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 these crop sections (pages 74 and 141 of the 2019 edition). Products labeled for corn include Aim, glyphosate, and paraquat. Products listed in the guide for soybeans include Aim, paraquat, glyphosate, and Sharpen. Some dicamba products are also approved for preharvest use in all types of soybeans, which escaped our notice until now, so it is not listed in the guide. The basic information for preharvest dicamba (for 4 lb./gal products):
Apply 8 - 32 oz/A 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; do not use preharvest-treated soybean for seed unless a germination test is performed on the seed with an acceptable result of 95% germination or better; do not feed soybean fodder or hay following a preharvest application of this product.
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 week or more following application. Differences 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 morninglory. Glyphosate is not likely to be effective on marestail and waterhemp, and many giant ragweed populations, whereas dicamba may with enough time between application and harvest. The first frost will usually provide the same results, so in a situation where crop maturity is delayed as is the case in many fields this year, consider whether an herbicide treatment is actually needed.
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.