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Horseweed Control in Soybean (06/28/18)

Horseweed (some call it marestail) is an annual broadleaf weed native to North America.

Horseweed Control in Soybean

Horseweed (some call it marestail) is an annual broadleaf weed native to North America. Horseweed is unique in that it may follow a winter or summer annual life cycle. The winter annual version forms a basal rosette after emergence in the fall for winter survival. The rosette ‘bolts’ in April and grows to a height of 2 to 6 feet by early summer. Horseweed leaves are alternate, linear, and simple with entirely or slightly toothed margins. Horseweed flowers in late July and may produce 200,000 seed per plant. Horseweed seed has a pappus like dandelion seed that enables it to be carried several miles by wind. Horseweed contains volatile oils, and acids that may cause skin irritation in livestock (especially horses) and humans. Early settlers used horseweed to treat diarrhea and dysentery and Native Americans used it as a coagulant to stop bleeding.

The summer annual version germinates in the spring and completes its lifecycle in one calendar year. The summer annual version is increasing in frequency in North Dakota.

Tillage practices and herbicide resistance have contributed to increased horseweed prevalence in the landscape. Horseweed thrives in no-till; tillage can reduce horseweed prevalence by 50%. Herbicide resistance to paraquat (SOA 22) was first reported in 1980 in Mississippi. Since then, resistance to triazine (SOA 5), ALS inhibitors (SOA 2) and glyphosate (SOA 9) has been reported in various states.

There are very limited in-crop options for effective horseweed control, especially if there are glyphosate resistant biotypes. Control is best when horseweed is six-inches tall or less. FirstRate is effective but only on susceptible biotypes (SOA 2). Liberty (SOA 10) will also provide effective control of small horseweed in LibertyLink soybean.

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Tom Peters

Extension Sugarbeet Agronomist

NDSU & U of MN

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