Mark VanGessel, Extension Weed Specialist; email@example.com
There are a lot of different issues going on in soybean fields. These are some thoughts based on recent phone calls and observations in the field.
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 may not have gotten the rain to activate those herbicides.
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. 2,4-D at a pint has a restriction of 15 days preplant, but the 1 pt rate is not going to be very effective on taller horseweed plants. 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.
A lot 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 the best approaches for weed management.
Finally, I have been asked why the burndowns may not have worked in some of the fields. First, and foremost consider how effective the burndown is on the plants/cover crop present. Paraquat is generally not very effective on grass species or on some of the broadleaf weeds once they are more than a few inches tall. Glyphosate is not very effective on legumes, mustards, henbit, or annual ryegrass. So maybe the herbicide used was not the right choice.
There can be issues with tankmixing. While a triazine herbicide such as atrazine, simazine or metribuzin can increase the control of paraquat, these herbicides can reduce the effectiveness of glyphosate under some circumstances. So be careful about tankmixing.
Was there adequate coverage? While glyphosate can perform well at volumes well under 15 gal/A, that may not be adequate coverage for heavy weed pressure or trying to control smaller weeds under the cover crop. And paraquat should be applied in higher spray volumes. Was the burndown sprayed with large droplet sizes? While increasing droplet sizes can reduce drift, it can also reduce coverage. So things like air-induction nozzles or drift control agents can reduce spray coverage, which in turn reduce performance. Finally, all these herbicides seem to work better when the sun is shining and that has not happened much in our area over the past month.
Many things can work against you when trying to control weeds. Planning ahead, scouting, and allow time to retreat under these challenging conditions will improve success.