Herbicide treatment of previously seeded rangeland on the Channeled Scablands of eastern Washington with indaziflam and imazapic

Authors: Stonecipher, Clinton; Welch, Kevin; Cook, Daniel

Submitted to: Society for Range Management Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: 10/8/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: Forage options on the Channeled Scablands are limited due to an increase in annual grasses, primarily cheatgrass (Downy brome) and medusahead (Taeniatherum caput-medusae). Revegetation is often necessary to change the flora from an annual grass dominated state and provide alternative forages for livestock and wildlife. Prolific seed production from annual grasses makes it difficult for reseeding effort to be successful. Herbicides can provide an advantage to seeded plant species and aid in success of revegetation. The herbicides, Plateau® and Rejuvra®, were evaluated on rangelands that were previously reseeded with perennial vegetation. The study was conducted at two locations in eastern Washington. Location S was planted November 2013 and location M was planted October 2015. Vavilov II Siberian wheatgrass, Hycrest II crested wheatgrass, Sherman big bluegrass, and Immigrant forage kochia were planted at each location. Herbicides were applied in the fall (September 2023) on a set of plots and in the spring (March 2024) on the remaining plots. Plots were evaluated the year of treatment (YOT; July 2024) and one year after treatment (YAT; July 2025). Annual grass biomass was reduced by the Plateau® and Rejuvra® spring treatments however, medusahead biomass did not differ from controls at the S location. At the M location, medusahead was reduced in the Plateau® and Rejuvra® fall treatments and annual grass biomass was reduced in the Plateau® spring treatment compared to controls. Perennial grasses increased in the spring treatment of Plateau® the YAT at the S location. Forage kochia increased in the Rejuvra® spring and fall treatment and the spring Plateau® treatment the YAT at the S location. Both herbicides reduced medusahead and annual grasses but there were differences among locations.

Technical Abstract: Forage options on the Channeled Scablands are limited due to an increase in annual grasses, primarily cheatgrass (Downy brome) and medusahead (Taeniatherum caput-medusae). Revegetation is often necessary to change the flora from an annual grass dominated state and provide alternative forages for livestock and wildlife. Prolific seed production from annual grasses makes it difficult for reseeding effort to be successful. Herbicides can provide an advantage to seeded plant species and aid in success of revegetation. The objective of this study was to evaluate imazapic and indazaflam on rangelands that were previously reseeded. The study was conducted at two locations in eastern Washington. Location S was planted November 2013 and location M was planted October 2015. Vavilov II Siberian wheatgrass (Agropyron fragile), Hycrest II crested wheatgrass (Agropyron cristatum), Sherman big bluegrass (Poa secunda), and Immigrant forage kochia (Bassia prostrata) were planted at each location. Herbicides were applied in the fall (September 2023) on a set of plots and in the spring (March 2024) on the remaining plots. Plots were clipped the year of treatment (YOT; July 2024) and one year after treatment (YAT; July 2025). Annual grass biomass was reduced in the indaziflam and imazapic spring treatments (P=0.0005) however, medusahead did not differ from controls (P>0.05) at the S location. At the M location, medusahead was reduced in the indaziflam and imazapic fall treatments and annual grass biomass was reduced in the imazapic spring treatment compared to controls (P = 0.0052). Perennial grasses increased in the spring treatment of imazapic the YAT at the S location (P=0.0005). Forage kochia increased in the indaziflam spring and fall treatment and the spring imazapic treatment the YAT at the S location. Both herbicides reduced medusahead and annual grasses but there were differences among locations.