Mark VanGessel, Extension Weed Specialist; mjv@udel.edu
As wheat harvest peaks, planting double-cropped soybeans is right around the corner. Even if the wheat field appears to be clean, there is likely to be small weeds that are ready to grow rapidly once they get more sunlight. So, it’s important to use a good burndown herbicide to control emerged weeds.
Where glyphosate-resistant horseweed, Palmer amaranth, or common ragweed are present you will need something in addition to glyphosate to control these weeds. Glyphosate plus Liberty or Liberty plus Enlist has been fairly good treatments for burndown prior to planting double-cropped soybeans.
At this time of the year, we are after the peak emergence period for most species. So, while weeds will continue to germinate, there will not be as many to deal with. Palmer amaranth, morningglories, jimsonweed, and common cocklebur are the main species that will germinate this late in the season. Generally, weeds emerging in July do not attain as much biomass because the shorter days cause the weeds to start flowering and putting resources into reproduction and not so much into stems and leaves.
Here are a few things to keep in mind:
- In non-irrigated fields with no soil moisture, wait until you get rain.
- Liberty needs good spray coverage to maximize effectiveness, including when its tankmixed with glyphosate or Enlist. A minimum of 20 gallons per acre should be used with medium to coarse spray droplets.
- Enlist and E3 soybean is the only 2,4-D formulation and soybean combination that will allow you to spray and plant immediately.
- Enlist is a formulation of 2,4-D developed to significantly reduce volatility, but drift is still a concern, so use the proper spray tips to ensure larger droplets and avoid spraying at high temperatures.
- I frequently see poor control of fall panicum with glyphosate and it is more likely to occur with large fall panicum plants. I have not seen this reduction of control with other annual weed species, but giant foxtail and large crabgrass are the only other grasses in most of my trials.
- Glyphosate, Enlist, and Liberty will not provide residual control, so in fields with Palmer amaranth it is important to include a residual herbicide. This time of year, you do not need the top-of-the-line residual herbicide program. If you have light weed pressure and no Palmer amaranth in the field, you might be able to get away with no residual herbicide. But since you need a burndown herbicide application, an inexpensive soil applied herbicide will get you a little assistance with slowing weeds while the crop gets established and maybe buying a little time before you need to apply your postemergence herbicides.
- Rainfall can be spotty to incorporate the preemergence herbicides so if you have irrigation consider using it to “activate” the herbicides.
- Pay attention to your rotations for the fall and next spring because some herbicides have a rotational interval that may not allow some crops to be planted for 10 months.