Conyza canadensis, commonly known as horseweed or marestail, is one of the most problematic weeds in agriculture, particularly in no-till farming systems. This broadleaf species has a unique life cycle, emerging both in the fall and spring, which allows it to be well-established before crops are planted. Its prevalence has been significantly amplified by its ability to rapidly develop resistance to many common herbicides. This resistance makes finding an effective chemical solution for horseweed a complex challenge that requires a multi-faceted approach.
Understanding Herbicide Resistance in Horseweed
Horseweed has earned a global reputation as a highly resistant weed species, drastically reducing the effectiveness of once-reliable control methods. The most widespread concern involves its resistance to Group 9 herbicides, specifically glyphosate, which was the first confirmed case of glyphosate resistance in a weed species in the United States in 2000. This resistance is not caused by a change in the herbicide’s target site, but rather by the plant’s ability to limit the chemical’s movement.
Instead of the glyphosate being translocated throughout the plant to the growing points, horseweed rapidly sequesters the herbicide into the cell’s vacuole, effectively trapping it. This reduced translocation prevents the glyphosate from reaching the site of action in sufficient concentration to kill the plant. Horseweed biotypes have also developed resistance to other chemical groups, most notably Group 2 (ALS-inhibitors), making the use of multiple herbicide modes of action necessary for control.
Strategies for Pre-Emergent Control
Since horseweed is easiest to control when it is small, the first line of chemical defense involves using pre-emergent herbicides, also known as residual herbicides. These products target the weed either before it emerges from the soil or when it is still in the small, vulnerable rosette stage. Applying a strong residual herbicide early in the season or in the fall significantly reduces the pressure on later post-emergent applications.
Chemical active ingredients like flumioxazin (Group 14), metribuzin (Group 5), and pendimethalin (Group 3) are components of an effective residual program. Flumioxazin and metribuzin, in particular, provide several weeks of control, keeping the soil surface weed-free for a longer period. To maximize effectiveness, these pre-emergent products are often tank-mixed with a burndown herbicide to kill any horseweed that has already emerged, ensuring a clean start.
Effective Post-Emergent Herbicide Options
For actively growing horseweed that has escaped the pre-emergent treatment, a different set of post-emergent herbicides must be used. Due to the high risk of resistance, these treatments must almost always be applied as a tank mix of chemicals with different modes of action to ensure a complete kill. The most effective options rely on herbicides that target the plant through contact or those with different systemic mechanisms than the Group 9 or Group 2 chemistries.
Herbicides such as 2,4-D and dicamba (both Group 4, synthetic auxins) are highly effective on small horseweed rosettes and are frequently used in burndown applications. Mixtures of dicamba or 2,4-D with other active ingredients can achieve near-total control of even glyphosate-resistant biotypes. Glufosinate (a Group 10 herbicide) is a contact-based product highly effective against multiple-resistant horseweed, particularly when the plants are less than six inches tall.
Paraquat (a Group 22 herbicide) is another contact herbicide that provides rapid burndown of emerged horseweed. However, once the horseweed begins to “bolt,” or elongate its stem, control becomes significantly more difficult with any product. Bolted plants have more biomass and are much less susceptible to systemic herbicides, often requiring higher rates or the use of contact herbicides. It is paramount that anyone using these chemicals always reads and strictly follows the specific product label instructions and adheres to all local regulations.
Maximizing Efficacy Through Application Timing and Rotation
A successful, long-term management program for horseweed depends not just on the herbicide chosen, but on when and how it is applied. The most important element is timing, with applications in the fall often being the most effective, as the horseweed is in its small, overwintering rosette stage. Fall treatments, often containing 2,4-D or dicamba, control emerged plants and reduce the population that will be present in the spring, complementing the spring burndown application.
Herbicide rotation is an equally important strategy to prevent the development of future resistance. This involves consistently alternating the chemical mode of action (the way the herbicide kills the plant) from year to year, for example, switching from a Group 4 herbicide one year to a Group 14 the next. This practice ensures that the same selection pressure is not continuously applied to the horseweed population, thereby slowing down the evolution of resistance.
The application process itself can be optimized through the use of adjuvants, such as non-ionic surfactants or crop oil concentrates. These additives improve the wetting and coverage of the leaf surface, which can increase the uptake of the herbicide into the plant, boosting the kill rate. Research shows that the time of day matters for some products, with systemic herbicides like 2,4-D and dicamba working best when applied around midday, while the contact herbicide paraquat is often more effective with a sunset application.