The bad news about herbicide resistant weeds is that the problem is likely to get worse. The good news is, there are steps farmers can take to reduce the problem. Jason Norsworthy, weed scientist from the University of Arkansas, told growers at the Nebraska Soybean Day, Dec. 15, combatting resistant weeds takes a more holistic approach than what farmers have become accustomed to in the era of glyphosate tolerant crops.
“Simplicity has a cost. Cheapness has a cost. We all have a tendency to do things the easiest and most efficient way. But ultimately, it cost us glyphosate tolerant corn, soybeans and cotton,” he said. “Crop rotation is part of the solution. But rotating Roundup Ready corn with Roundup Ready soybeans and Roundup Ready cotton is not sustainable.”
In the High Plains, glyphosate tolerant kochia, pigweed and marestail are big challenges. Add in that these same weeds are also often resistant to acetolactate synthase-inhibitor herbicides and protoporphyrinogen oxidase herbicides, and the weed control toolbox is pretty lean. But Norsworthy said there are six steps farmers can take beginning with the 2017 crop year to help combat these weeds.
1) Identify the enemy
A given crop field will have 25 or so weeds. Don’t create a weed control program for all of them. Instead, develop a program for the two or three worst weeds, and pray that the program works on the rest. You’ll have to know when these weeds emerge, their growth rate and how much seed they produce. You’ve got to learn the weaknesses of these worst weeds.
2) Start clean, stay clean
Strategic herbicide application is necessary. This will require a combination of a burndown a month or two prior to planting, application of a residual herbicide, a post-emerge herbicide application and then follow-up herbicide applications until crop canopy.
3) Apply the correct rate at the correct size
Glyphosate used to control weeds of impossibly large size. Those days are gone. You must spray herbicide when weeds are less than 4 inches tall (preferably smaller), and get good coverage with the proper nozzle package. Furthermore, use the correct rate! Less-than-lableled rates hasten the weed’s ability to infer resistance to the herbicide.
4) Use multiple effective modes of action
Farmers are aware of the benefits of tank mixing herbicides to get good control. However, let’s say you want to use glyphosate in combination with another product on Palmer amaranth. Well, since glyphosate no longer works on palmer amaranth, you effectively have just one mode of action. It’s important to think big picture on herbicide programs, Norsworthy said.
5) Exploit biological weaknesses
Cultural and mechanical practices should be adopted. For instance, in soybeans, narrow rows help intercept light more quickly than wide rows. The canopy closes more quickly and helps thwart weed emergence. In some cases, long-term no-tillers plow 20 percent of their acreage every five years. That helps reduce Palmer amaranth emergence by 97 percent, which reduces selection pressure on herbicides, the weed scientist said.
6) Prevent weed seed production
Let one weed go, and the weed population grows exponentially until fields become physically overwhelmed with resistant weeds. Norsworthy said there were soybean fields in Nebraska that could not be harvested this year due to a solid stand of Palmer amaranth. You can’t prevent all weeds from producing seed, but you can make a good dent by physically removing Palmer amaranth weeds (a big job, no doubt). Save your worst fields until last, and clean out the combine after problem fields.