Halauxifen-methyl is a new synthetic auxin herbicide for control of broadleaf weeds, including preplant applications for corn (Zea mays L.) or soybean [Glycine max (L.) Merr.]. The objective of this study was to investigate the efficacy of halauxifen-methyl in comparison to the current auxin standards, 2,4-D and dicamba, on glyphosate-resistant (GR) horseweed (Erigeron canadensis L.) at different plant heights. In field experiments, a foliar application of halauxifen-methyl at the recommended use rate of 5 g ae ha^-1 resulted in 81% control. Dicamba applied at 280 g ae ha^-1 provided a comparable level of efficacy of 80%, while 2,4-D at 560 g ae ha^-1 resulted in 49% control. The addition of glyphosate improved GR E. canadensis control with 2,4-D more than with halauxifen-methyl or dicamba, possibly due to the higher level of control observed with halauxifen-methyl or dicamba alone. Even though applied at 50 to 100 times lower application rates, the efficacy of halauxifen-methyl on E. canadensis was similar to dicamba and greater than 2,4-D. Thus, halauxifen-methyl should be an effective tool for management of GR E. canadensis before planting both conventional and herbicide-resistant soybean varieties, and it precludes the extended preplant application interval required for dicamba in some soybean management systems.