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1 March 2008 Glyphosate-resistant Horseweed (Conyza Canadensis) Emergence, Survival, and Fecundity in No-till Soybean
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Abstract

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

Vince M. Davis and WILLIAM G. JOHNSON "Glyphosate-resistant Horseweed (Conyza Canadensis) Emergence, Survival, and Fecundity in No-till Soybean," Weed Science 56(2), 231-236, (1 March 2008). https://doi.org/10.1614/WS-07-093.1
Received: 22 May 2007; Accepted: 1 October 2007; Published: 1 March 2008
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