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.
- Group 2 (ALS)
- Group 9 (glyphosate)
- Group 2 + Group 9
- Group 22
- Group 9 + 22
Locations of resistant populations:
Arkansas, Delaware, Illinois, Iowa, Maryland, Mississippi, Missouri, Nebraska, North Carolina, Pennsylvania, Tennessee, Virginia