Seeds of a suspected glyphosate-resistant giant ragweed biotype from Lauderdale County, TN, were collected from a continuous cotton field in fall 2007 after plants were nonresponsive to multiple glyphosate applications. The objectives of this research were to (1) confirm resistance by quantifying the response of the putative resistant biotype to glyphosate compared to a susceptible biotype from a nonagricultural area, (2) quantify shikimate accumulation over time in both biotypes, and (3) determine the effectiveness of POST-applied herbicides labeled for use in cotton in controlling both biotypes at three growth stages. The susceptible biotype had a 50% lethal dose of 407 g ae/ha of glyphosate compared with 2,176 g/ha for the resistant biotype when treated at the four-node stage, a 5.3-fold level of resistance. The resistant biotype accumulated 3.3- to 9.8-fold less shikimate than the susceptible biotype at 1 to 7 d after treatment. The resistant biotype was less responsive to glyphosate as treatment was delayed past the two-node stage, much more than the susceptible biotype. Glufosinate, MSMA, and diuron controlled both biotypes by at least 90%, regardless of size at application. Prometryn, flumioxazin, carfentrazone-ethyl, fomesafen, and trifloxysulfuron controlled both biotypes by at least 89% when applied at the two-node stage, but control generally diminished with later application timings. Pyrithiobac was not effective in controlling either biotype, regardless of size at application. Hence, there are effective herbicide options for controlling glyphosate-resistant giant ragweed in cotton, and the resistant biotype does not appear to exhibit multiple resistances to other herbicides.

Nomenclature: Carfentrazone-ethyl; diuron; flumioxazin; fomesafen; glufosinate; glyphosate; MSMA; prometryn; pyrithiobac; trifloxysulfuron; giant ragweed, Ambrosia trifida L. AMBTR; cotton, Gossypium hirsutum L