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.
Preharvest treatments are not intended to be used to speed up crop maturity, and largely do not accomplish this. The restrictions on preharvest treatments that specify how mature the crop must be at time of application are designed to minimize any effect of herbicides on crop maturation. Applying earlier than specified could interfere with that process. The residue tolerances for this use are also based on a certain application timing, and failure to follow label guidelines could result in illegal herbicide residues in grain. For crops being grown for seed, and for sweet corn and popcorn, be sure to check with the seed company/processor for approval prior to using any preharvest treatments. In general, though, labels prohibit feeding of treated hay or straw or application to crops grown for seed. Consult the weed control guide and labels for more info. The basics are as follows:
Dicamba - soybeans: Apply 8 - 32 oz/A (4 lb./gal products) as a broadcast or spot treatment after soybean pods have reached mature brown color and at least 75% leaf drop has occurred; soybeans may be harvested 14 days or more after a pre-harvest application.
Sharpen – soybeans: Apply 1-2 oz/A at least 3 days prior to harvest, or 10 days for most effective desiccation. Soybeans should have at least 65% brown pods and 70% leaf drop with seed moisture of 30% or less.
2,4-D - corn: Labels vary with regard to types of corn that can be treated (some indicate no sweet corn) and based on whether crop is being grown for seed. Apply after the hard dough (or dent) stage when silks have turned brown. Weed seed production can be suppressed if applied prior to the flowering stage. Allow 14 days between application and grain harvest. Do not forage or feed corn fodder for 7 days after application.
Aim/Longbow – corn and soybeans: Apply 1-1.5 oz/A at least 3 (soybeans) or 7 (corn) days before harvest.
Paraquat (Gramoxone etc.) – corn and soybeans: For corn, apply up to 1.3 pts/A 7 days before harvest, after black layer has formed at base of kernels. For soybeans, apply up to 10.7 oz/A 15 days before harvest, when at least 65% of seed pods are brown or when seed moisture is 30% or less.
Glyphosate – corn and soybeans: Rates vary with product formulation – see labels. Apply to corn 7 days before harvest when grain moisture is 35% or less, and black layer has formed with maximum kernel fill complete. Apply to soybeans 7 to 14 days before harvest (varies with product) after pods have lost all green color.