Preharvest Herbicide Treatments for Weed Desiccation

By Mark Loux

There are plenty of fields with late season weed problems this year.  Weeds that come through the crop canopy late may be small or spindly or sparse enough to be handled easily by a combine.  Other fields can benefit from a preharvest herbicide treatment to kill/dissociate weeds, which makes harvesting easier and can reduce weed seed production and foreign matter in harvested grain.  Information on preharvest herbicide treatments for field corn and soybeans can be found in the “Weed Control Guide for Ohio, Indiana, and Illinois”, at the end of those crop sections (pages 75 and 146 of the 2022 edition).  Products listed for corn include Aim, glyphosate, 2,4-D, and paraquat, and for soybeans include Aim, dicamba, paraquat, glyphosate, and Sharpen.  Keep in mind that Aim and Sharpen have relatively narrow spectrums of activity, and will be less effective than the others across a broad range of weed species (i.e. make sure the target weed is something that they actually control).

Preharvest herbicide treatments are primarily intended to suppress/kill and desiccate weeds that can make harvest more difficult.  Products with contact activity will cause faster desiccation and leaf drop of weeds but may be less effective at killing weeds compared with systemic products.  Effective desiccation with contact herbicides may still require a wait of a week or more following application, and this can vary by weed.  The maximum paraquat rate is well below the rate required to actually kill large weeds, but it is still probably most effective for desiccation of morning glory (allow vines to rot for a few weeks after herbicide application).  Glyphosate is not likely to be effective on marestail and water hemp, and many giant ragweed populations, whereas dicamba or 2,4-D may with enough time between application and harvest.  The first frost will usually provide results similar to herbicides, so in a situation where crop maturity is delayed or the infested field can be harvested later in fall, consider whether a herbicide treatment is actually needed.  Preharvest treatments can also be effective for control of warm season perennials, and the systemic herbicides will be most effective where this is the goal.  Keep in mind also that for weeds with fruits that can contaminate harvest, such as black nightshade, the preharvest treatment can desiccate the foliage but will not affect the fruits, except that desiccation of weeds may result in fruits closer to the soil.