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The Good, the Bad, and the Ugly of Waterhemp Management

It’s here, it’s ugly, and it won’t go away, but you can still fight waterhemp in soybeans with existing technologies, according to University of Illinois (U of I) weed experts.

The dark side is that nothing new and highly effective is on the horizon to combat waterhemp with multiple herbicide mode resistance.

Once waterhemp developed resistance to glyphosate, chemical options were already limited. With today’s multiple mode resistance, some growers have near-zero chemical options.

“The easiest answer to that situation is to do something different,” says Aaron Hager, U of I Extension weed scientist. “You’ll need to switch to glufosinate-resistant (Liberty) soybean varieties, include tillage, or do both because you have no other postemergence options to control it now.”

If your farm isn’t at that stage yet and if you plan to fight waterhemp with dicamba-tolerant soybeans, it’s likely that you still have some worry about herbicide moving off-target. Recent research shows, in the case of waterhemp, huge spray droplets can be just as effective as drift-prone small droplets.

“We’re recommending that you use some type of nozzle and adjuvant combination that makes a really large droplet spectrum,” says Scott Bretthauer, U of I Extension sprayer technology specialist.

The dark side

Waterhemp’s most significant challenge is that it resists multiple herbicides and has rendered ineffective many options that you use in soybeans, says Hager. It’s just a species that is well adapted to modern farming practices. 

From the time glyphosate resistance was identified more than 12 years ago in Missouri, resistance has spread to several states and to several classes of chemicals.

“We have populations in Illinois where individual plants resist herbicides from four different site-of-action classes,” Hager says.

If you’re a soybean grower, you really have only four classes of postemergence herbicides to throw at waterhemp.

“The ALS inhibitors (Pursuit, Scepter) are almost completely ineffective against waterhemp in Illinois now,” Hager says.

“You could use glyphosate, but resistance continues to increase every year. Your other option would be foliar-applied diphenyl ethers (PPO inhibitors including Flexstar). That resistance also is increasing at a very alarming rate,” he says.

“If a three-way resistant waterhemp emerges, chemically, you cannot control it in conventional or glyphosate-resistant soybean. If you have waterhemp with all three resistances and grow soybeans, the only viable chemical option is glufosinate,” he says.

What about tillage?

It is an option for the waterhemp battlefield. In time, tillage will reduce the weed seed bank, and it has the potential to win the battle against this native weed. Waterhemp can produce more than 1 million seeds per plant, but it has an Achilles’ heel: seed longevity.

“We’re trying to convince people that this is how we’re going to effectively manage the long-term problem,” Hager says. “The way we’re going to ultimately continue farming without a lot of yield loss is by reducing the seed bank.”

The estimated lifespan for seed viability is generally less than 10 years.

“We will not see new herbicides in the next five years – probably not in 10 years – that are effective with new modes of action against waterhemp,” he says. “We’ve tried to say this for years. If you lose the effectiveness of what you have by accumulating resistances, there’s nothing coming from industry.”
 
The bright side

Happily, most of you have time to adapt before your waterhemp gets that far. Bretthauer began evaluating nozzle and adjuvant combinations in 2010 for the arrival of dicamba-tolerant soybeans. The first seed is expected to be released in 2016. His work began after Monsanto and BASF partnered to produce the new soybean and a new low-drift formulation of dicamba for in-crop applications against waterhemp and other weeds.

Bretthauer focused on particle drift reduction because he knew the new dicamba formulation would offer low volatility. However, particle drift could occur with some nozzles and droplets. Certain formulations of dicamba and 2,4-D have a history for causing vapor drift issues.

“Drift is a concern with spraying dicamba. We’ve been looking at drift-reduction nozzles and adjuvants for two things: effectiveness at weed control and effectiveness at reducing drift,” Bretthauer says. 

There was good news when his Illinois research team treated glyphosate-resistant waterhemp with new formulations using dicamba.

“We had effective weed control with all the treatments,” he says. “Treatments that make the biggest droplet sizes in nozzle-adjuvant combinations are just as effective at controlling the waterhemp as are the smaller droplets.”

One surprise

Bretthauer expected that very large droplets would lead to reduced waterhemp control. Fortunately, it didn’t happen on tall morningglory treatments from 2010 through 2012 or with waterhemp treatments at Southern Illinois University in 2013 through 2014.

“One potential consequence of drift-reduction technology is, if you have really large spray droplets, they will bounce or roll off the plant leaves. They may make it onto the leaves, but they may not stay and retain on the leaf,” he says.

Bretthauer didn’t directly measure how well the droplets stuck to plants, but coworkers rated the effectiveness of the applications.

“We had effective weed control with all the treatments. That’s good news for the use of this technology. It means that, whatever we have available for drift reduction by adjuvant selection or nozzle selection, we can focus on the drift reduction and still be confident that we’re going to get good weed control,” he says.

Formulations varied, but results were similar. Glyphosate was packaged as Roundup PowerMax or WeatherMax. The first year, his group used Clarity and later a low-volatility formulation of dicamba.

In 2013 and 2014, they used a numbered glyphosate-dicamba premix formulated for ultra-low volatility. The adjuvant making the largest droplet size and lowest percentage of fines in 2014 was Border. Border is a crop-based, guar additive. Control, a polyvinyl polymer additive, did a really good job in reducing fines the first year.

“Drift-reduction additives do work to reduce drift by reducing the fines. We see a reduction in small driftable droplets when we add a drift-reduction additive to even an air-induction nozzle, so there is a benefit by combining these technologies,” he says.

“I don’t know exactly what Monsanto and BASF will put on their labels, but I do know that only the larger droplet-size classes will be allowed, including ultra-coarse,” Bretthauer says. “Applicators will want to select nozzle types, sizes, and pressures along with an adjuvant that will give them a really large droplet so they can comply with label droplet-size requirements.”

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