Herbicide Control Strategies for Ventenata dubia in the Intermountain Pacific Northwest

Ventenata dubia is an exotic annual grass that has become increasingly invasive in various perennial grass systems throughout the Intermountain Pacific Northwest. Currently, little information is available to landowners about herbicide control options. In our first field study, we evaluated V. dubia control efficacy and perennial grass tolerance of herbicides applied pre-emergence (PRE) at two locations and as an early postemergence (EPOST) application at four different conservation reserve grasslands, with each grassland dominated by different perennial grass species. Treatments included flufenacet plus metribuzin (303 + 76 g ai ha⁻¹ [0.27 + 0.07 lb ai ac⁻¹]), propoxycarbazone-sodium (49 g ai ha⁻¹ [0.04 lb ai ac⁻¹]), rimsulfuron (53 g ai ha⁻¹ [0.05 lb ai ac⁻¹]), sulfosulfuron (53 g ai ha⁻¹ [0.05 lb ai ac⁻¹]), and imazapic (105 g ai ha⁻¹ [0.09 lb ai ac⁻¹]). Rimsulfuron and flufenacet plus metribuzin applied PRE provided > 90% control 10 mo after treatment (MAT). Rimsulfuron and sulfosulfuron applied EPOST provided > 90% control 9 MAT. Herbicide injury to bluebunch and intermediate wheatgrass was negligible across treatments. Imazapic and sulfosulfuron applied EPOST resulted in significant injury to smooth brome and timothy. In our second study, we addressed the following question: Will fall herbicide plus fertilizer treatments improve V. dubia control compared with herbicide treatments alone? We imposed fall herbicide treatments in main plots and fertilizer treatments (fall N, fall P, fall K, fall PK, spring N, NPK) in split plots at three study locations. Herbicide treatments resulted in high levels of V. dubia control. Differences in V. dubia abundance among fertilizer treatments were negligible 9 MAT. Within herbicide control plots, spring N and NPK treatments resulted in significant increases in perennial grass cover and decreases in V. dubia cover (9 MAT). This result indicates that spring N applications timed to the onset of perennial grass growth could be utilized as a component of an integrated management strategy for V. dubia in invaded perennial grass systems.Nomenclature: Flufenacet; imazapic; metribuzin; propoxycarbazone-sodium; rimsulfuron, sulfosulfuron; ventenata, Ventenata dubia (Leers) Coss. VENDU; bluebunch wheatgrass, Pseudoroegneria spicata (Pursh) Á. Love PSESP; intermediate wheatgrass, Thinopyrum intermedium Host THIIN; smooth brome, Bromus inermis Leyss. BROIN; timothy, Phleum pratense L. PHLPR.Management Implications: Ventenata dubia is an increasingly problematic invasive annual grass in the Intermountain Pacific Northwest. In the Palouse Bioregion, V. dubia has reduced forage production in hay and pasture, can negatively affect wildlife habitat and soil erosion management goals within Conservation Reserve Program (CRP) lands, and has invaded canyon grasslands and Palouse Prairie remnants. Currently, little information is available to land managers about how to control V. dubia across the range of perennial grass systems in which it invades. We used two field experiments to identify effective herbicide control options that also limit potential herbicide injury to perennial grass species and promote perennial grass competition. We evaluated flufenacet plus metribuzin (Axiom DF) at 303 + 76 g ai ha⁻¹, propoxycarbazone-sodium (Canter DG) at 49 g ai ha⁻¹, rimsulfuron (Matrix DG) at 53 g ai ha⁻¹, sulfosulfuron (Outrider DG) at 53 g ai ha⁻¹, and imazapic (Plateau SL) at 105 g ai ha⁻¹ applied at PRE and EPOST application timings at two study sites and an EPOST application timing at four study sites. Rimsulfuron and flufenacet plus metribuzin provided the most consistent V. dubia control (> 90%) at the PRE timing. Rimsulfuron and sulfosulfuron provided the most consistent control (90%) at the EPOST timing. Sulfosulfuron can be used in grass hay, grazed rangelands and CRP lands, but our study demonstrated the potential for perennial grass injury to smooth brome and timothy. In comparison, rimsulfuron provided greater safety to perennial grasses but is restricted to nongrazed perennial grass systems. Our study also indicates that PRE applications of flufenacet plus metribuzin provide high levels of V. dubia control without injury to timothy, which provides an effective control option for timothy hay producers who have limited labeled control options. In our second field experiment, we evaluated various fertilizer treatments (NPK) with or without a fall herbicide application. Fertilizer treatments did not result in lower ventenata abundance (% cover) within the first growing season when used with herbicide applications. However, in the absence of an herbicide application, spring N only and NPK applications reduced V. dubia abundance because of increases in perennial grass competitiveness. These results suggest that N fertilization timed to the onset of perennial grass growth in the spring can be an important tool for managing V. dubia–invaded perennial grass systems.