PARAQUAT-RESISTANT BIOTYPES OF HORDEUM-GLAUCUM FROM ZERO-TILLAGE WHEAT

There has been a significant increase in the area seeded to minimum-an d zero-tilled crops worldwide over the past two decades. These croppin g systems rely primarily on the non-selective herbicides glyphosate or paraquat/diquat to control weeds before seeding the crop. Both glypho sate and paraquat/diquat are regarded as low-risk herbicides in the ab ility of target weeds to develop resistance to them. Following 10-15 y ears of once annual applications of paraquat and diquat for weed contr ol in zero-tilled cereals, failure of these herbicides to control Hord eum glaucum Steud. in two separate fields occurred. Dose-response expe riments demonstrated high-level resistance to paraquat and diquat in b oth populations; however, the resistant biotypes are susceptible to ot her herbicides. This is the first report, worldwide, of paraquat resis tance following the use of this herbicide in zero-tillage cropping sys tems and is therefore a harbinger of future problems in minimum-tillag e systems when there is exclusive reliance on a contact herbicide for weed control.