Growth stage influences level of resistance in glyphosate-resistant horseweed

While glyphosate-resistant horse- weed has not previously been re- ported in California, we suspected that it might exist, especially in noncrop areas. We collected horse- weed seeds from two locations in the San Joaquin Valley and treated greenhouse-grown plants at differ- ent stages with different amounts of glyphosate. This study showed that a glyphosate-resistant biotype of horseweed exists in the noncrop areas of Dinuba, in Tulare County, and that the level of resistance may be infl u- enced by the plant's growth stage at the time of glyphosate application.