Horseweed has generally been considered a winter annual weed species, but efforts to control horseweed as a winter annual weed in no-till soybean production with glyphosate have routinely failed in southeast Indiana. The objective of this study was to determine emergence timing, plant survival, and fecundity of a glyphosate-resistant (GR) horseweed biotype in the presence or absence of other winter annual weeds or soybean. A field study was conducted from October 2003 to October 2004 and repeated from October 2004 to October 2005 in fields following no-till soybean production. Horseweed emergence was not observed in the fall of 2003. Winter survival of plants that emerged in the fall of 2004 was 20% by late April 2005 and was inversely related to fall rosette size. Horseweed population densities were the highest in mid-May of both years, and over 90% of the plants observed at this time emerged in the spring. Plant survival from mid-May to mid-October was 3% and 21% in 2004 and 2005, respectively. Horseweed with flower heads above the soybean canopy by early August had greater late-season survival and produced more seed than plants growing below the canopy. Horseweed with flower heads above the soybean canopy produced an average of 27,200 and 58,320 seeds plant⁻¹ in 2004 and 2005, respectively. Our research indicates that this southeast Indiana horseweed biotype behaves primarily as a summer annual weed and produces significant amounts of seed when uncontrolled in no-till soybean production.

Nomenclature: Glyphosate; horseweed, Conyza canadensis (L.) Cronq. ERICA; soybean, Glycine max (L.) Merr