Hit the Weed Window or Pay a Price
Residual reliance as weed resistance builds
Planting with no pre-emerge herbicides or residuals is a dance through a minefield, particularly in an age of herbicide resistance. Far from formulaic, every grower’s approach to preplanting chemical application is contingent on specific circumstances, but a common thread remains: Hit the window or pay a price.
THE BACKBONE
On most Midwest farms, multiple weed species force producers into a wallet- draining game of whack-a-mole, with the most prolific early spring and summer annuals centered on marestail, giant ragweed, waterhemp, common lambsquarter and giant foxtail.
“Looking at our present tools, we’ve become so much more limited in terms of postemerge herbicides,” says Bill Johnson, a weed scientist with Purdue University Extension. “Many weeds have developed resistance to the main postemergence herbicides.”
Residuals were once the backbone of weed control programs, and Johnson hopes for a return.
“I’d like to see us going back to residuals like we used to do, and then using postemerge to clean up escapes,” he says.
With a baseline focus on marestail, waterhemp and lambsquarter, Johnson cites an extensive list of premix options, but draws down on three key chemicals: sulfentrazone, flumioxazin and metribuzin.
“Bottom line, if you have those three weeds, at least use one of those active ingredients,” he says. “There are certainly other products with residual activity on something like giant ragweed, provided the ragweed isn’t ALS-resistant.”
When does Johnson recommend spray activity? “For a residual, ideally we want to see that put down the day of or shortly after planting.”
The overwhelming majority of Ohio soybean fields are sprayed with pre-emerge herbicides, initially driven by an upswing in marestail, according to Mark Loux, a weed scientist with Ohio State University Extension.
“I’d say we once got down to about 40% of our soybean acres getting pre-emerge, but now we’re back to the far majority getting a pre-treatment,” he says. “We’ve got better post options, but the bottom line is we’ve got weed species that are just not manageable without pre-emerge herbicides.”
The biggest resistant weed issues in Loux’s geography come from giant ragweed, common ragweed, marestail, waterhemp and Palmer amaranth. “Take away residuals and you get big problems with lambsquarter,” he says. “Some weeds are relatively easy to control pre, but much harder to control post.”
A BALANCING ACT
The application window, Loux says, is a balancing act. “Go on too early and you reduce longevity into the season. If you plant beans at the beginning of May, and put on a pre in mid-April, you really want residual activity into June, and if you apply too early, you squander some of that reach into June.”
If you’re going after a specific driver weed like giant ragweed, Loux says, then the only herbicides with residual activity are premix products that contain chlorimuron, cloransulam or imazethapyr.
“Something like metribuzin is not strong enough by itself, and we always add something else,” he says.
Loux warns about the possibility of plant injury. “We can put certain products on at planting that cause injury, but there was less potential for that back when we put them on a week or two ahead. To me, that is on the seed companies and the companies that sell those traits. They tell people they can apply anytime they want because you don’t have to wait, but if you put them on after planting, that increases your potential for crop injury.”
As with all chemical decisions, the financial factor weighs heavy. “Look at waterhemp,” Loux says. “It gets control from a residual in the post. Where are you going to pull the trigger and how are you going to divide your chemical dollars between pre and post? That’s a tough choice.”
