High Avena fatua control costs have caused some Hordeum vulgare growers to use reduced rates of herbicides without fully understanding the consequences. Field studies near Moscow and Genesee, ID, were conducted to determine the effect of A. fatua density and PP-604 rate on A. fatua seed production in H. vulgare and on H. vulgare yield. PP-604 treatments were 25, 50, 100, 150, and 200 (minimum labeled rate) g ha⁻¹, and five A. fatua densities ranged from 0 to 386 plants m⁻². Visual A. fatua control was greater than 85% with 100 g ha⁻¹ PP-604 at all locations. Data from 1998 were used to construct nonlinear exponential decay and parabolic models to describe the effect of reduced herbicide rates on viable A. fatua seed production and relative H. vulgare grain yield, respectively. At A. fatua densities of 42 to 138 plants m⁻², 46 to 71% of the minimum labeled rate of PP-604 reduced seed production 95%. However, an estimated 140 to 235 seeds m⁻² were produced at this level of control, which may not ensure a decline in the A. fatua population over the long-term. Hordeum vulgare grain yield was maximum when 70 to 85% of the minimum labeled rate was applied to A. fatua densities of 42 to 138 plants m⁻². A higher rate of PP-604 likely will be required to ensure a decline in A. fatua populations over the long-term than needed to obtain maximum H. vulgare grain yield in a single growing season.

Nomenclature: PP-604 (proposed common name, tralkoxydim), 2-[1-(ethoxyimino)propyl]-3-hydroxy-5-(2,4,6-trimethylphenyl)-2-cyclohexene-1-one; Avena fatua L. AVEFA, wild oat; Hordeum vulgare L. ‘Baronesse’, spring barley.