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Mid-season weed management in soybeans

By Dr. Mark Loux, OSU Extension Weed Specialist, adapted from C.O.R.N. 2020-21

A few weed-related observations while we try to stay cool and hope for a day of rain or at least popup thunderstorms.

One of the frequent questions during extended dry weather is — do I wait for rain before applying POST herbicides, or just go ahead and apply before the weeds get any larger and tougher to control.  Our experience has been that it’s best to go ahead and apply when weeds are still small, even if it’s dry, and herbicides will usually do what they are supposed to.  Letting them get larger without any sure forecast for rain can make for a tough situation that requires higher rates or a more injurious mix.  On the other hand, waiting to apply can be fine if there is a good chance of rain within the next few days.  It’s not always an easy decision.

The deadline for applying dicamba to Xtend soybeans was June 30.  Tavium can still be applied where the soybeans were planted less than 45 days ago and have not exceeded V4, an alternative to dicamba will have to be used.  We should point out that very hot days and warm nights are not appropriate conditions for applying dicamba anyway.

Mark Loux OSU Soybean Researcher Field Leader
Dr. Mark Loux, OSU Extension State Weed Specialist

The replacement for dicamba on Xtend soybeans is usually going to be glyphosate or a mix of glyphosate with either fomesafen (Flexstar, etc), Cobra/Phoenix, or Ultra Blazer.  Will they cause soybean injury?  Yes.  Will the injury be worse under hot conditions?  Probably.  Do you want weed control?  We assume yes.  Using a less aggressive adjuvant approach can reduce the injury.  Example – applying fomesafen with MSO + AMS will be less injurious than COC + UAN.  Be sure to use adjuvants appropriate for the weed species and size though.

Applying POST herbicides early or late in the day may have some potential to reduce injury.  Keep in mind however that the activity of most POST herbicides on weeds is reduced during overnight hours.  In previous OSU research where we applied herbicides at 3-hour intervals from 6 am to 9 pm, activity was substantially reduced from 9 pm through 6 am.  So activity was decreasing after 6 pm and ramping back up after 6 am.  Our studies included fomesafen, glyphosate, Firstrate, 2,4-D, and glufosinate.  Of these herbicides, 2,4-D was the only one not affected by time of day.   Giant ragweed was the only broadleaf weed in the 2,4-D study, which occurred in wheat stubble.

Applying a mix or premix that contains a site 15 herbicide — acetochlor, metolachlor, pyroxasulfone, dimethenamid — often increases the risk and severity of soybean injury.  It’s late enough in the season that we would question the value of including residual herbicides. Weed emergence is tapering off, and the dry forecast will prevent these herbicides from being active anyway.

While it has not been much of an issue in Ohio, fomesafen can carryover and injury corn.  This is most likely to occur for late-season applications followed by dry conditions that reduce the rate of degradation.  We are at this point now, so consider a switch from fomesafen to another group 14 herbicide.  There is no risk of carryover to corn for Cobra/Phoenix or Ultra Blazer.

POST cutoff restrictions for a few soybean herbicides (DBH = days before harvest; from Table 18 of Weed Control Guide):  clethodim – 60 DBH; Cobra/Phoenix – 45 DBH; Enlist Duo/One – no later than R2; fomesafen – 45 DBH; Fusion – prior to bloom; glyphosate – through R2; glufosinate – up to R1 and 70 DBH; Ultra Blazer – 45 DBH.

We have had discussions with growers about double-crop soybeans – whether to use a residual herbicide approach or just use POST herbicides.  Herbicides that cause much injury and slow down growth should be avoided in double-crop soybeans since time from planting to harvest is short.  So the argument for a residual herbicide approach is probably best made in nonGMO or RR soybeans, where use of a site 14 herbicide might be required to control glyphosate-resistant weeds.  Planting a LL, LLGT27, or Enlist soybean would allow use of effective POST herbicides without risk of injury.  The argument against a residual herbicide approach is the possible lack of rainfall to get them working soon enough, their lack of activity on some glyphosate-resistant weeds, and possible increased carryover risk from applying this late.  There are cost considerations also when making this comparison.

The dry weather forecast has some growers abandoning plans for doublecrop soybeans.  This is just a reminder to implement some type of weed control measures in wheat and barley stubble, with the goal of preventing weed seed and increases in the soil seedbank.  Marestail, foxtail, and ragweeds are common weeds in stubble, and waterhemp could be an issue.  In previous OSU research on control of marestail with herbicides in stubble, which can be difficult, applying before the end of July resulted in the most effective suppression of seed.  Mowing or tillage can also be used to control weeds.  Where mowing and herbicides will be combined, herbicides should be used first.

Wheat stubble does provide the opportunity to work on Canada thistle, curly dock and dandelion, among other perennials.  Most effective control of these weeds will occur where they are left undisturbed from late summer into October or early November, which allows them to reach a size when herbicides are most effective.  In other words – don’t mow or treat them so late in summer that they don’t have enough time to regrow prior to a fall herbicide treatment.

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