Farmers are getting better at fighting herbicide resistant weeds, but there's still a lot they can learn and do better. That was the conclusion of a survey of corn and soybean growers conducted by DuPont Crop Protection at the 2015 Farm Progress Show. The survey showed 85% of farmers worry about herbicide resistant weeds affecting production, but only about half of them could name the herbicide modes of action used on their fields. And uncertainty remains about how to best use of variable herbicide modes of action or sites of action to fight growing herbicide resistance among weed populations.

Almost everyone has gotten the wake up call to manage herbicide resistant weeds by not overusing one herbicide mode of action (MOA). But the best way to vary MOA is still a question for farmers and industry experts.

Number matter when it comes to herbicide mode of action
A recent study of glyphosate resistant waterhemp by University of Illinois researchers seems to confirm that tank mixtures of more than one mode of action work better to manage weeds and prevent resistance than simply rotating single modes of action from one season to the next.

Jeff Carpenter says he's encouraged by farmers' increasing awareness of how to manage herbicide resistant weeds.

Jeff Carpenter says he's encouraged by farmers' increasing awareness of how to manage herbicide resistant weeds.

On the 105 farms in the University of Illinois study, those where farmers used multiple MOAs in herbicide tank mixtures were much less likely to have resistant weeds on their farms than those who simply rotated herbicides from season to season. The difference was significant. When using an average of 2.5 MOA per application, fields were 83 times less likely to have herbicide-resistant weed populations, compared to growers who averaged only 1.5 MOA per application. Tank-mixing herbicides for each application was by far the most effective way to manage resistance.

Study suggests herbicide mixing is better than herbicide rotation
Pat Tranel, a University of Illinois weed scientist and co-author on the study, said this is not the first time researchers have presented evidence that herbicide rotation is a poor resistance management strategy, but the work on-farm makes the results harder to ignore. "This confirms previous conclusions that farmers should use herbicide mixing rather than rotation."

Another caution is not becoming too reliant on simply rotating herbicide-tolerant crop varieties. "New tools like 2,4-D and dicamba-tolerant soybeans are in development, so some people may think 'I'll throw in 2,4-D with glyphosate, because that's using two modes of actions,' but if you already have glyphosate resistance, then you are not using two effective modes of action," says Tranel. "We don't say that mixing is the end-all solution. But if success is measured by lack or lower frequency of resistance, this study shows that successful farmers use multiple herbicides per application."

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Farmers are increasingly aware of how to manage resistant weeds
Jeff Carpenter, herbicides portfolio manager for DuPont Crop Protection, says he's encouraged by farmers' increasing awareness of how to manage resistant weeds. "It is expensive to hand-weed fields where herbicide-resistance has taken hold," says Carpenter. "From a big-picture standpoint, straight glyphosate use has created the problem, so clearly, history shows dependence on a single herbicide MOA is risky."

Exactly how many herbicide MOA are needed to address resistance depends on local factors, Carpenter adds, but using two, three or more MOA in a herbicide program is relatively easy to do today. Companies like DuPont have introduced herbicide products to address resistant weeds in fields planted with herbicide-tolerant crops.

"But you need to understand local weed issues, not only on your farm, but on neighbors' farms, too," he cautions. "Weed seed moves easily across geographies. Wind, animals, machinery, and harvested and transported crops all are ways weeds can spread, and can do so very quickly."

Improve your herbicide resistance strategy
Carpenter offers these tips on herbicide use to help address resistance:

•Incorporate residual herbicides into your weed-control program.

•Use herbicides and herbicide tank mixes that contain more than one MOA.

•Consider using multiple herbicide applications in a season, starting with a strong preemergence herbicide program.

•Layer residual herbicide products across multiple application timings, such as burndown, at planting, preemergence and postemergence.

"The primary addition I recommend for most herbicide-tolerant cropping systems is inclusion of a preemergence residual herbicide," says Carpenter. "Ideally, that herbicide should contain at least two active ingredients or there should be multiple products in the spray tank to meet that criteria. In soybeans, for example, using PPO, long-chain fatty-acid, ALS or PSII-inhibitors are the most common MOA herbicide classifications that can help prevent early season weed growth and address resistance."

He notes that DuPont already offers several products that are combinations of two or more of these four classes of herbicides. Examples include DuPont Afforia, Canopy Blend, Enlite, Envive and Trivence herbicides. All are dry formulations that are easy to measure, mix and store."

In addition to always reading the herbicide label, Carpenter recommends growers refer to the "Take Action" herbicide resistance management website. All herbicides registered in the U.S. are listed on the site, clearly showing each herbicide's mode of action and site of action on the target weed.