Herbicide Resistance: 7 Steps For Gaining Control — With A New Twist

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DF, sprayer, spray, pre-emerge, herbicide, ferguson, griffith

Okay, you’ve seen plenty of lists that suggest ways to prevent or cope with herbicide resistance.

This one is different.

The first of the 7 points takes into account antagonism, a process that weakens a herbicide’s effect when it’s in specific tank-mix combinations. That weakening effect, in turn, might give grasses a huge head start towards glyphosate resistance.

It’s a point that rarely surfaces in these discussions, but things have changed with the wide adoption of dicamba-based technology. This resistance-fueling effect has been identified with certain grasses when dicamba and glyphosate ride along together.

All seven points are important. They form what weed scientists agree is a realistic approach to resistance management. But if you’re short on time, at least read point #1.

Here’s the list:

#1. Mix herbicides but understand how to do it right.

Mixing two or more effective herbicides in a tank mix is better for managing weeds and resistance than rotating crops or technologies.



But do your homework before you mix. Things have changed, especially when dealing with new herbicide technologies. Herbicide antagonism has thrown an extra complication into tank-mix planning. That’s the word from Larry Steckle, University of Tennessee Extension weed scientist.

Check for possible antagonism between products, or you could make resistance worse, Steckel emphasizes.

Mixing Roundup and dicamba is a prime example, he says, because the combination “impedes the Roundup just enough that we’re starting to see more (grass) resistance.”

Steckel says about 15% of barnyardgrass he recently tested in Tennessee “showed increased glyphosate tolerance compared to what we saw a decade ago. Obviously, this resistance is scaling up, and part of that has got to be the antagonism we’re getting from dicamba.”

Steckel noted that other barnyardgrass populations with less resistance could still be killed with a full rate of glyphosate, but when dicamba is added to the tank, that is no longer the case.”

Junglerice now rates as another weed slipping through due to this antagonism.

The bottom line, says Steckel: “Don’t mix Roundup and dicamba together and expect to gain good grass control.” And down the road, that combination could trigger more Roundup resistance in certain grass species.

#2. Do something different.

“In some parts of the United States, weed resistance isn’t as much of a problem because producers build much more diversity into their crop rotation.

Says Kevin Bradley, Extension weed scientist at the University of Missouri. “For us in the Midwest, diversity would be adding more wheat to the rotation. Unfortunately, that isn’t done much anymore. We’re mostly corn and soybeans, and in some cases just soybeans.”

#3. Don’t shave rates.

“With the current economy, we’re always looking for ways to cut costs,” notes Bob Hartzler, Extension weed scientist at Iowa State University. “But this isn’t the time to shave herbicide rates. Don’t be pennywise and pound foolish.”

Short-term economics may dictate shaving rates a bit, Hartzler adds, “but if you look at it over a 10- to 15-year period, this ends up costing you money because you’ve selected for resistance.”

#4. Keep weeds guessing.

Don’t use the same herbicide program 2 times in a row, whether you had good control or not, Hartzler advises. Also, verse yourself on herbicidal modes of action. Don’t just rotate products – rotate how herbicides actually kill weeds, and modes of action designate those differences.

“Every field differs and farmers know that, but we’ve gotten into the habit of trying to use uniform programs everywhere, and that can hurt.”

#5. Aim high.

Go into every season with a zero-tolerance objective for controlling weeds and seeds. In the language of baseball, “Always swing for the fence.”

But be realistic, too.

From a scientific standpoint, Hartzler isn’t too keen on the idea of zero threshold or zero tolerance because it’s impossible to achieve.



On the other hand, setting one’s weed control goals too high may not be a bad thing,” he notes. “That’s because even if you fall short, you can still attain a level of weed control that is sustainable. “If you go in thinking 95% control of weeds is good enough, that program may hold together for three or four years, but eventually it’s going to collapse.

“Strive for perfect weed control,” Hartzler adds, “but know when to throw in the towel, as well. And if you’re losing repeatedly with the same strategies, change things up from what you’ve been doing.”

#6. Use practices you don’t pour out of a jug.

Steckel advises producers to use tillage where appropriate and also consider planting a cover crop.

Cover crops can provide decent weed control, Steckel says. “We recommend a grass cover crop such as cereal rye or wheat, with a legume like vetch or crimson clover. The keys are achieving good ground coverage and producing plenty of biomass.”

Xtend and Enlist technologies help make cover crops a better weed management tool, Steckel says.

“With these technologies, we can terminate the cover crop closer to planting or even after planting. When we do that, we’re reducing the number of emerging Palmer amaranth by up to 50%.”

#7. Consider the long-term.

A big impediment to farmer adoption of resistance management tools is the extra cost.

“The number one reason why these practices aren’t being implemented are the tight margins in corn, soybean and cotton production,” Bradley states.

On the other hand, “It is possible to get weed resistance under control,” Bradley adds. “I deal with many growers who are not going to let pigweeds produce seed in their fields. That pays dividends later. I think it’s a 2- to 3-year proposition to regain control in a really bad field, but it can be done.”