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Double down on weeds

Gil Gullickson Updated: 10/04/2012 @ 2:25pm Crops Technology Editor for Successful Farming magazine/Agriculture.com

So what's caused weeds to resist glyphosate and other common herbicides?

Well, let's first trot out all the usual suspects:

• Repeated use of the same herbicide year after year.

• Use of below-label rates that lead to sub-lethal weed kills. This practice permits resistant weed biotypes to survive and to thrive.

• Treating weeds past their recommended growth stage. Killing enormous weeds is fun. And most of the time, glyphosate herbicide does kill them. Still, there's nothing worse than making a weed angry. Stunted weeds can snap back, produce seed, and greatly haunt you in subsequent years.

Application woes will key complexity

There's another culprit, believes Bob Wolf, co-owner of Wolf Consulting and Research in Mahomet, Illinois. “My contention is that application played a role in weed resistance to herbicides due to poor coverage and not getting enough product on the target for effective coverage,” he says.

Long-term, this will create an increasingly complex world of weed management. Here's why.

“Killing weeds is the first goal of applying a herbicide,” says Wolf, who recently retired as an Extension agricultural engineer at Kansas State University. “Minimizing drift is the second goal. Typically, small droplets improve coverage. Unfortunately, smaller droplets increase drift potential.”

Starting in the early 1990s, drift-reduction nozzles came onto the scene. They were able to make droplets bigger. Later on, this was combined with drift-control additives for stopping drift dead in its tracks. Still, this came at a price.

“With bigger droplets, you got less coverage,” notes Wolf. This was compounded by use at lower-than-recommended pressures.

“My observation is that applicators used them (drift control nozzles) just like the nozzles they used before at the normal pressures of 30 psi to 40 psi,” he says. “We found through research we did at Kansas State University that drift-control nozzles needed to be 60 psi to 80 psi to be effective.”

Subpar herbicide application also coincided with spraying at higher-than-recommended speeds. With new sprayer costs topping off close to $400,000, there's lots of pressure to get more acres to make it pay. Pressure is particularly acute for commercial applicators.

“Usually, applicators are paid to cover acres,” says Wolf. “It might not be the best incentive, but it is a challenge we have.”

Coverage vs. drift potential cuts

Weighing in on coverage vs. slicing drift potential while covering more acres will continue to be a trade-off. This is particularly true now that technology (which enables crops to resist 2,4-D, dicamba, and HPPD inhibitors) is entering the scene. Drift is particularly a concern with 2,4-D and dicamba, although manufacturers say new formulations are easing this worry.

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