Differential response of horseweed (Conyza canadensis) to halauxifen-methyl, 2,4-D, and dicamba

摘要

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 (GR(50)) 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 GR(50) 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 GR(50) 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, GR(50) values for horseweed root length for 2,4-D and dicamba were 0.16 and 0.19 mu M, respectively, whereas only 0.004 mu M 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.