New Technology Doesn’t Eliminate Fall Herbicides

In this photo a combination of Weedar 64 at a rate of 24 fl oz/A, Clash at 4 fl oz/A and Panther at 2 oz/A was applied in the fall of 2014 to control winter annual weeds. This photo was taken on June 10, 2015. Compare the clean row middles in the treated area with the well-established marestail in the untreated border rows.

In this photo a combination of Weedar 64 at a rate of 24 fl oz/A, Clash at 4 fl oz/A and Panther at 2 oz/A was applied in the fall of 2014 to control winter annual weeds. This photo was taken on June 10, 2015. Compare the clean row middles in the treated area with the well-established marestail in the untreated border rows.

There’s a lot of discussion about new herbicide-tolerant traits that are coming on the market soon. But University weed scientists urge growers to keep these new technologies in perspective – they are additional tools in the fight against weed resistance but are not silver bullets.

So, don’t abandon proven practices like fall herbicide applications if they’re warranted. For a number of reasons they simplify planting, keep things on schedule and also add different modes of action that stall resistance.

“We started doing fall herbicide applications in our no-till fields for a lot of different reasons,” says Dr. Mark Loux with Ohio State University. “Not just because of marestail but because we had plenty of winter annuals that were making a mess in the spring. The ground wouldn’t dry out and the weed cover harbored insects and nematodes. Weeds often were so thick that growers couldn’t till through them. The spring herbicide applications didn’t work fast enough and didn’t desiccate weeds fast enough, which meant the ground couldn’t dry out soon enough, either.”

Loux says growers still need to scout fields and control heavy populations of winter annuals, especially marestail, with fall burndown treatments. Yes, the new technologies allow using 2,4-D or dicamba in spring burndown, plus those postemergence treatments are enticing. But for the sake of planting efficiency, getting that growth out of the way in the fall often pays big dividends.

In this photo a combination of Weedar 64 at a rate of 24 fl oz/A, Clash at 4 fl oz/A and Panther at 2 oz/A was applied in the fall of 2014 to control winter annual weeds. This photo was taken on June 10, 2015. Compare the clean row middles in the treated area with the well-established marestail in the untreated border rows.
In this photo a combination of Weedar 64 at a rate of 24 fl oz/A, Clash at 4 fl oz/A and Panther at 2 oz/A was applied in the fall of 2014 to control winter annual weeds. This photo was taken on June 10, 2015. Compare the clean row middles in the treated area with the well-established marestail in the untreated border rows.

“If you have large populations of deadnettle or chickweed by the spring, they’re big and hard to manage,” he explains. “With early spring applications it’s cold, so herbicides aren’t working that fast. That means you can run out of time to get everything desiccated and dried out ahead of when you need to plant.

“Entomologists also say that if you have more vegetation in the field in the spring that you’re more likely to face insect issues. Other weeds like purple deadnettle also can host soybean cyst nematode. It’s much easier with a fall application to control these weeds and minimize later pest pressure.”

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Stewardship of these new technologies is another concern for many weed scientists. That’s yet another reason to keep weed management approaches as diverse as possible.

“These new traits and technologies could begin losing effectiveness within as little as 3 years if we don’t steward them correctly,” says Dr. Aaron Hager with the University of Illinois. “Weeds adapt, something we can’t afford to forget.”

Loux says the lessons learned over the past two decades will be invaluable in stewarding and protecting new trait technologies.

“If we don’t take the right approach with any herbicide, we risk weeds developing resistance to it,” Loux emphasizes. “We’ve eliminated many of the weeds we were talking about as problems 15 or 20 years ago. The weeds we’re fighting today, on the other hand, are the bad actors – the ones that quickly develop resistance to multiple sites of action. They have more complex life cycles and can come at you all year. These weeds have to be sprayed when they’re 2 inches tall or less and if they escape then each plant can potentially produce a million seeds.

“The weeds we’re left dealing with require a pretty complex control system if we expect to avoid resistance issues. If we go backwards to an overly simplified weed control system, they’ll simply adapt to it.”

  • To see the entire article by Dr. Loux, click here.
  • University of Missouri weed scientist Dr. Kevin Bradley also urges growers to keep new technologies in perspective. Read the article here.