After harvest, weeds just don’t stop growing. Come spring, corn and soybean farmers find weeds in the field that need attention. Post-harvest herbicide application is often a critical piece of a weed management strategy to control herbicide-resistant weeds, prevent weed seeds from spreading and get a head start on the next spring season. Field conditions typically allow easier and more efficient application in the fall compared to cold, wet springs. Whether or not to apply an herbicide in the fall depends on a number of factors.
A post-emergent fall application can halt growth of an existing weeds or, combined with a residual herbicide, prevent emergence. Fall application is all about the weeds that made it through the season. In our area, for instance, marestail and palmer pigweed are major post-harvest culprits that many row crop farmers have to deal with. Winter annuals ‒ like resistant ryegrass, chickweed, henbit and pennycress ‒ can be problems.
Catching weeds at the right time is key. A single palmer pigweed plant can produce between 600,000 and 1 million seeds. Eliminating the weed before it reproduces can save farmers a lot of time and effort by minimizing the weed seed bank to get ahead of growth. Fall weed control treatments help make sure fields are clean for spring planting.
After harvest, it is important to scout the field to see what survived to determine if a herbicide application is warranted. In certain conditions, weeds can grow vigorously. Treating when they are small—between 4 to 6 inches or smaller in some cases—allows for much more control than when they get bigger.
Farmers who decide that a fall herbicide application works in their management program should be sure to use the right product and be aware of the product’s active ingredient, mode of action and residual activity. Things to take into account in determining the best product or products to use include potential resistance problems, tank mix compatibility and future crops. Products such as DuPont™ Canopy®, DuPont Leadoff®, and Roundup PowerMAX® are a few to consider depending on cropping plans and weed pressure.
Herbicide resistance has become more of a problem in recent years, so it is something to be especially aware of. Weeds treated with the same mode of action repeatedly can build up resistance to the active ingredient, rendering it ineffective. Farmers can avoid resistance by changing up the active ingredient in their herbicides. There are more than 25 different modes of action available to row crop farmers.
What a farmer plans to grow in the future in the field also needs to be a factor when deciding on a herbicide application. Carefully reading the label before taking action will identify any sensitivities for future crops along with timelines for planting.
Along with herbicide applications, farmers have multiple options for weed control they can use throughout the seasons. Crop rotation, cover crops (keeping something growing on the field at all times) and starting clean in the field are all means of a year-round weed control program that should be practiced in addition to herbicide applications.
For more information on the benefits of year-round weed control and how a fall herbicide application can help, contact your Southern States agronomy expert