Glyphosate-resistant horseweed (Conyza canadensis) emergence, survival, and fecundity in no-till soybean

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 I canopy. Horseweed with flower heads above the soybean canopy produced an average of 27,200 and 58,320 seeds plant 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.