Paraquat‐resistant biotypes of Hordeum glaucum from zero‐tillage wheat

There has been a significant increase in the area seeded to minimum‐ and zero‐tilled crops worldwide over the past two decades. These cropping systems rely primarily on the non‐selective herbicides glyphosate or paraquat/diquat to control weeds before seeding the crop. Both glyphosate and paraquat/diquat are regarded as low‐risk herbicides in the ability of target weeds to develop resistance to them. Following 10–15 years of once annual applications of paraquat and diquat for weed control in zero‐tilled cereals, failure of these herbicides to control Hordeum glaucum Steud. in two separate fields occurred. Dose–response experiments demonstrated high‐level resistance to paraquat and diquat in both populations; however, the resistant biotypes are susceptible to other herbicides. This is the first report, worldwide, of paraquat resistance following the use of this herbicide in zero‐tillage cropping systems and is therefore a harbinger of future problems in minimum‐tillage systems when there is exclusive reliance on a contact herbicide for weed control.