Reminders for postemergence herbicide applications in soybean

T.J. Adkins

Nearly three quarters of Kentucky soybean were planted as of the June 6, 2021 crop progress report with over 60% being emerged. Now that the majority of soybean are in the ground and emerging it is time to start evaluating and thinking about postemergence herbicide applications. Assuming that residual herbicides were applied at planting, farmers can typically expect to need to make postemergence application four to six weeks after planting. Those who have not applied a residual will need to be making applications much sooner if they haven't already. In this article we want to give you a few reminders and tips to assist in maximizing those soon to be applied postemergence soybean herbicides.

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. 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 to weeds that are a maximum of 6-inches tall.

Add another Residual if you're making a postemergence application, then it is likely that either your residual herbicide has broken and is no longer active or you did not apply a residual herbicide.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. Even when using a robust residual herbicide at planting the first postemergence application will likely occur several weeks prior to soybean canopy. 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 canopy and eliminate the need for a second postemergence application. 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.

Double Check Herbicide Traits, It may seem silly or even redundant, but double check the soybean variety and herbicide traits prior to postemer- 2 gence 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 double checking the soybean herbicide trait can go a long way in preventing a replant situation.

Check your surroundings, 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 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 effecting those susceptible plants. Glufosinate likes it Hot and Sunny The use of glufosinate for postemergence applications is likely to be on the rise this season with numerous trait packages now available with glufosinate tolerance. Those who have not applied glufosinate in the past need to keep a few things in mind to maximize this herbicide. 1. Glufosinate is maximally effective when applied on hot sunny days, which is typically achievable in Kentucky. 2. Avoid applications of glufosinate during extended periods of overcast cloudy conditions. 3. Avoid applications of glufosinate in the late afternoon and evening. 4. Glufosinate is a contact herbicide and thus coverage is critical for maximum performance. Apply with nozzles that produce medium to coarse droplets and apply at a spray volume of at least 15 gallons per acre.

June 30th Cutoff for Dicamba, there were a number of additional restrictions placed on the latest registration of the dicamba herbicides for postemergence applications to dicamba tolerant soybean. One of the restrictions was a cutoff date of June 30th, after this date regardless of soybean growth stage dicamba is no longer allowed to be applied to soybean.

Sprayer setup is as important as product selection 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 in 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 herbicide such as glyphosate, 2,4-D, and dicamba do not have to have maximum coverage to perform and thus can be applied with low-drift nozzles to reduce the potential of off target movement. Products such as dicamba and 2,4-D can only be made with nozzles that are listed on the product label that reduce the potential for drift. 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, such as with contact herbicides, applicators should strive to apply 15 to 20 gallons per acre to assure adequate coverage.

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