Roy Wendte aims to hit his problem weeds, waterhemp and marestail, with a one-two punch every time he sprays.

“I try to use multiple modes of action with each herbicide application — at least two or three modes,” says the Altamont, Ill., farmer. He puts down soil-applied residuals on all his crop acres, and deploys “as many modes of action within a season as I can.”

This strategy is the best way to curb the evolution of herbicide-resistant weeds, according to new research from the University of Illinois. Farmers must expose weeds — both before and after emergence — to more than one effective mode of action in every spray pass, says study co-author Pat Tranel, a University of Illinois weed scientist.

“You’ll always have a certain percentage of a weed population that is naturally resistant to a herbicide mode of action,” says Amy Asmus, an agronomist at Asmus Farm Supply, Rake, Iowa. But if your herbicide application has two effective modes of action, “instead of selecting for that weed, you’ll kill it.”

It’s the same approach used in medicine, Tranel says, where pathogens such as HIV are subjected to a cocktail of multiple drugs, halting the evolution of resistance traits.

Herbicide rotation fails

Many growers rely mainly on herbicide rotation to manage resistance. But the Illinois research showed that rotating chemistry failed to prevent resistance, Tranel says. In fact, rotation actually increased the frequency of glyphosate-resistant waterhemp in central Illinois fields.

Researchers tracked the incidence of glyphosate-resistant waterhemp on 105 central Illinois grain farms, examining nearly 500 site-years of herbicide application records from 2004 to 2010. They also analyzed landscape, soil, weed and farm management data — more than 60 variables in all, says Aaron Hager, University of Illinois weed scientist and study co-author. “Anything we could measure in a field, we did.”

The goal: “We were trying to understand why, on one side of the blacktop, we see a soybean field overrun with glyphosate-resistant weeds, and on the other side of the blacktop, a weed-free soybean field. Is there a common denominator in fields that are clean versus those with resistant waterhemp?”

Herbicide rotation won’t stop resistance

Using herbicide rotation to curb the evolution of resistant weeds only works if there’s a penalty associated with being able to survive a chemical, explains Tranel.

The fitness cost means resistant weeds won’t be as vigorous as herbicide-susceptible weeds. That puts resistant weeds at a competitive disadvantage in the rotation year, when the population is not exposed to the herbicide that it’s becoming resistant to. This leads to depletion of resistant weeds as a portion of the total weed population.

But for most herbicides, including glyphosate, resistance carries little or no fitness penalty, Tranel says, so resistant weeds grow and reproduce just as well as susceptible strains. Therefore, the herbicide used in rotation kills the same proportion of weeds that are resistant and susceptible to the previous herbicide. The result is that the share of resistant weeds in the field does not go down.

Your management is key

Study results found that the best predictor of glyphosate-resistant waterhemp was the farmer’s own management practices.

Other factors — landscape or soil characteristics, high weed densities, proximity to infested fields — were much less important. This last finding was a bit of a surprise, Tranel adds, because growers often believe that they can’t manage resistance effectively unless their neighbors do, too. “That’s what’s encouraging,” he says. It shows that resistance is not inevitable. “It’s what you did in your field that matters.”

Glyphosate-resistant waterhemp was most common in fields:

By contrast, farmers who used properly targeted tank mixes every time they sprayed were much less likely to have resistant weeds in their fields, Hager says. Increasing the number of effective modes of action (MOA) per application by just one — from 1.5 to 2.5 — cut the risk of selecting for glyphosate-resistant waterhemp by 83 times.

Roy Wendte’s weed management plan for soybeans in 2016 started with an application this fall of Authority XL plus 2, 4-D (MOA 2, 14, 4). Next spring, he will apply Anthem plus Sencor (14, 15, 5) before emergence, followed by Flexstar GT (14, 9) or, if Roundup Ready 2 Xtend technology is available, XtendiMax (4, 9). Regardless of seed technology, he adds, “a residual is very important,” and forms the foundation of his resistance management program.

Know effective modes of action

However, this resistance management strategy works only if each component of the tank-mixture is effective against target weeds, Tranel stresses. Growers must look beyond the number of modes of action in a tank mix to the number of effective modes of action — a distinction that is often lost, he says. “To say there are three modes of action in a jug is not to say there are three effective modes of action.”

