Many growers know a total-post weed control program is too big a gamble in any year. But with potential shortages of some postemergence products likely in 2022, this is not the year to risk forgoing residual herbicides.
“We understand that growers sometimes have time-management factors that make total-post an attractive decision to consider. But it’s an even bigger gamble with a limited supply of glyphosate or glufosinate,” says Sarah Lancaster, Kansas State University Extension weed science specialist. “We’re hearing predicted supply issues from retailers who don’t know how available these and other products might be, along with not knowing the price point.”
If you can’t count on postemergence products being available, residual herbicides become critically important. “By using residual herbicides in 2022, growers are buying themselves some insurance to make sure what limited post products we have will work,” she adds.
“By using residual herbicides in 2022, growers are buying themselves some insurance to make sure what limited post products we have will work.”
Growers know many weed seedbanks contain herbicide-resistant weeds, resulting from an over-reliance on specific herbicides and herbicide applications. “Farmers who are on top of their weed control game have started using more residuals,” Lancaster says. “Just like we talk about with our postemergence applications, it’s important to think about applying multiple active ingredients with multiple sites of action in a preemergence herbicide application,” she adds.
In a corn system, atrazine has been and will be, for the foreseeable future, the backbone of those early season programs. “Adding to that, you may want to add something to control triazine-resistant weeds, like a Group 15 product. And maybe you want to mix in a Group 27, like a mesotrione-type product,” she adds. “Those mixes are pretty common, along with some products that contain Group 4 herbicides like clopyralid.”
Good residual combinations like these are helping control major weed problems like Palmer amaranth. Lancaster reports that growers are generally seeing good control of Palmer amaranth using atrazine with a Group 15 plus a Group 27 tank-mix. “But we must continue to pay attention to larger-seeded broadleaf weeds like velvetleaf, morningglory and cocklebur, which can be easier to manage with a postemergence herbicide.”
“But we must continue to pay attention to larger-seeded broadleaf weeds like velvetleaf, morningglory and cocklebur, which can be easier to manage with a postemergence herbicide.”
Another critical aspect of selecting the right residual mix is to consider a product's environmental impact. “While we often talk about the weed control spectrum of different products, we need to think about an herbicide that is more water-soluble and able to control weeds right away versus a product that will stick around in the soil longer,” Lancaster says.
For example, it’s best to combine a quicker-acting product with a longer-acting product. “As we think about trying to manage multiple flushes of weeds, it’s important to have different products out there that can control weeds at different times to help carry us through to a timely postemergence herbicide application,” she adds.
To lengthen control, a Group 15 or a Group 2 ALS-inhibiting type of herbicide does provide a longer half-life in the soil if more rainfall is received for activation. For more details on the strengths and weaknesses of the weed spectrum of residual herbicides by Group number, Lancaster points to a good resource on pages 24-26 of the 2021 Chemical Weed Control for Field Crops, Pastures, Rangeland and Noncropland.
Finding success with a residual program provides a great weed control foundation. However, Lancaster believes it’s just as important to remember to change the program to prevent selecting for tolerant or resistant weeds. One example is the recent confirmation of S-metolachlor resistance found in waterhemp and Palmer amaranth.
“Herbicide rotation is just as important as a good crop rotation. Make sure that when you're rotating crops, you are rotating herbicides,” she says. “If we're going to have Group 15 products, in all likelihood, used as part of our preemergence program in every crop, we must consider the supporting modes of action that we add to the mix. If you're relying on atrazine in corn, make sure that you're using Group 14 products in your soybeans. The same can be said for layered residual products. That's why you want to keep vigilant records to prevent overuse of products.”
Weeds get comfortable in their habits and routines, just as farmers do when using a similar mix of herbicides that are working. “It’s best to keep weeds guessing, and not just from a chemical standpoint. Consider options like more diverse crop rotations, along with changing planting dates, row spacings, cover crops and tillage regimes,” Lancaster says.
One of the best weed control tools you have is footprints in the field. “You can't underestimate the importance of just scouting in a meaningful way, regularly, to keep those later-emerging weeds from adding to the weed seedbank,” she adds. “Weather in a given year is going to affect how and when various weed species emerge with the potential to produce seed.”
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