When resistant weeds are concerned, herbicide modes of action don't come back from the grave.
At Bayer CropSciences' Respect the Rotation field day, University of Arkansas' Jason Norsworthy explains in the mid-1990s many fields developed weed populations with resistance to ALS chemistries. Along came the Roundup Ready system, and everything was wonderful. Today, glyphosate is losing its effectiveness. Why not give ALS another try?
"Once you lose a herbicide mode of action, it's gone," Norsworthy says. "And we're losing herbicides at an alarming rate."
Globally, Norsworthy estimates 11 weeds evolve resistance to herbicides every year. Even worse, there are no new modes of action in the pipeline, he adds. Point is: it's time to get serious about protecting effective chemistries.
This particular Respect the Rotation was held at Collinsville, Ill. The site was chosen because there is a massive population of glyphosate-resistant Palmer amaranth growing unchecked. Southern Illinois University weed specialist Bryan Young estimates there's around 2,000 acres of glyphosate-resistant Palmer amaranth currently in Illinois. It will spread.
Hailing from Arkansas, Norsworthy has seen Palmer amaranth (he calls it pigweed) take over in the mid-south. From the time a farmer sees a single escape, Norsworthy estimates it will take only two to three years before glyphosate is completely ineffective.
To control these super weeds, it's going to take better management and a bigger herbicide budget. Young and Norsworthy agree the first key is starting with a clean field.
"If you don't start clean, those escapes will be a thorn in your side for the rest of the year," Young adds.
To start clean, growers must train themselves to spray smaller weeds. Gone are the days of spraying 12-inch weeds. Young says the max size is three inches for Palmer amaranth.
If glyphosate or glufosinate are working on your farm, switch chemistries now! Young says the worst thing growers can do is continue to challenge herbicides, i.e. use a one-pass post system to kill seven-inch weeds. This is what caused problems with glyphosate in the first place.
Additionally, both weed specialists say a liberal use of soil-residual herbicides is now necessary. Young recommends overlapping residual herbicides by tank-mixing a residual with the post-emergence pass.
Norsworthy reminds growers that multiple modes of action are no longer a luxury; they're a necessity. He posed this little scenario at the event. Say a farmer applies a pre-emergence pass of Valor and a post-emergence pass of Flexstar and Roundup. How many modes of action did this grower utilize?
The answer is one. He notes the weed science community is working toward a numerical system to make it easier for growers and applicators to recognize different modes of action.
Finally, Young says it may be time to swap spray nozzles. Roundup made large, drift-resistant droplets all the rage. It's time to get back to medium droplets, Young says.
"Most of the Roundup we've been spraying for years has been applied with droplets between 600 and 800 microns," Young notes.
The label for the Liberty Link system, which utilizes glufosinate as its active ingredient, specifies a droplet size of 300 microns. Yeah, a medium-size drop is more susceptible to drift, but Young says it's necessary to get control out of these chemistries that are still effective on Palmer amaranth.
Bottom line is the days of a one-pass Roundup system are over. If it's still effective on your farm, you better switch it up soon before you lose Roundup forever.