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Resistance management dilemma in soybeans

It is often said by those in the agricultural community that the problem with weed resistance management is that it costs as much as the cure. In other words, why should I pay for an additional herbicide to prevent resistance, when I can go ahead with my current program until I have resistance and then absorb the cost of the additional herbicide?

The problem with that line of thinking is that once you get resistant biotypes on your farm, they never go away. For example, I believe cockleburs in Arkansas fields that were resistant to Scepter herbicide and other herbicides with that mode of action are still resistant, even though this weed has not been a problem and has been effectively controlled since the introduction of Roundup Ready soybeans.

If cocklebur were to suddenly develop resistance to glyphosate, Scepter would not be an option on those farms. If resistance had been prevented, then Scepter might still be a useful tool on those farms. This is a speculative example — I am not saying that we have a glyphosate-resistant cocklebur.

I believe we are on the verge of the same thing with Palmer amaranth, or Palmer pigweed, and glyphosate today. We are aware of at least around 500 acres in Arkansas that may be infested with Palmer amaranth that is more tolerant to glyphosate than other biotypes in Arkansas.

Since the discovery of this weed was announced, many of you have let me know that you also have had some pigweed that you have had to re-spray to control. While many populations of Palmer amaranth are still being controlled with glyphosate alone, we may be getting a wake-up call that now is the time for prevention.

In the end, producers will have to decide if it is worth spending the additional dollars needed for resistance management. If you want to be proactive on your farm to stop the spread of glyphosate-resistant weeds in soybeans, the following are some general suggestions:

Crop rotation. A good crop rotation for us in Arkansas soybean production is soybeans and rice. Not many resistant weeds have developed in this one- to one-year rotation.

In addition to crop rotation, you must rotate herbicide modes of action. A list of herbicide modes of action is available in our (University of Arkansas) MP-44 publication and is available on the Internet at

Applying tank-mixtures is also a good way to prevent the buildup of resistance. Tank mixtures must provide efficacy on the same weeds and both herbicides must be applied at rates high enough to kill weeds.

Use tillage at some stage in crop production or in some sort of rotation with reduced tillage systems. Most of the glyphosate resistance we are working on evolved under reduced-tillage systems and in single Roundup Ready crop systems that had been in continuous glyphosate-only programs for at least five years.

These are general guidelines for avoiding resistance in general. For control and prevention of glyphosate-tolerant Palmer amaranth and other species of pigweed in soybeans, we recommend the following:

Begin with a clean field using either tillage or a burndown program that includes glyphosate or Gramoxone plus Valor.

Where tillage is used, a true pre-emergence treatment of either Valor or Dual will provide residual control of pigweed.

Tank-mixtures of 1.25 pints per acre of Flexstar in-crop will provide postemergence control of Palmer amaranth.

In-crop and prior to pigweed emergence, Sequence herbicide (glyphosate + Dual) can be used to extend residual control of pigweed.

Crop rotation of soybeans to rice is a good resistance management strategy for pigweed, but care should be taken to control pigweeds on levees. This can be difficult if pigweeds are allowed to get above 4 inches tall. Propanil combinations with Grandstand or Aim or Storm will control small pigweeds. 2,4-D is pretty good later in the season if they do not get too large.

This information is available in a new fact sheet “FSA2152 Prevention and Control of Glyphosate Resistant Pigweed in Roundup Ready Soybean and Cotton” and can also be found on the Internet.

Bob Scott is the University of Arkansas Extension weed specialist. e-mail: [email protected]

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