By Mark Loux
Options to try to make weedy fields more harvestable include preharvest application of certain herbicides or waiting until after frost to harvest, or both. Weedy fields should generally be harvested as late as possible to give maximum time for weeds to die and dry down and rot. Vines can be especially problematic, and burcucumber seems to be having a good year based on comments we have received. Our experience is that a combination of herbicide, time, and frost can be the most effective way to mitigate problems with vines. They need to become rotted and brittle enough to be prevent them from tangling up harvesting equipment.
One of the problems with preharvest herbicide applications is how to apply in mature crop, and how to maximize coverage of weeds with low-volume aerial applications. Low spray volume limits the effectiveness of contact herbicides labeled for this use – Gramoxone, Aim, Sharpen (soybeans only). Gramoxone can be effective for dessication of some weeds when applied by ground equipment in the recommended volume of 20 gpa. Sharpen and Aim are not likely be effective enough across a range of weed species unless mixed with another herbicide. Systemic herbicides will generally be the better choice in low-volume applications. Glyphosate and dicamba are labeled for use in corn and soybeans, and 2,4-D is labeled for use in corn only.
Information on preharvest herbicides is listed at the end of the corn and soybean herbicide description sections in the Weed Control Guide for Ohio, Indiana, and Illinois, and here. The crop must be physiologically mature at time of herbicide application. Labels specify what the measure of this is – black layer formed, dented, % of pods with mature brown color, etc. The minimum interval between application and harvest to avoid residue problems is also specified, and ranges from 3 to 15 days. Source : osu.edu