What You’ll Learn...
- Weed management is often more difficult in the spring because of weed size and weather conditions.
- Fields that are heavily infested with weeds such as marestail, henbit, dandelion, and chickweed are good candidates for a fall burndown herbicide application.
- Burndown and residual herbicides can provide control of most winter annual weeds.
- Fall burndown is an opportunity to use herbicides with different sites of action on difficult weeds such as Palmer amaranth.
Fall Burndown Herbicide Benefits
Post-harvest herbicide applications, in conservation tillage or no-till fields, can help to greatly minimize weeds in seedbeds in the spring.
- Fields with heavy weed populations are the best candidates for a fall application.
- Fall burndown helps to spread out the workload in the spring.
- Fall burndown typically provides better control of marestail (horseweed) than spring burndown.
- Palmer amaranth should be managed after harvest up to frost to help prevent seed production.
- Fall conditions are more favorable for control of winter annual weeds than early spring because of smaller weed size and more suitable days for herbicide applications.
- Controlling winter annuals in the fall may improve soil temperature and soil moisture at planting.
- Fall burndown can reduce the potential for cutworms and soybean cyst nematodes (SCN) by removing weed hosts where these pests overwinter.
Fall applications will not eliminate the need for a residual herbicide program near or at planting. Fall burndown will not provide in-season control of summer annual weeds, particularly tough-to-control weeds such as waterhemp or Palmer amaranth.
Winter Annual Weeds, Marestail (Horseweed), and Palmer Amaranth
Winter annual weeds will emerge in the fall after harvest and complete their life cycle in the spring and early summer. Weed control is often more difficult in the spring because of larger weeds and weather conditions. 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 soil warming from the sun.
A single marestail plant can produce as many as 200,000 seeds, so it is vital that growers initiate a herbicide control program when marestail is small. Marestail is generally easier to control in the fall when they are small before they bolt or shoot a main stem in the spring. Bolted marestail is difficult to control with a spring burndown herbicide application.
Palmer amaranth can emerge and produce seed in the fall in as little as 5 to 6 weeks. Tillage or mowing may be needed to control or cut-off newly emerged plants that exceed 4 inches in height after harvest.
Roundup PowerMAX® and Roundup PowerMAX® II Herbicides are effective on most grass and broadleaf weeds; however, the addition of dicamba or 2,4-D is recommended for broad-spectrum burndown of weeds. Some marestail or Palmer amaranth populations may be resistant to glyphosate (the active ingredient in Roundup® brand agricultural herbicides) requiring the addition of dicamba for control.
Check individual product labels to determine crop planting restrictions for residual herbicides, just in case planting intentions change. Any crop can be planted 30 days after an application of Roundup PowerMAX or Roundup PowerMAX II, or 4 months after an application of dicamba. The rotation interval for corn can be up to 10 months for Valor® XLT Herbicide and up to 18 months for Authority® Herbicide products. Tillage after application may reduce residual weed control.
Fields with a diverse array of winter annual, biennial, or perennial weeds plus tough-to-control weeds may require multiple herbicide application timings in additions to a fall burndown (i.e., spring burndown, at-planting, or in-crop) for effective weed management. The application of residual herbicides in the fall does not replace the need for residual herbicide applications in the spring.
Fall herbicide applications should be part of a comprehensive weed management program. Programs should be designed to minimize the risk of weed resistance and weed species shifts. Get crop and weed specific recommendations at www.RoundupReadyPLUS.com.
Bradley, K. 2013. Considering fall herbicide applications: It’s not just about the weeds. University of Missouri Integrated Pest Management. http://ipm.missouri.edu
Bradley, K. 2008. Considering fall herbicide options in corn and soybean. University of Missouri Integrated Pest & Crop Management. http://ipm.missouri.edu
Websites verified 11/1/2018.