As soybean crops move rapidly through their early growth stages, one of the greatest risks to your yield potential over the next few weeks is a well-intentioned herbicide pass made just a few days too late.
Because so much of a soybean crop’s yield potential is determined during pod fill, Missy Bauer stresses that what happens now with herbicide passes is about setting up the plant in advance to handle that critical period well.
“We need to be controlling the timing of that in-season application, making sure that we’re not stressing beans with that pass,” says Bauer, Farm Journal Field Agronomist, based in south-central Michigan.
The Danger Of Spraying Into Bloom
Heavy rain, high winds, or prolonged stretches of bad weather can easily push your spraying schedule back by days or even weeks. When that happens, the same pass meant to protect yield potential can chip away at it.
“Spraying beans once they’ve already started flowering will have a negative impact on yield. Try to target being out there to apply herbicides before we get to flowering,” Bauer advises.
Flowering in soybeans is typically triggered by day length (photoperiod) and often begins across many parts of the Midwest just after the summer solstice (June 21 for this year), though early-planted crops can begin sooner.
According to Iowa State University Extension, the R1 stage (beginning bloom) starts the moment you find at least one open flower on any node of the plant’s main stem.
A Messy Spring Can Mean Tough Summer Choices
Granted, getting out in fields ahead of R1 to make post applications could be a challenge this summer for those growers who had an erratic planting season, due to weather.
To make matters tougher, many soil-applied residuals (like Valor, Authority, Sharpen, and metribuzin) cannot be applied over the top due to crop injury concerns, reports Dwight Lingenfelter, Penn State Extension weed science associate.
Beyond the yield loss potential, Isaac Ferrie, a field agronomist with Crop-Tech Consulting in central Illinois, reminds growers that their herbicide toolbox shrinks the further soybeans advance into the reproductive stages.
If you have to spray going into reproduction, Ferrie says to keep these factors in mind:
- R1 & R2 Stages: More labeled herbicide options are available now. Soybeans usually still have time to recover from most of the chemical stress and potential flower abortion.
- R3 Stage (beginning pod): The priority shifts entirely to holding onto every single pod once you reach R3. Pushing a herbicide pass now is heavily discouraged by agronomists, is often off-label, and will deliver poor weed control results at best.
Consider The Labels For Product Mainstays
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 notes.
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.
If you are evaluating alternative chemistries, Lingenfelter offers a list of herbicides with foliar broadleaf activity and/or residual activity, along with their labeled application timings and strengths. Learn more details from his recent article, available here.
