Italian ryegrass is one of the most troublesome weeds worldwide because of the rapid evolution of herbicide resistance in this species. Oregon tall fescue seed production requires high seed purity, demanding good control of Italian ryegrass. The necessity to control herbicide-resistant Italian ryegrass and maintain tall fescue seed purity created interest in new chemical management options. The objectives of this study were to assess the effects of synthetic auxin herbicides on seed viability of Italian ryegrass biotypes and the feasibility of this management strategy for use in tall fescue seed production. Eight treatments of synthetic auxin herbicides were applied to Italian ryegrass and tall fescue at two growth stages (boot and anthesis): dicamba (1.0 and 2.2 kg ae ha^–1), 2,4-D (1.1 and 2.2 kg ae ha^–1), aminopyralid (0.5 kg ae ha^–1), dicamba + 2.4-D (0.8 + 1.1 kg ae ha^–1), 2.4-D + clopyralid (1.1 + 0.3 kg ae ha^–1), and halauxifen-methyl + florasulam (0.4 kg ae ha^–1 + 0.4 kg ai ha^–1). Aminopyralid applied at boot and anthesis stages of Italian ryegrass reduced seed viability. Aminopyralid treatments reduced seed viability and weight of Italian ryegrass more than 50% compared to the control. Four biotypes from different locations in western Oregon with different types of herbicide resistance were sprayed, and differences in aminopyralid effect among Italian ryegrass biotypes were documented. Aminopyralid reduced the speed of germination by 1 to 2 d. Aminopyralid treatments had a greater effect when applied at the anthesis stage and had a greater negative impact on tall fescue. Tall fescue plants were more susceptible to aminopyralid, so this management practice is not feasible for tall fescue seed production. Future studies are needed to understand the physiological mechanisms involved in the reduced seed viability and to define an optimum aminopyralid rate for different Italian ryegrass biotypes.

Nomenclature: 2,4-D; aminopyralid; clopyralid; dicamba; florasulam; halauxifen-methyl; Italian ryegrass, Lolium perenne L. ssp. multiflorum (Lam.) Husnot; tall fescue, Schedonorus arundinaceus (Schreb.) Dumort