The Seven-Year Cycle

Weed scientist says the evolution of herbicide resistance has spun out of control

Whether you’re battling kochia out West, pigweed down South or waterhemp in the Midwest, herbicide-resistant weeds have declared war with farmers across the country. And with the resistance army continuing to grow, farmers have had no choice but to pull out the latest weed-control weapons available to defend their farms.

But replacing one herbicide chemistry with the next won’t win the resistance battle, says Bob Scott, Ph.D., University of Arkansas Extension weed scientist.

Spinning and spinning

For decades, farmers have used chemical control methods necessary for weed management. But one by one, weeds have developed resistance to a number of herbicide chemistries, eliminating them as viable control options for the future.

“Our data and other literature show that herbicides have about a seven-year lifespan, give or take, if no resistance management efforts are made with a new product from day one,” says Scott.

Scott’s seven-year cycle proves true in nearly every instance of pigweed resistance in soybeans in the Mid-South, which began in the 1980s when pigweed developed resistance to the popular herbicides Prowl® and Treflan™ (Group 3).

When ALS chemistry (Group 2) was introduced in the 1990s, there was an explosion of farmers using herbicides like Scepter® to control pigweed populations in Arkansas. But by ’97, Scott says the state was already seeing fairly widespread resistance to this chemistry.

Next on the resistance cycle for pigweed was glyphosate (Group 9), commonly known as Roundup®. Roundup was introduced in 1974 as one of the first broad-spectrum systemic herbicides available for agricultural production. The Roundup Ready® trait was introduced in the late-1990s, increasing the post emergence use of glyphosate in-season over tolerant crops until resistance began setting in around 2005.

“Even with the extreme pressure that the industry placed on this technology, it still followed the same seven-year cycle of resistance,” says Scott.

And once glyphosate-resistant pigweed took over fields in the Mid-South, farmers turned to PPO herbicides (Group 14), like Flexstar® and Valor®, for control around 2010.

“Farmers relied heavily on PPO chemistry to fix their weed problem,” says Scott. “Now, here we are in 2017, and we’re seeing an explosion of PPO resistance throughout the region.”

With pigweed resistance confirmed at varying levels to herbicide groups 3, 2, 9 and 14, farmers in the Mid-South have very few chemical control options left for such a problematic weed.

What caused the cycle to spin so quickly?

When considering the factors that aided in this cycle of resistance, many like to use the words “over-use” or “overreliance,” and are quick to point a finger at farmers. But Scott says there are a number of factors that contributed to the resistance issues we face today, aside from producer practices, which include herbicide companies and their product marketing.

“You could say the economics drive the decision making,” says Scott.

Compared to other products, glyphosate was priced so low that farmers had no incentive to use alternative herbicide options. And since it was needed so badly for weed-control, farmers relied on it heavily.

“We’re working with chemical companies now to encourage herbicide reward programs be in place to make sustainable weed control more affordable for farmers,” says Scott.

Forward steps to slow the cycle

As new herbicide products like Engenia™, FeXapan™ and XtendiMax® become available, Scott and his colleagues have had to rethink their recommendations on how to slow the resistance cycle.

“We do not consider herbicide programs that are just going to get us by for one year,” says Scott. “We create year-round, diversified programs geared toward preventing pigweed from going to seed in hopes that this seven year cycle we’ve seen will be extended.”

Scott’s wholesome approach to weed control includes non-chemical practices like crop rotation and harvest weed-seed destruction, and focuses on two key practices when it comes to herbicide applications:

  1. Use residual herbicides with at least two effective modes of action (MOAs) in each application.
  2. Use the full recommended labeled rate.

Effective MOAs for pigweed-control are dwindling, and no new herbicide chemistries are set to hit the market for many years. For this reason, Scott says farmers must admit they have a resistance problem early and make the right decisions, for the right reasons, when it comes to managing herbicides.

For more information on Scott’s research, visit


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