Abstract Materials and MethodsA 4200 ft² area was chosen for the study. The entire study area was measured and staked. The study was set up as a completely randomized block containing five treatments and four replications. Each plot (treatment) was 7 feet wide by 30 feet in length. The treatments included two spring and two fall treatments at 4 and 8 ounce/acre rates, as well as an unsprayed check (Table I). Methylated seed oil (MSO) was added to the early spring applications for increased plant activity. A CO2 knapsack sprayer was used for the herbicide applications. The knapsack sprayer had a 7-foot boom and was calibrated to deliver 20 gallons per acre (gpa) at 4 miles per hour (mph). Prior to applications, air and soil temperatures were recorded along with relative humidity and wind speed. Weed control and off-target vegetation injury evaluations were collected in May and July. Fall applications of imazapic at the 4 and 8 ounce rates were applied on September 27, 2007. Moisture levels were well below normal and precipitation did not occur until November. Fall applications did not have the addition of methylated seed oil. Data will continue to be collected until the fall of 2008. Additional imazapic applications will be applied in the fall of 2008 to identify if 2007-08 results were representative or if weather conditions impacted the results.

Ventenata grass (Ventenata dubia), commonly called wiregrass or hairgrass, is an invasive, introduced annual grass. Ventenata is highly invasive in bluegrass, alfalfa, small grains, pasture and rangeland. Little is known about the impacts of this invasive species that is rapidly spreading across the Pacific Northwest. Ventenata has shown resistance to the herbicides glyphosate and sethoxydim. In the early spring of 2007, a research study was initiated on rangeland in Anatone, Washington, using the herbicide imazapic. Imazapic was applied at 4 and 8 ounce per acre rates in the early spring and fall. Spring applications at the 4 ounce per acre rate provided an average of 68% control, while the 8 ounce per acre rate provided 93% control. Early spring applications resulted in 25% or less injury to existing rose species at the 4 ounce rate, compared to 55% injury at the 8 ounce rate. The herbicide imazapic is typically applied to invasive plants in the fall because of greater control, but data from fall applications in the Anatone study has shown mixed results due to unusual weather conditions.

Results and ConclusionsVentenata grass became chlorotic 6 days after spring imazapic application. The 4 ounce per acre spring application of imazapic provided 68% ventenata grass control (Table II, Figures I and II), while the 8 ounce per acre rate provided 94 percent control (Table II and Figure II). Imazapic also provided 65 and 95 percent Japanese brome control at the 4 and 8 ounce rates, respectively. The bunchgrasses in the study area did not show any sign of injury at the 4 or 8 ounce rate. Upon controlling the ventenata grass, cattle heavily grazed the bunchgrasses left within the treated plots. Early spring applications did cause some injury to existing rose species. The 4 ounce rate resulted in less than 25 percent injury, compared to 55 percent injury from the 8 ounce rate (Table II & Figure III). Some of the resultant off-target injury may have been due to the addition of methylated seed oil. Fall applications did not perform as well as expected. Imazapic applied at the 4 ounce rate provided 57 percent control of ventenata grass, while the 8 ounce rate provided only 73 percent control. Weather conditions may have been a factor.

IntroductionVentenata grass has been spreading throughout the Pacific Northwest at an alarming rate over the past decade. Ventenata is infesting pasture and rangeland, cropland, and lands enrolled in the Conservation Reserve Program (CRP). In early 2007, BASF Corporation agreed to fund a study to research control of ventenata grass using the herbicide imazapic (Plateau®). NRCS assisted in locating a research site near Anatone, Washington. The selected site was a high elevation native pasture periodically grazed by livestock. Ventenata grass had infested over 90 percent of the area (see photo below). Other vegetative species present were: prickly lettuce, wild rose, cinquefoil, bulbous bluegrass, other bunchgrasses, marestail, Japanese brome and yarrow (Figure IV). Two field tours were conducted at the site in 2007 to educate local landowners about ventenata grass and its potential control using the herbicide imazapic.

VENTENATA GRASS CONTROL WITH IMAZAPIC Stephen M. Van Vleet¹

¹Washington State University, Extension Educator, Agriculture and Natural Resources 310 N. Main St., Colfax, Washington 99111, svanvleet@wsu.edu

Table I Ventenata Study Treatment Area401SpringPlateau+MSO8 oz/a

402FallPlateau4 oz/a

403Untreated Check

404SpringPlateau+MSO4 oz/a

405FallPlateau8 oz/a

301FallPlateau4 oz/a

302FallPlateau8 oz/a

303SpringPlateau+MSO4 oz/a

304SpringPlateau+MSO8 oz/a

305Untreated Check

201Untreated Check

202FallPlateau4 oz/a

203FallPlateau8 oz/a

204SpringPlateau+MSO4 oz/a

205SpringPlateau+MSO8 oz/a

101SpringPlateau+MSO 4 oz/a

102SpringPlateau+MSO8 oz/a

103FallPlateau4 oz/a

104FallPlateau8 oz/a

105Untreated Check

* Fall data evaluations will continue to be collected through September 2008

Injury—Rosa sp.

Ventenata, Japanese Brome and Marestail

204

205

July 10, 2007

401

204

404

May 8, 2007

Figure I Imazapic at 4 and 8 oz/A (spring application)

Figure II Imazapic at 4 and 8 oz/A (spring application)

Figure III Spring injury with 8 oz/A imazapic

Figure IV Vegetation species Site at Anatone, Washington

Table II Grass Control and Injury Application Timing Date Ventenata

controlJapanese brome

controlRosa spp.

injury

Spring4 ounce/acre

May 2, 2007 100 …. 25

Spring4 ounce/acre

July 10, 2007 69 65 5

Spring8 ounce/acre

May 2, 2007 100 …. 55

Spring8 ounce/acre

July 10, 2007 94 95 15

Fall*4 ounce/acre

June 27, 2007 57 …. 0

Fall*8 ounce/acre

June 27, 2007 73 …. 0