Field studies were conducted at Fayetteville, Arkansas, to determine whether 47 Palmer amaranth accessions from different areas of the southern United States varied in response to postemergence applications of the registered rates of the isopropylamine salt of glyphosate (840 g ae/ha), fomesafen (420 g ai/ha), and pyrithiobac (70 g ai/ha). Glyphosate controlled all Palmer amaranth accessions at least 99% 21 d after treatment (DAT). Palmer amaranth control with fomesafen was equivalent for all accessions and at least 96% 21 DAT. Percent dry weight reductions were at least 92 and 94% for glyphosate and fomesafen, respectively. Palmer amaranth control with pyrithiobac was variable and ranged from 20 to 94% 21 DAT, but differences could not be attributed to accession origin. Herbicides with alternate modes of action from pyrithiobac should be utilized for Palmer amaranth control in regions where pyrithiobac has been used continuously.
Nomenclature: Fomesafen, glyphosate, pyrithiobac, Palmer amaranth, Amaranthus palmeri (S.) Wats. #3 AMAPA.
Additional index words: Accession, dry weight reductions, ecotype, herbicide response variability.
Abbreviations: ALS, acetolactate synthase (EC 184.108.40.206); DAE, days after emergence; DAT, days after treatment.