Mark VanGessel, Extension Weed Specialist; email@example.com
With small grain harvest underway, there are some questions about preplant weed control in the double cropped soybeans. This has always been a challenging question, but with the presence of herbicide-resistant weeds, it has become even more complicated. 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.
A non-selective burndown herbicide and a residual herbicide for broadleaf weeds are needed for most 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. 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. Since most of the small grain fields where treated with Group 2 herbicides (Harmony Extra, Osprey, PowerFlex, Finesse, etc.) it is good to diversify your herbicide mode of action. 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 nice 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. Liberty 280 is going to be more effective as a herbicide sprayed over the top of emerged soybeans (postemergence) than as a burndown herbicide.
Control of horseweed (marestail) preplant is going to be quite challenging. I do not recommend 2,4-D because it is not effective on these large and damaged plants and due to the risk of off-target movement. 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. 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 such as Canopy, Envive or Valor XLT will suppress horseweed plants if used at the full rate (although probably will not kill them). Another complicating factor is that there are biotypes of horseweed that are resistant to chlorimuron in the region.
Palmer amaranth that is resistant to glyphosate is also going to be an issue. Glyphosate will not control these plants so a product that is very effective for postemergence of Palmer amaranth is needed. Options include Synchrony XP, Canopy, Envive, Valor XLT, or Pursuit/Extreme. Gramoxone (paraquat) also is an option. The grasses that are not controlled with Gramoxone could control with early postemergence application of Select or Poast or glyphosate. After the beans are planted your options for postemergence control of Palmer amaranth would include Reflex or Blazer. If you used any of those Group 2 products mentioned earlier for burndown (Canopy, Valor XLT, Pursuit etc.) do not use another Group 2 herbicide postemergence.