Dose-response experiments were conducted on a biotype of prickly lettuce collected from Whitman County, WA, to determine the level of resistance to 2,4-D. Initially, progeny of prickly lettuce that survived two applications of glyphosate and 2,4-D in mixture were collected to determine if antagonism of the 2,4-D or glyphosate was occurring. Prickly lettuce survival was determined to not be due to antagonism of 2,4-D or glyphosate when the two herbicides were applied in mixture. The doses required to reduce growth 50% (GR50
) for resistant and susceptible field-collected prickly lettuce were 150 and 6 g ae/ha 2,4-D, respectively, indicating the resistant biotype was 25 times more resistant to 2,4-D than the susceptible biotype. The resistant biotype expressed injury but produced regrowth following application. A dose of 2,4-D at 220 g/ha was required to reduce regrowth frequency 50% (FR50
) for resistant field-collected prickly lettuce. Regrowth was also observed with the susceptible biotype, although the FR50
was much lower (10 g/ha), resulting in an R/S ratio of 22 based on the respective FR50
values. A rate of 4,300 g/ha 2,4-D (10 times the maximum labeled rate in wheat) was required to reduce the regrowth frequency in the resistant biotype to zero.