Yellow starthistle is the most widespread broadleaf invasive plant in the western United States, and it is particularly prevalent in California. Prior to the registration of aminopyralid in 2005, the standard for chemical control of yellow starthistle was the herbicide clopyralid. We report on a compilation of several independent trials comparing the efficacy of aminopyralid and clopyralid on yellow starthistle. Treatments were applied at several rates and timings at 11 locations in four states between 2001 and 2007. Treatments were made pre-emergence and postemergence at the seedling and rosette stages of yellow starthistle. Results showed that aminopyralid, even at the low rate of 18 g ae ha⁻¹, provided nearly complete control of yellow starthistle when treatments were made at the seedling stage. However, less consistent control (80 to 100%) resulted with applications made at the pre-emergence and rosette stages. At the seedling stage, aminopyralid is about four times more effective on yellow starthistle compared to clopyralid, based on the rate of acid equivalent. In the Central Valley of California, complete control was obtained at the lowest registered rate (53 g ae ha⁻¹) when applications were made from December through February. At two locations we also evaluated control of the poisonous native plant coast fiddleneck. Although clopyralid does not adequately control coast fiddleneck, aminopyralid provided almost complete control when applied in the winter growing season. Applications of aminopyralid at the rosette stage resulted in a two-fold increase in annual forage grass biomass the following year. These results indicate that aminopyralid is a valuable tool for land managers and can play an important role in integrated management strategies for yellow starthistle and coast fiddleneck.

Nomenclature: Aminopyralid; clopyralid; yellow starthistle; Centaurea solstitialis L.; coast fiddleneck; Amsinckia menziesii (Lehm.) A. Nels. & J. F. Macbr. var. intermedia (Fisch. & C. A. Mey.) Ganders.

Interpretive Summary: Yellow starthistle is one of the most invasive species of rangelands and natural areas in the western United States. Several control options have been developed including mowing, grazing, burning, biological control, and herbicides. Until 2005, the herbicide clopyralid was considered the most effective chemical option for yellow starthistle control. However, clopyralid does not provide effective control of coast fiddleneck, which often co-occurs with yellow starthistle in rangelands of California. Coast fiddleneck is toxic to livestock and often increases in cover following clopyralid treatment. Aminopyralid was registered for use in noncrop areas in 2005. In this study we compared the efficacy of aminopyralid and clopyralid for yellow starthistle and coast fiddleneck control at several rates and timings at 11 locations in four states during the time period ranging from 2001 to 2007. Our results indicate that aminopyralid requires about one-fourth the active ingredient compared to clopyralid to achieve similar control of yellow starthistle at the seedling stage. Unlike clopyralid, aminopyralid also provides excellent control of coast fiddleneck. Although pre-emergence applications and late-season applications (rosette stage) also control yellow starthistle, those timings were less consistent at lower rates and therefore required higher rates as per label instructions. Late-stage applications were more effective given higher rainfall. We did find that late-season application resulted in increased annual forage grass production the following year. In the Central Valley of California, the optimal timing of application was from December through February. In cooler climatic regions the optimal application timing window might be somewhat later. Our results indicate that aminopyralid is a very