Resistant pigweed threat to Mid-South crops

At the recent Southern Weed Science Society meeting we had an excellent symposium on Palmer pigweed resistance to glyphosate. I commend the weed science community for finally coming together and saying, “We have a problem.”

I believe that glyphosate-resistant pigweed is a bigger threat to our cropping systems in Arkansas than soybean rust. My concern is we are very poorly armed for the fight.

Glyphosate resistance cannot be compared to other herbicide resistance issues we have had in the past. With most of those we simply had to switch herbicides and we often had even better herbicides close to registration at the time.

However, the Roundup Ready technology changed the entire way most farmers farm. It allowed for efficiency most could only dream of before. It allowed more conservation tillage systems and more broadcast plantings, and it eliminated the need for a lot of cultivation and time-consuming operations such as post-directed sprays.

I do not believe many farmers will or even can go back to farming soybeans and cotton the way they did before Roundup Ready. That is what we are facing if we cannot turn around the resistance threat right now.

Some speakers at the symposium attempted to take what has been learned about glyphosate-resistant marestail to find solutions to the Palmer pigweed problem. Those two weeds do not have much in common, and very little of what we have learned about marestail will help.

One thing the Tennessee guys pointed out about marestail is they have been forced to use more conventional tillage systems to solve the problem. This further illustrates my point that glyphosate resistance will not only change the herbicide program but also the entire farming system.

Some speakers at the meeting offered different resistance management programs for situations where the Palmer pigweeds were not yet resistant to either glyphosate or the ALS inhibitor herbicides, where they were resistant to glyphosate but not the ALS inhibitors, and where they were resistant to both.

My philosophy is all cotton and soybean fields that have Palmer pigweed need to be managed as if you have glyphosate resistance, and most of ours are resistant to the ALS inhibitors.

I believe we will be much more successful preventing the problem than ever controlling it after it occurs.

Every company is going to have a resistance management program — usually involving its herbicides. Some are already advertising on television. Not all of these are bad, but not all are going to be successful.

Most of the resistance management programs involve alternative herbicides. That is all we have, so we must do the best we can with them.

In a lot of situations in Arkansas, however, all existing herbicides were failing on Palmer pigweed before Roundup Ready came along.

At our on-farm research site at Newport, Ark., when I was a university scientist, I could take a program of Treflan or Dual preplant incorporated followed by Sencor, Canopy XL or Authority pre-emergence followed by two timely applications of Reflex postemergence and get a plot with 80 percent control that could not be harvested because it was overgrown.

Therefore, adding one of these to the tank with glyphosate or as a separate treatment in a glyphosate program will not correct a problem once it happens.

On the positive side, I think programs with other herbicides can help prevent or delay the onset of resistance in a lot of situations.

Also, there are some real opportunities with corn and even grain sorghum rotations this year, if the acreage of these crops increases as predicted. Atrazine and some of the other corn herbicides are much better on Palmer pigweed than most that are used in cotton or soybeans.

Take advantage of those rotations where possible in some of your worst pigweed fields.

I am not conceding the fight by any means. However, if we allow resistant Palmer pigweed to get the momentum it is going to be very difficult to stop.

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