Kentucky Corn, Soybeans: Postemergence Herbicide Applications

    Postemergence dicamba spray in soybeans. Photo: Debra Ferguson

    Kentucky soybean planting is quickly wrapping up and the majority of corn has or is receiving a postemergence herbicide application as it quickly advances in growth stages. Postemergence applications on soybean will soon begin if they have not already begun.

    In this article, we want to give farmers a few reminders when making postemergence applications in soybean. This year, more than ever before, farmers and applicators will need to focus on the small details as there may not be many second chances this year with the ongoing herbicide shortage.


    Weed size matters and the smaller the better. All postemergence herbicides work best when applied to 2- to 4-inch tall weeds and failures are more likely to occur when applications are made to weeds larger than this size. This is especially important this year, with many farmers relying on tank mixes of selective herbicides to achieve weed control in the absence or lack of availability of glyphosate and glufosinate.

    Glyphosate can be very forgiving and can control weeds much larger than the recommended 2- to 4-inch height, but many of the alternative herbicides that will be going out this year are not as flexible and forgiving. While the 2- to 4-inch rule should apply to all weed species, it is especially important to remember when dealing with waterhemp and Palmer amaranth.

    While it is understandable that weather can keep sprayers out of the field for extended periods of time, applications should be made as soon as possible to capture the weeds at their smallest size.


    As discussed above many applicators and farmers will be forced to use tank mixtures of selective herbicides in the shortage of broad-spectrum herbicides such as glyphosate and glufosinate.

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    If you are using a tank mixture of selective herbicides, make sure to check that the products are both physically compatible and will not antagonize each other’s activity. A combination that is often considered in the absence of glyphosate and glufosinate in auxin-resistant soybean (RR2Xtend, RR2XtendFlex, and Enlist E3 soybean) is a group 1 herbicide (i.e. Select or Assure) with a group 4 herbicide (Xtendimax or Enlist One).

    In this combination, the group 4 herbicides can antagonize the herbicidal effects of the group 1’s (loss in control as compared to when applied alone) on certain grass weed species. Although the antagonism can be overcome by choosing either the correct rate structure or separating the application of products. The appropriate rates to overcome the antagonism can be found in the herbicide labels.


    If you’re making a postemergence application, then it is likely that either your residual herbicide has broken and is no longer active or you did not apply a residual herbicide. Either way, adding a residual herbicide to your postemergence tank mix can bring significant value to the application.

    A postemergence application with foliar active herbicides will only control what is emerged and allow additional weeds to emerge in the unshaded space between crop rows. The addition of a residual herbicide to the tank mix will suppress further weed emergence in these spaces and can potentially get the field to the canopy and eliminate the need for a second postemergence application.

    This can be especially important with the ongoing herbicide shortage.

    Residual herbicides that can be applied postemergence in soybean include the group 15 herbicides: S-metolachlor (Dual II Magnum, Prefix, and many others); pyroxasulfone (Zidua, Anthem Maxx, and Perpetuo); dimethenamid-P (Outlook); and acetochlor (Warrant and Warrant Ultra). The group 15 herbicide can be especially beneficial on fields dealing with small seed broadleaves and grass species.


    Residual herbicides all have maximum cumulative rates that can be applied per growing season. If you plan to apply a residual herbicide postemergence that contains the same active ingredient as was applied in your preemergence application, make sure you will not be exceeding this limit.


    It may seem silly or even redundant, but double-check the soybean variety and herbicide traits prior to postemergence application in every field. We are all humans whose memories can fail us, especially when trying to remember things from a busy time of year such as planting season. An extra minute of double-checking the soybean herbicide trait can go a long way in preventing a replant situation.


    While you are checking the herbicide traits of the soybean in the field to be sprayed, also take a moment to check your surroundings and the weather. While dicamba has certainly received a lot of attention in the last five years when it comes to off-target movement, all herbicides have the potential to move off-target.

    Double-check surrounding fields and identify any potential susceptible plants and if the current weather conditions will allow for an application to be made without affecting those susceptible plants.


    While selecting the right herbicide(s) and applying at the right time are all very important, many times nozzle selection and sprayer setup can be the difference between a successful and unsuccessful herbicide application.

    Selecting a nozzle for postemergence herbicide application depends on the type of herbicides being applied and the need for drift reduction. Systemic herbicides such as glyphosate, 2,4-D, and dicamba do not have to have maxi-mum coverage to perform, although coverage is still important.

    Products such as Xtenimax, Engenia, Enlist Duo and Enlist One can only be made with nozzles that are listed on the product label that reduce the potential for drift. There are a number of nozzles now available that can reduce drift potential while still achieving the cover-age needed for maximum systemic herbicide activity.

    Applications that contain a contact herbicide such as glufosinate should be made with nozzles that produce medium to coarse droplets. Contact herbicides need maximum coverage and thus nozzles that produce extremely coarse and ultra-coarse droplets should be avoided.

    In either case, if coverage is a major concern, then spray volume should be increased to help increase coverage. Research has consistently found that spray volume has as much if not more of an influence on coverage as compared to nozzle selection and droplet size. So, if coverage is needed, applicators should strive to apply 15 to 20 gallons per acre to assure adequate coverage for maximum control.

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