Fall Herbicide Applications in No-Till Systems

As summer gives way to fall and harvest rapidly approaches, planning for fall field work becomes top of mind for many growers. Across the U.S., fertilizer applications, residue management, cover crop seeding, soil sampling, and a host of other work is undertaken during the fall to prepare for the next growing season. For growers who've implemented no-till practices, fall weed control also becomes an additional consideration; winter annual weeds ordinarily controlled by a spring tillage pass require growers in no-till operations to take a more proactive approach to ensure season-long weed control.

Winter annual weeds, which generally emerge in the fall and finish their lifecycle in the spring, can be problematic for growers if left unchecked after harvest. Common examples of these weeds include henbit, chickweed, marestail (horseweed), and pennycress, all of which can interfere with planting operations in the spring and rob essential nutrients, moisture, and sunlight from emerging row crops; further illustrating this point, a study from Kansas State University found that winter annual weeds averaged an uptake of up to 16 lbs. of available N per acre at 14 different research sites (1). For growers who've adopted no-till practices, a fall herbicide application can offer an economical weed control method that provides relatively clean fields at planting and reduces competition for essential resources.

When considering fall herbicide applications, use a prescriptive approach to determine which fields need to be sprayed. As Meaghan Anderson at Iowa State University points out, not all no-till fields need a fall herbicide application. Focus on areas that have a history of winter annual weed pressure, high weed pressure at harvest, demonstrated resistance to herbicides, or may be particularly difficult to access during the spring (such as low-lying, wet fields) (2).

When planning your fall herbicide applications, be sure to consider the following:

1. Like summer annual weeds, winter annual weed populations are impacted by moisture, temperature, and crop residue cover. Scout fields during and after harvest to determine the weed pressure in your area, especially during warm, wet autumn conditions.
2. Variable fall temperatures can often impact the speed at which herbicides take effect, especially products that require translocation through the plant, such as glyphosate or 2,4-D. If temperatures dip too low, the overall efficacy of your herbicide application may be negatively impacted due to poor uptake by the target weeds. Many winter annual weeds are well-adapted to hard frosts but still need time to recover before a herbicide application can be effective (2); giving plants at least 48-72 hours to recover from a hard frost, then scouting for new growth before applying a herbicide is essential to maintain the efficacy of your herbicide application (3). Generally speaking, it's best to cease herbicide applications altogether if the extended forecast calls for nighttime temps below 40 degrees F and daytime highs below 55 degrees F.
3. Ensure the herbicides you plan to use are labeled and suitable for fall applications; while many are labeled for fall applications, not all are suitable. A common and effective mix for fall applications includes labeled rates of Roundup PowerMAX® 3 (glyphosate) with labeled rates of 2,4-D or dicamba, plus a label-approved adjuvant to ensure efficacy. Other popular choices include Autumn™ Super or metribuzin, which can offer some residual control of marestail and other weeds later into the fall.
4. Labeled rates of herbicides and the use of adjuvants are still required with fall applications to ensure proper efficacy. While growers may be tempted to reduce rates or skip the use of adjuvants, lower fall temperatures can negatively impact the effectiveness of herbicides, especially if labeled rates are reduced or adjuvants aren't correctly utilized.

One final and often overlooked consideration growers must make is that a fall herbicide application should not be intended to replace spring applications. Fall applications generally limit winter annual pressure going into the spring but do little to inhibit the emergence of summer annual weeds, including waterhemp and spring-germinated marestail that begin showing up after planting (5). Attempting to save money by eliminating spring residual applications will only cost in the long run, especially where herbicide-resistant weed species are present. Fall herbicide applications should be in addition to, not a replacement for, spring and summer herbicide applications.

As harvest commences and leads into fall fieldwork, scout early and often for weed pressure and be prepared with a plan of action to manage troublesome winter annuals proactively. Knowing your risk factors and having a plan, especially as part of a no-till system, will ensure a successful 2024 and beyond.

Resources: (1) Mueller, Nathan & Ruiz Diaz, Dorivar & Dille, J. & Shoup, Doug & Mengel, David & Murray, Leigh. (2013). Winter Annual Weed Management and Nitrogen Rate Effects on Corn Yield. Agronomy Journal. 105. 1077. 10.2134/agronj2012.0344.

(2) Fall Burndown Treatments for Winter Annual Weeds | Integrated Crop Management. (n.d.). https://crops.extension.iastate.edu/cropnews/2022/09/fall-burndown-treatments-winter-annual-weeds. Accessed 08/18/2023.

(3) Low Temperature and Frost may Affect Efficacy of Burndown Herbicides. (2019, July 22). CropWatch. https://cropwatch.unl.edu/2017/low-temperature-and-frost-may-affect-efficacy-burndown-herbicides. Accessed 08/18/2023

(4) Cool Weather & Burndown Herbicides | Nebraska Extension. (n.d.). https://extension.unl.edu/statewide/burt/cool-weather-burndown-herbicides/. Accessed 08/18/2023

(5) Division of Plant Sciences, plantsciweb@missouri.edu. (n.d.). Considering Fall Herbicide Applications: It’s not Just About the Weeds (Kevin Bradley). https://ipm.missouri.edu/cropPest/2013/9/Considering-Fall-Herbicide-Applications-Its-not-JUST-about-the-weeds/. Accessed 08/18/2023.

Legal Statements ALWAYS READ AND FOLLOW PESTICIDE LABEL DIRECTIONS. Performance may vary, from location to location and from year to year, as local growing, soil and weather conditions may vary. Growers should evaluate data from multiple locations and years whenever possible and should consider the impacts of these conditions on the grower's fields.