Mark VanGessel, Extension Weed Specialist; firstname.lastname@example.org
Fall herbicide treatments have a nice fit for many situations such as small grain weed control, assisting with cover crop management, and reducing the severity of weed infestations for no till corn or soybeans.
Let’s look at each of these situations. Treating small grain fields with a late fall herbicide application has worked well in our trials. As discussed in earlier newsletters there are no herbicides labeled for applications at planting (a preemergence application). As a result we have lots of winter annuals that emerge with or shortly after the crop. Waiting until spring to control these weeds often results in poor control because the weeds are large, and often stressed from the winter weather. On the other hand, an application in the late fall is made while the weeds are relatively small and actively growing. Remember these weeds are winter annual and will continue to grow after a few hard frosts, and the soil temperatures allow for significant growth through the month of November. If there is a lot of spring emergence, then those weeds can be controlled with a herbicide applied in combination with spring nitrogen applications.
Controlling weeds in cover crops that will be used for early-season vegetables can be challenging in some springs (particularly henbit and chickweed). One way around this with a grass cover crop is using a herbicide in the fall to “clean up the cover crop”. Using a broadleaf herbicide such as Harmony Extra or 2,4-D in the fall will control many of the broadleaf weeds and not limit crop rotation in the spring (replant intervals are 1.5 to 3 months). Then when burning down the grass cover crop in the spring, the concern is killing the cover crop, and not worrying about the winter annual broadleaves that can be tough to control that time of year.
Finally, fall treatments for fields that will be planted to no-till corn or soybeans next spring. We have looked at a number of products that could be tankmixed with glyphosate or paraquat with the idea they would provide residual control for spring emerging plants and these fields will not need a burndown herbicide. UD Weed Science Research has not found a consistent herbicide program for this approach. Furthermore, for effective weed control in corn or soybean most fields need a residual herbicide applied prior to or at planting, so a trip across the field for a herbicide application is needed in the spring. Fall herbicide applications of glyphosate or paraquat with 2,4-D are an excellent way to limit the amount of weed biomass in the spring, which in turn allows the soil to warm up faster and possibly conserve moisture. In our experiences, the addition of residual herbicides to the tankmixture of glyphosate, paraquat, and/or 2,4-D has limited utility in most situations.