The drought has continued to intensify in the Upper Midwest region during the past few weeks, impacting both herbicide effectiveness and weed control options.
While spotty rains have given some relief in parts of Minnesota, precipitation has been inconsistent. As a result, corn and soybean development has been slower than usual, and crop canopies aren’t closing quickly enough to shade out emerging weeds.
Preemergence (PRE) herbicides are the foundation of a robust weed management program, but they require rainfall to be activated. University of Minnesota weed management plots planted in early May got enough precipitation to start the PREs, and those fields look outstanding, according to Dr. Debalin Sarangi, Extension weed scientist. Other plots planted when it was dry two weeks later didn’t receive the needed rainfall. Consequently, there was limited activation of the herbicides, and the stories are now full of foot-tall weeds. While small-seeded weeds, like water hemp, are limited in dry soils, they will start emerging again when it rains. With short soybeans and slow canopy development, weeds could get ugly quickly.
In areas where PRE activation was limited, Sarangi hopes that a postemergence (POST) herbicide application was made early enough to take care of the weeds the PRE missed. However, if a clean-up option is needed, glufosinate (Liberty) can be applied until the R1 (beginning flower) stage in soybean, while Enlist One (2,4-D choline) can be used through the R1 stage. If glufosinate is selected, herbicide activity will be better if applied on a sunny, hot day with some humidity. They are treating weeds when small becomes even more critical when conditions are harsh.
With water hemp reemerging, adding a residual Group 15 herbicide, like Warrant (acetochlor), to a tank mix with either POST herbicide can help control. Outlook (dimethenamid-P) may also be an option, but V5 must apply it.
What about the PPO-inhibitor herbicides (Group 14)? Flexstar (fomesafen) has a 10-month rotation restriction to corn, so we’re quickly approaching the application cutoff for this herbicide. However, older chemistry PPOs, such as Ultra Blazer (acifluorfen) and Cobra (lactofen), may be POST options in soybeans.
In dry, hot conditions, some adjuvants can boost herbicide absorption into the plant. Read the herbicide label, and if an adjuvant is mentioned, consider adding it. Row cultivators could also play this year, mainly since the soybean canopy is not doing its job.
Herbicide carryover can become a severe concern in dry years. After dry conditions last fall and this spring, Sarangi has seen corn and soybean injuries from previous year’s herbicide applications. As we approach the end of June, the odds increase that herbicides could carry over to next year’s crop, especially under dry conditions.
“Although corn ear development is starting to be determined now, conditions during pollination play a much more important role,” explains Tom Hoverstad, University of Minnesota researcher at the Southern Research and Outreach Center in Waseca. The most critical time is the two weeks from about V12 to pollination when the potential number of kernels per row is affected.
The most recent rain will carry parts of southern Minnesota for several days, but more rain will be needed.
Corn plants pick up moisture from the bottom of the root zone when the top few inches of soil are very dry. Without water, nitrogen at the top of the root zone is not readily available to the crop. Now that the area has received some rain, Brad Carlson, University of Minnesota Extension educator, doesn’t expect to see nitrogen stress immediately. However, deficiencies may appear in August if early season N loss occurs.
UMN research has shown that side-dress N applications past V10 or V12 are ineffective. While the trend toward side-dressed N has increased during the past decade, unfavorable weather, too wet or too dry, can put those applications and profits at risk.
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Thanks to the Minnesota Corn Growers Research and Promotion Council and the Minnesota Soybean Research and Promotion Council for their generous support of this program!
