2,4-D is often used as a preplant burndown herbicide to help control horseweed and other broadleaf weeds before planting in no-till corn and soybean production. Isolated instances of poor horseweed control have occurred in production fields. The objective of this research was to evaluate the response of various horseweed populations to 2,4-D. In the first study, 478 horseweed populations from Indiana were subjected to 280 g ae ha⁻¹ of 2,4-D amine in the greenhouse. This rate of 2,4-D caused visible injury and prevented all biotypes from forming new leaves for 28 days. There were specific populations where all plants sprayed were alive at 28 days after treatment (DAT), and approximately 10% of all populations had a least one plant that survived 280 g ae ha⁻¹ 2,4-D, resumed growth, and produced seed. In a dose-response study, we observed populations with three-fold more tolerance to 2,4-D. The most tolerant population had a GR₉₀ of 513 g ae ha⁻¹ and the most susceptible population had a GR₉₀ of 121 g ae ha⁻¹ based on dry weights. Growth suppression with 2,4-D was not affected by rosette size for rosettes between 0.5 and 10 cm in width.

Nomenclature: 2,4-D; horseweed, Conyza canadensis (L.) Cronq. ERICA; corn, Zea mays L.; soybean, Glycine max (L.) Merr.