With small grain harvest underway, there are some questions about preplant weed control with double cropped soybeans. This has always been a challenging situation, and with the presence of herbicide-resistant weeds, it has become even more complicated and results are often less than satisfactory. Larger plants that have been cut off or damaged by the combine are going to be less susceptible to herbicides and achieving 100% control will require a lot of environmental factors falling in your favor — the recent dry spell does not work in your favor.
A non-selective burndown herbicide and a residual herbicide for broadleaf weeds are needed for some situations. Remember, you want to start “clean” and if the field has weed seedlings already present, they will have a growth advantage over that of the soybeans. In situations where grasses are present, glyphosate will be the best choice.
Residual products such as Canopy, Valor XLT, Envive, Prefix, and the Authority products are all options to assist with burndown control, but only in some situations due size of the weeds and limited spectrum of control. I list these products because they either do not have active ingredients that are Group 2 (ALS-inhibiting herbicides) or they do not rely only on Group 2 products. If your soybean planting is delayed, remember that Prefix, Valor XLT, Envive, Canopy, and the Authority products have a ten-month rotation to field corn.
Liberty Link soybeans are a tool for double cropped soybeans because they allow for a different mode of action and Liberty 280 has some activity on marestail (see below) and it is effective on small Palmer amaranth plants.
Control of horseweed (marestail) preplant is going to be quite challenging. I do not recommend 2,4-D because of off-target movement at this time of year and it is not effective on these large and damaged plants. I do not have experience with Sharpen under these conditions, but it can be used on medium textured soils at 1 oz/A, or 1.5 oz/A with a 14-day interval before planting (30-days for coarse-textured soils). The Sharpen label recommends horseweed height at 6 inches tall, and that is before it is cut off by the combine. Likewise, Liberty 280 will injure or suppress large horseweeds but often not kill them. Products with chlorimuron or cloransulam such as Canopy, Envive, Valor XLT, Authority First or Sonic may suppress horseweed plants if used at the full rate (although it will not kill them). Another complicating factor is that there are biotypes of horseweed that are resistant to chlorimuron and cloransulam in the region.
If you have Palmer amaranth that is resistant to glyphosate and/or Group 2 herbicides, your options are limited as well. If the Palmer amaranth plants are resistant to both glyphosate and Group 2, we do not have any products that will consistently control them. Gramoxone (paraquat) or Liberty 280 are two options but regrowth is likely. These products will probably not control established grasses. After the beans are planted your options for postemergence activity on Palmer amaranth would include Blazer (use of Reflex will probably limit next year’s rotation so be sure to check the label). If you used any of those Group 2 products mentioned earlier for burndown (Canopy, Valor XLT, Pursuit, Sonic etc.) do not use another group 2 herbicide postemergence.