Effects of the sulfonylurea herbicide metsulfuron methyl on growth and reproduction of five wetland and terrestrial plant species

Two wetland plant species (Mimulus ringens L. [monkey-flower] and Bidens cernua L. [bur-marigold]), two terrestrial species (Sinapis arvensis L. [wild mustard] and Phaseolus vulgaris L. [beans-variety, Kentucky blue]), and one species found in both wet and dry habitats (Echinochloa crusgalli [L.] Beauv. [barnyardgrass]) were exposed to 1% (0.015 g a.i./ha) and 10% (0.45 g a.i./ha) of recommended label rate of metsulfuron methyl, a sulfonylurea herbicide used in western Canada. The objective of the study was to investigate the effect of metsulfuron methyl on these plant species and to determine the most sensitive phenological stage. Chemical analyses of herbicide residues showed that there was good correspondence between the quantity measured in the tank mix compared with that detected on glass fiber papers, the latter representing the dose reaching the test plants during the spray event. All species exhibited marked effects on the vegetative growth and reproductive performance when sprayed at 10% label rate. Less pronounced but significant effects were shown at 1% label rate. Seed weight was reduced for B. cernua and S. arvensis. The seedling stage was the most sensitive period for all species tested, although surviving plants sprayed at later stages showed considerable effects on the reproductive parts. All species tested would be at risk from small doses of metsulfuron methyl drifting away from the sprayed areas. This study highlights the shortcomings of the current testing schemes required prior to pesticide registration.