Weeds like waterhemp, marestail and Palmer amaranth are a worrisome problem in soybean fields this summer. But as the calendar turns to August, beware of tackling weeds too late for labeled herbicide applications and, in the process, cutting your soybean yield potential, advises Isaac Ferrie, Crop-Tech Consulting field agronomist in central Illinois.
He wants to remind growers that the number of herbicides available to address weeds in soybeans rapidly drops off as the crop advances into early reproductive stages.
“There are more labeled herbicide options to control weeds in soybeans that are in the R1 and R2 growth stages, and there’s usually still adequate time for beans to recover from stress and flower abortion,” Ferrie says. “Once we get into R3, we need to hold onto as many pods as possible to protect yield potential, and that can create some complications for any herbicide use.”
In most cases, herbicide use once soybeans reach R3 is discouraged by agronomists and, equally important, not labelled and provide reduced control outcomes at best.
Here are some brief guidelines for herbicide application timing in soybeans for three of the main chemistries in the marketplace. Bear in mind some of these products can be used only on specific seed technologies.
1. Liberty herbicide (glufosinate) – It can be sprayed on LibertyLink soybeans from emergence up to the R1 growth stage (first bloom). Learn more via the label.
“That means once 50% of those plants out there have one flower on them, we can’t spray Liberty,” Ferrie says.
2. Enlist One and Enlist Duo – They can be applied through the R1 growth stage and up to the R2 growth stage. Learn more here about application timing.
“So, as soon as we have a flower in the top two nodes, we can’t be spraying Enlist,” Ferrie says.
3. Roundup (glyphosate) – It can be applied up to R3 in soybeans. The R3 growth stage starts when one of the four uppermost nodes with a fully developed trifoliate leaf on the main stem has a pod that is 3/16 inches long. According to this definition, glyphosate can be applied through the R2 growth stage, but applications after the R3 stage begins will be off-label, notes Christy Sprague, with Michigan State University Extension.
Ferrie offers some great insights on the pros and cons of late-season soybean herbicide applications in a brief video, available here.
For more insights on late-season herbicide applications and options in soybeans, check out this resource written by Extension educators at the University of Minnesota:
Updated soybean growth stage cutoffs for postemergence herbicide application.
Stop Seeds From Going Into The Bank
Weeds that are popping through the soybean canopy are hard to ignore and are worth pulling out of the field by hand. Pulling weeds can help reduce weed seed banks and their impact on future crops.
University of Minnesota research shows over 50% of lambsquarters seeds can be viable in 12 years. For waterhemp, 50% of the viable seeds will be degraded in three years. Giant ragweed seeds are degraded relatively quickly, with over 95% of the weed seeds being degraded in two years (Goplen et al. 2017).
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