down a pigweed “curve of tolerance” showing the least-resistant populations to the most glyphosate-tolerant. Doing so will allow a baseline to be established besides helping identify area problem weeds.

“That's a program Monsanto and the university is looking forward to initiating…Everyone has their radar screens turned on looking for (potential problem pigweeds).”

Monsanto is changing things up a bit when approaching potential tolerant pigweeds. Historically, if there were escapes the first recommendation was to “spray it down and see what happens,” says Jennifer Ralston, the company's Roundup technical manager.

“We all know that it's a problem and there are so few options to control it in-crop. If it appears that it might be resistant, the recommendation will be to use something other than glyphosate or glyphosate plus another product.

“You won't have time to make the second application to find out if it's truly resistant. So recommendations will be much more proactive when it comes to Palmer amaranth control.”

If there's a positive about tolerant pigweed it's that compared to marestail its seed isn't as mobile.

Producers can do “everything in their power to keep horseweed off the farm,” says Shappley. “But a vacant lot or someone down the road not doing the right thing means they're fighting a losing battle. You're at others' mercy.

“When you think about that, you try and promote a concerted effort. It's almost like pushing an area-wide eradication effort. Everyone, pretty much, has to be a player in the system for it to be effective. That's one of the challenges.”

While Palmer amaranth is a prolific seed producer, it moves easily only through hitching rides on farm machinery, flooding, birds and the like. Unlike marestail, “it isn't going to move in an out-of-control manner,” says Ralston.

When harvest arrives, it is important growers are aware of fields where pigweeds survived or weren't controlled well. “They need to take precautions such as making those fields the last harvested,” says Shappley. “If that's one of the first you have to harvest, take the time to clean your harvest equipment out.

“That can reduce the transmission of that seed to another field.”

For producers who have tolerant pigweeds, Monsanto offers more intensive management recommendations. “We've developed about a dozen weed control sheets,” says Ralston. “They're available on our Web site for download. There's one on Palmer amaranth containing recommendations (for several Roundup Ready crops).”

Ralston admits the resistant pigweed situation is “much bigger” than was anticipated. “We're in the process of sending a letter to…cotton growers east of the Texas region…emphasizing the need to be proactive in managing this weed. We want the growers to be as successful with the technology as possible.”

When Roundup Ready crops first hit the market there were concerns the technology would boost chances for resistant weeds to develop. Monsanto said such worries were overblown. Since the concerns have proven legitimate, is it difficult for Monsanto reps to deal with the aftermath?

“At the time Roundup Ready crops came out, the ‘Roundup alone’ recommendation was great,” says Ralston. “It worked very well and made many growers successful. But the reality is the situation has changed. We're in a different place than we were five or 10 years ago. We have to make new recommendations to meet those changing conditions. The best we can do is come up with recommendations for the situation we're in. We can't really look back and say what we should have done differently.

“You can't predict the future. All you can do is develop the best recommendations for the situation you're in. That's what we're trying to do. People say, ‘I can't believe Monsanto is recommending a residual.’ But we want our growers to be successful, and if that's what it takes, that's what the recommendation will be.”

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