Horseweed (marestail)

Photo

Young horseweed seedling with lobed leaves beginning to form a rosette.

Horseweed management is challenging because the weed can emerge in both fall and spring. In addition, herbicide resistant populations are spreading fast. In this video you will learn how to use a high biomass cover crop to decrease horseweed seed stand and production. Dr. John Wallace, PSU, outlines how to achieve excellent horseweed control with this tool

Horseweed (also called marestail) is of rising concern in many states due to its rapidly spreading herbicide resistant populations. It is a winter annual, and its seeds are wind-dispersed. It should be controlled when seedlings are small in order for herbicide applications to be effective, and not allowed to go to seed. Because seedlings emerge in both the fall and the spring, fields should be scouted for horseweed in the fall and treated with a fall burndown and/or a fall cover crop to prevent those seedlings from re-emerging in the spring.

 

Resistant populations:

  • Group 2
  • Group 7
  • Group 9
  • Group 5 + 7
  • Group 2 + 9
  • Group 22
  • Group 9 + 22

Integrated Weed Management Strategies for Control

 

Cultural Tactics: Competitive cash crops will delay or reduce germination, growth, and seed set of many weed species. For horseweed, the spring emergence window is ending or has already ended at the time of cash crop planting so efforts for control are better focused prior to planting. In some areas, where the horseweed germination window extends into the growing season, narrow row spacing can help the cash crop reach canopy more quickly and compete with young horseweed plants.

Crop rotation is one cultural tactic that can impact horseweed density. Crop rotation provides the opportunity to diversify weed management tactics, preventing and combating herbicide resistance. Studies have shown that a soybean-corn crop rotation can reduce horseweed density compared to planting continuous soybean.  Corn is typically planted earlier than soybeans so this rotation allows for herbicide application earlier in the spring while the horseweed plants are smaller; residual herbicides applied to corn also prevent much of the spring emergence flush. There are also different herbicide options available for these crops.

Crop residues can play a role in suppressing fall-germinating horseweed. Studies in Tennessee have found that corn residue providing 75% ground cover reduced horseweed density the following May by 79 to 81% compared to where there was no crop residue and 49 to 75% compared to fields with soybean residue. Any tactic that will increase the amount of ground cover such as not removing crop stover or planting a cover crop, will help suppress horseweed. However, relying on these tactics alone will not keep horseweed at bay for a full cropping season.

With horseweed, repeated scouting is necessary because of the weed’s long, unpredictable germination periods and its ability to spread long distances. It can be easy to miss a flush of germination or a new infestation without repeated scouting.

Cover Crops: Winter cover crops have the unique ability to suppress horseweed in both fall and spring. In the fall, actively-growing cover crops prevent light from reaching the soil (decreasing horseweed germination) and compete with existing weed seedlings for light, moisture and nutrients; in the spring, the residue from terminated cover crops likewise discourages horseweed germination and emergence.

Generally, 7,000 lbs per acre of cover crop biomass is needed to suppress weeds, but it is not known how much biomass is needed to suppress horseweed specifically. High biomass cover crops have the potential to suppress horseweed early in the cash crop growing season but as the cover crop residue degrades, horseweed can emerge and still be a weed of concern in the field.

Mechanical: Tillage can be used as a control tactic to manage horseweed since seeds are small and require light for germination, so germination is greatly reduced once the seeds are buried by even a little bit of soil. The increase in reduced tillage acreage has favored horseweed spread and establishment.

Aggressive tillage practices will have the greatest effect on horseweed by burying weed seeds or uprooting plants. Reduced tillage practices, such as vertical tillage, are not as effective plowing or aggressive disking for seedlings and often will not have much of an effect on larger horseweed or bury seed enough to keep them from germinating.

Studies from Nebraska demonstrated tillage alone was very effective in controlling horseweed, with fall tillage reducing densities by 88% and spring tillage reducing densities by 94%. Tillage was more effective than any single herbicide application in the fall or spring. Herbicide use in combination with tillage or sequential herbicide applications provided up to 99% reduction in horseweed plants. Mechanical weed control was effective in this study, but an integrated weed management strategy is critical for managing herbicide resistance.

Studies have shown that no seedlings germinate when buried more than 0.2 inches.

While tillage can contribute heavily to horseweed management, in some instances tillage for weed management is not desirable because of potential trade-offs, such as cost or the reduction in erosion control. In those instances, there are alternative tactics that are effective for controlling horseweed.

Chemical: Horseweed should be controlled when small, preferably in the rosette stage. Once the plants begin to bolt, control becomes more difficult. Control timing can be problematic because of the multiple and prolonged germination windows.

Control is very important prior to planting crops as there aren’t always options for adequate control in-crop, especially with soybeans and cotton. Burndown herbicides, such as 2,4-D, dicamba, glufosinate (Liberty), saflufenacil (Sharpen), paraquat (Gramoxone), or tank mixes of these herbicides, can be applied in the fall or spring to control horseweed. Additionally, waiting until after planting to manage horseweed typically means the plants are too large for adequate chemical control.

Glyphosate will control susceptible horseweed but resistance to glyphosate is widespread across the United States. Including residual herbicides in a preplant application can extend weed control until crop canopy closure. Examples of soybean herbicides include flumioxazin (Valor), sulfentrazone (Authority products), or metribuzin; corn herbicides include atrazine and mesotrione (Callisto); and cotton herbicides include flumioxazin (Valor) and fluometuron (Cotoran). Refer to herbicide labels for appropriate uses and rates in your area. Due to herbicide resistance, there may be fewer options to control horseweed postemergence in many crops. Refer to your local extension’s weed management guide for effective postemergence options.

Biological: Currently, there are no commercially available biological control agents for horseweed. However, research has identified bacteria that can control horseweed; a commercial product has yet to be formulated.

Resources:

Take Action on Weeds – Horseweed

Biology and Management of Horseweed – Part of the Glyphosate, Weeds, and Crops Series

Horseweed Management factsheet – Penn State University

Herbicide recommendations for horseweed in no-till soy – Iowa State University

Control Horseweed Prior to Crop Emergence – Penn State University