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1 July 2008 Response of Northeastern Arkansas Palmer Amaranth (Amaranthus Palmeri) Accessions to Glyphosate
Jason K. Norsworthy, Robert C. Scott, Kenneth L. Smith, Lawrence R. Oliver
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Palmer amaranth accessions were collected from 21 fields in northeastern Arkansas in the fall of 2006 to determine if they differed in response to increasing doses of glyphosate and to determine the survival frequency following treatment with the label rate (870 g ae/ha). The herbicide dose required to kill 50% of individuals in an accession (LD50) ranged from 41 to 339 g/ha glyphosate, with most accessions responding similarly to glyphosate. The AR18 and AR19 were the least-sensitive accessions, with LD50 rates of 312 and 339 g/ha glyphosate, respectively. The mean survival frequency was 2.2% across all accessions when 870 g/ha glyphosate was applied to five- to seven-leaf plants. Sixteen of the accessions had at least one plant survive glyphosate at 870 g/ha. AR18 and AR19 accessions had a survival frequency of 6.3 and 11.8%, respectively. Following an additional cycle of selection with glyphosate, 44.3% of the progeny from the AR19 accession survived glyphosate at 870 g/ha, and its LD50 value significantly increased to 646 g/ha glyphosate. This research shows that there is a low percentage of Palmer amaranth plants currently present in production fields throughout northeastern Arkansas that are capable of surviving a single glyphosate application at the labeled rate and that further selection with glyphosate can increase the frequency of survival.

Nomenclature: Glyphosate; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA

Jason K. Norsworthy, Robert C. Scott, Kenneth L. Smith, and Lawrence R. Oliver "Response of Northeastern Arkansas Palmer Amaranth (Amaranthus Palmeri) Accessions to Glyphosate," Weed Technology 22(3), 408-413, (1 July 2008).
Received: 6 December 2007; Accepted: 1 April 2008; Published: 1 July 2008

Glyphosate resistance
Herbicide resistance
herbicide tolerance
selection for resistance
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