Say you’re targeting waterhemp with a premix such as Flexstar GT, a combination of glyphosate and Flexstar, which provides two modes of action, Groups 9 and 14. But if waterhemp is resistant to glyphosate (9), “you really only have one effective mode of action,” Tranel says.

It’s the same story with the coming dicamba- and 2, 4-D-tolerant soybean systems from Monsanto and Dow, says Asmus, who is chair of the North American Certified Crop Advisers IPM Rapid Response Team. The new technologies rely partly on glyphosate. But if you already have glyphosate-resistant weeds, these systems “still provide only one mode of effective action. Even if you have a second mode of action in the tank, if it’s not effective on that weed, it doesn’t count.” Relying on dicamba or 2,4-D alone to control glyphosate-resistant broadleaves will only speed the evolution of resistance to these herbicides, she says.

Likewise, many growers are rotating from a Roundup Ready system to a Liberty Link system in soybeans. “Liberty hasn’t been used routinely, and so its effectiveness is still good,” says agronomist George Soltwedel, retired manager of Effingham Equity, Effingham, Ill. But the same selection pressures that have undermined Roundup will compromise Liberty, too, if the herbicide is sprayed by itself, Hager warns.

“It’s the mixing of chemistries that’s important,” agrees Soltwedel, who worked on the Illinois study. “We know we’re not doing a good job with just one product in the field.”

Current costs versus future benefits

What does this mean for weed management in 2016?

You should plan to use more than one effective chemical on your problem weeds each time you spray, Hager says, “and not as separate applications over the course of a growing season.” The Illinois research shows that “lack of effective tank-mixtures — not lack of residual herbicides — was the best predictor of glyphosate-resistant waterhemp.” Sequential attacks with multiple herbicides, such as “a soil-residual herbicide applied before or after planting, followed by a single postemergence herbicide,” are not sustainable, he says.

However, tankmixing adds expense, Asmus says, and the payback may not be seen for several years. “If what farmers are using is working, they will be reluctant to spend more money.” Yet, growers should understand that the long-term benefits of managing weed resistance will exceed the additional short-term costs, she says.

That’s confirmed by a 2015 report from the USDA Economic Research Service. With horseweed, for example, economic simulations found that managing resistance cut returns in the first year, but boosted returns in the second year and all subsequent years. Resistance management practices included tank-mixing more than one mode of action and not using glyphosate two years in a row.

Projected over a 20-year period, resistance management increased farmers’ returns 14% to 17%, the USDA-ERS estimates. And the economic advantage increased with the number of years of consecutive use.

Wendte agrees that growers can’t afford to ignore resistance management. Yes, it costs more, he says, “but the alternative is worse.”

Farmer succeeds with intensive scouting

For the past five years — ever since giant ragweed and waterhemp became harder to control with glyphosate — Pat Sullivan has been working hard to ramp up weed management.

Sullivan and his father and two brothers grow corn, soybeans and canning crops near Franklin, in south-central Minnesota.

 The Sullivans apply full rates of preemergence herbicides on both corn and soybeans, use Liberty Link technology, and have added non-chemical tactics to their arsenal.

One of the most important is intensive scouting.

The Sullivans scout after each herbicide application, using Precision Planting’s FieldView Plus to pin the locations of surviving weeds. GPS field coordinates, notes and photos automatically sync to everybody’s iPads “so we can all see it,” Sullivan says. “Then we do a lot of spot- and hand-spraying” of escapes. “We also go out with Rangers and pull weeds by hand.”

Growers might not think of a herbicide application followed by hand-pulling survivors as “two modes of effective action” on target weeds, says Pat Tranel, University of Illinois weed scientist — but it is.

The Sullivans pay attention to other weed management details that often go overlooked. They hand-spray fence lines and field borders, and mow road ditches twice a year to prevent weeds in non-cropped areas from producing seeds.

They’ve also intensified sprayer management. “We switch out tips for different products, instead of compromising on a tip that’s a ‘happy medium,’” Sullivan says. “We adjust water volume, pressure, and speed according to what products we are using.” They also added injection systems on their sprayers for better drift control.

These refinements matter, Hager says. “Don’t treat tank mixes like glyphosate. If the tank mix partner has to carry the load, do what you can to optimize its performance.”

The Sullivans also focus on timely postemergence applications. “We’ve found that earlier is better. Any weed over about three inches, we can’t reliably kill.”