Rattail fescue, a winter annual grass weed, has been increasing in Pacific Northwest (PNW) dryland cereal producing areas. Although rattail fescue is not a new weed species in the PNW, its incidence is expanding rapidly in circumstances where soil disturbances are minimized such as in direct seed systems. Options for effective rattail fescue control in winter wheat cropping systems have not been adequately investigated and need to be developed. Rattail fescue control with herbicide treatments was investigated in imidazolinone-resistant winter wheat using imazamox and other herbicides. Across multiple sites and two growing seasons, crop injury from herbicide treatments was minor to negligible with some exceptions. Treatments containing imazamox or mesosulfuron produced minor, transient winter wheat crop injury at some locations in some years. With the exception of flufenacet applied preemergence (PRE), control of rattail fescue in wheat was variable with single herbicide applications, but improved with sequential herbicide treatments. Rattail fescue biomass was greatly reduced by several treatments especially those containing flufenacet or from sequential herbicide application. Crop yield varied among sites due to growing season precipitation, and in some cases from rattail fescue control or herbicide related crop injury.

Rattail fescue infestations are increasing in dryland conservation-tillage winter wheat cropping systems in the inland Pacific Northwest (PNW) region of Idaho, Oregon, and Washington. Rattail fescue typically is controlled with cultivation in conventional tillage farming systems. However, reduced soil disturbance has allowed infestations to increase significantly. The objectives of this research were to determine the effectiveness of glyphosate rates and application timings on control of rattail fescue during a chemical-fallow period in winter wheat cropping systems. Chemical-fallow field studies were conducted during two growing seasons at nine sites throughout the PNW. Glyphosate was applied early POST, late POST, or sequentially in early plus late POST timings. Additionally, paraquat + diuron was applied early and late POST alone or sequentially with glyphosate. Sequential application treatments (glyphosate followed by [fb] glyphosate, paraquat + diuron fb glyphosate, and glyphosate fb paraquat + diuron) controlled rattail fescue (∼ 94% in Idaho and Washington, ∼ 74% in Oregon) and reduced panicle number (∼ 85% in Idaho, ∼ 30% in Oregon and Washington) equivalent to or greater than one-time treatments. Rattail fescue control and panicle reduction generally increased with increasing rates of glyphosate within application timings. Paraquat + diuron usually provided similar control and reduced rattail fescue panicle number compared to glyphosate treatments applied at the same application timing. Although not completely effective, sequential applications of either glyphosate or paraquat + diuron, fb glyphosate will provide effective control during chemical fallow.