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Fall herbicide treatments may be applied anytime after harvest depending on weed species and size. Applications should be made to actively growing weeds before a heavy frost or freeze. Review the cut-off date and soil temperatures on the herbicide labels before application. The majority of herbicide labels recommend application while the soil is cool, but not frozen.
Fields that are heavily infested with winter annual weeds such as marestail, henbit, and chickweed are good candidates for a fall burndown herbicide application. Winter annual weeds will emerge in the fall after harvest and complete their life cycle in the spring and early summer. Generally, winter annuals are easier to control in the fall than in the spring. If allowed to grow in the spring, winter annual weeds can form a thick mat on the soil surface which can interfere with tillage, crop establishment, and slow soil warming.
Figure 1. Marestail rosette (left) and seedling (right).
A single marestail plant (Figure 1) can produce as many as 200,000 seeds, and can germinate in the fall or spring. Fall-germinated marestail that overwinters is difficult to control with a spring-only application. Marestail is generally easier to control in the fall when they are small before they bolt or shoot a main stem in the spring. A fall and spring burndown application may be the best control option.
Glyphosate-resistant marestail is of particular concern. Cropping systems with glyphosate-resistant weeds like marestail should include an effective tank-mix partner with glyphosate for fall burndown. Consider applying Roundup® brand glyphosate-only agricultural herbicide plus 2,4-D and/or dicamba. Tank mixing dicamba may provide up to 14 days of soil activity during the fall for later emerging marestail. Adding a second herbicide such as 2,4-D or dicamba with an effective site of action will improve weed control and reduce selection for glyphosate-resistant weeds. Applications should be made during periods when marestail is actively growing (50 °F and above).
Fall herbicide applications can be the first phase of a comprehensive weed management plan, but should never replace a spring residual herbicide program. Benefits of a fall weed management plan include: