Halauxifen-methyl is an auxin herbicide for broadleaf weed control in preplant applications to corn and soybean. Our objective for this research was to characterize the phytotoxicity of halauxifen-methyl on horseweed, relative to 2,4-D and dicamba, in terms of weed height, the response to an auxin synergist, and root activity. The 50% reduction in plant growth (GR50) value for halauxifen-methyl on rosette-sized plants was 0.05 g ae ha−1, 100 times less than the labeled use rate of 5 g ae ha−1, compared with 36 and 31 g ha−1 for 2,4-D and dicamba, respectively. In a whole-plant bioassay, 240 g ae ha−1 of 2,4-D was calculated as the GR50 value on horseweed 20-cm tall, whereas applications of only 53 and 0.40 g ae ha−1 were necessary for dicamba and halauxifen-methyl, respectively, to achieve the same response. As weed size decreased, there was a concomitant reduction in the estimated herbicide dose for the GR50 with similar differences observed between halauxifen-methyl and the other two auxin herbicides. The addition of diflufenzopyr, an auxin synergist, to 2,4-D and dicamba resulted in a synergistic response on horseweed. However, the addition of diflufenzopyr to halauxifen-methyl resulted in an additive or antagonistic effect, depending on rate of diflufenzopyr, demonstrating a distinctive physiological pathway for halauxifen-methyl compared with 2,4-D and dicamba. In the agar-based bioassays, GR50 values for horseweed root length for 2,4-D and dicamba were 0.16 and 0.19 µM, respectively, whereas only 0.004 µM halauxifen-methyl was required for a comparable root response. These results indicate that horseweed exhibits a high level of sensitivity to halauxifen-methyl and suggest the activity of halauxifen-methyl is different from that of 2,4-D and dicamba. These differences in herbicide activity may reflect differential absorption, translocation, metabolism, or targeting of auxin receptors found in horseweed.