Russian thistle is a severe problem in fields after crop harvest in the Pacific Northwest (PNW) and is controlled either by tillage or broadcast applications of various herbicides. A study was conducted in Washington in 2000 and 2001 at four sites to compare the efficacy of two herbicide treatments applied with a light-activated, sensor-controlled (LASC) sprayer and a conventional broadcast sprayer for postharvest Russian thistle control. Additionally, simple economic comparisons, excluding fixed costs, were made among herbicide treatments and application methods. Both herbicide applicators controlled Russian thistle similarly within each herbicide treatment. Weed control was unacceptable (≤ 75%) when glyphosate plus 2,4-D was applied with either applicator. In contrast, Russian thistle control was > 90% with paraquat plus diuron regardless of applicator. The overall reduction in chemical use was 42% with the LASC compared with the broadcast applicator when averaged over the four sites. Herbicide and surfactant cost savings, using 2007 prices for the LASC compared with the broadcast applicator, ranged from $6.68/ha to $18.21/ha with the paraquat plus diuron treatment and averaged $13.27/ha less for the four sites. The use of the LASC for postharvest Russian thistle control can reduce growers' input costs, increase growers' profits, and improve environmental quality by reducing the amount and area of a restricted-use chemical.

Nomenclature: Glyphosate, 2,4-D, paraquat, diuron, Russian thistle, Salsola tragus L