By Andi Anderson
Horseweed, also known as marestail, continues to frustrate farmers across the United States and Canada despite decades of management efforts.
Ranked among the top five most troublesome weeds in winter cereal grains, fruits, nuts, and soybeans, horseweed’s success stems from its adaptability, high seed production, and widespread herbicide resistance.
According to the Weed Science Society of America, horseweed thrives in both conventional and organic systems, especially in no-till or reduced-tillage fields.
Each plant can produce thousands of lightweight seeds that spread easily by wind. Many populations have also developed resistance to glyphosate and ALS-inhibiting herbicides, making control even more challenging.
Researchers at Michigan State University (MSU) recently discovered that horseweed’s varying growth habits are not caused by genetic differences but by environmental conditions.
Seeds respond to temperature and moisture cues, determining whether they grow as winter annuals or summer annuals. This flexibility allows a single seed to potentially produce two generations per year.
MSU professor-led studies found that cold and moist conditions, known as seed vernalization, influence which growth type develops. In cooler conditions, seeds are more likely to become summer annuals, growing upright immediately instead of forming a winter rosette.
The hormone gibberellic acid, often used to speed up crop germination, also encourages horseweed seeds to develop summer growth traits—indicating why the weed thrives in diverse conditions.
For farmers, this life cycle flexibility means that both fall and spring management are essential. Fall strategies, such as tillage, cover crops, or preemergence herbicides, prevent winter annual emergence, while spring control targets summer annuals.
Because seeds from either type can give rise to both life cycles in the next season, management must be continuous across seasons.
Summer annual horseweed poses an even greater challenge due to stronger herbicide resistance and rapid growth.
Experts predict that as winters grow warmer, horseweed’s germination window will expand, demanding more frequent and timely control measures.
Ultimately, understanding horseweed’s behavior—from seed to seed shed—can help improve management strategies. As MSU researchers conclude, environmental cues, not genetics, drive this weed’s survival—making it one of agriculture’s most persistent and adaptable adversaries.
Photo Credit: gettyimages-zoomtravels
Categories: Michigan, Rural Lifestyle, Farm Safety