Some weeds, like horseweed/marestail, can be very hard to control in POST if they’re not controlled prior to planting.
If you have already burned down your soybean fields, be sure to look at them before planting and decide if you are going to need another application of a burndown herbicide (glyphosate or paraquat) due to newly emerged weeds. Even if you included a residual herbicide with your burndown, you need to check to be sure that no weeds have started to emerge.
For those fields that have not been burned down, you have a few things to consider. If you have marestail/horseweed you have two options, 2,4-D or Sharpen to tankmix with glyphosate. Most formulations of 2,4-D used at a pint have a restriction of 15 days preplant, but the 1 pt rate is not going to be very effective on taller horseweed plants. Some formulations of 2,4-D will allow 1 lb acid equivalent (1qt of a 4lb gal) as close as 15 days to planting. Be sure to check the labels. Sharpen use on coarse-textured soils with less than 2% organic matter needs to be applied 30 days before soybean planting due to potential crop injury. Medium to fine textured soils treated with 1 oz of Sharpen has no waiting period, while there is a 15-day interval with the 1.5 oz rate. Horseweed plants beginning to bolt will need at least the 1.5 oz rate for effective control.
Be aware that if you use Sharpen, the label does not allow another group 14 herbicide (Valor, Authority product, or Reflex) within 30 days on coarse-textured soils with low organic matter or 14-days for all other soil types.
Depending on your area, many of the soybean fields may have had a rye cover crop in them. While the rye will help with weed control by suppressing the growth of weeds or preventing weed emerging, it requires very thick mulch of a cover crop to be highly effective. So scout your cover crop fields to determine what the best approaches for weed management are.
From Mark VanGessel, Extension Weed Specialist, University of Delaware. Originally published in the Weekly Crop Update.