Glyphosate hormesis in broad-leaved weeds: a challenge for weed management

Little is known of glyphosate-induced hormesis in weeds and how this might influence weed management. To test the hormetic effect of low doses of glyphosate on broad-leaved weeds, two experiments were conducted, in the laboratory and the screenhouse. The hormetic effects of glyphosate solution in growth media (0, 65, 130, 250, and 500 g acid equivalent (a.e) ha(-1)) and foliar spray (0, 4, 8, 16, 32, and 64 g a.e. ha(-1)) were tested on four broad-leaved weeds (Coronopus didymus, Chenopodium album, Rumex dentatus, and Lathyrus aphaca). Glyphosate solution in the range 65-250 g a.e. ha(-1) stimulated the germination and seedling growth of all tested weeds. However, at 500 g a.e. ha(-1) inhibition of germination and growth was observed. Foliarly applied glyphosate in the range 4-32 g a.e. ha(-1) increased root and shoot length, dry biomass, and seed production ability of all four weeds species; however, the stimulatory response was species dependent. These results indicate that glyphosate hormesis could play a significant role in altering crop/weed competition and might influence weed management.