Medusahead (Taeniatherum caput-medusae) is one of the most problematic invasive grasses on many California rangelands. It is difficult to control selectively in grasslands. Prescribed burning, grazing, and herbicides have been tested with some success but are not practical in all situations. The selective herbicide Milestone (aminopyralid), normally used for control of certain broadleaf species such as thistles, suppresses some annual grasses when applied pre- or early postemergence. We tested the efficacy of the aminopyralid for medusahead control in preemergence applications at three foothill rangeland sites in northern California. Treatments were applied in early fall 2009 and we evaluated the plots in May 2010. Our results indicate that high label rates of aminopyralid applied in fall, before medusahead emergence, can help to suppress this weed in sites similar to annual grasslands of California. Medusahead control at the highest rate (14 oz product/acre) of aminopyralid was consistent across the three sites, averaging 89% reduction in cover. Aminopyralid also provided some selectivity among grasses, resulting in increased cover of more desirable annual forage species, such as slender oat (Avena barbata) and Italian ryegrass (Lolium perenne ssp. multiflorum) at both 7 and 14 oz product/acre. Though our study showed that control was less effective at the 7 oz product/acre rate, studies in other areas of California have shown this rate to also be effective. The key to optimum results is the timing of application, which should be made in late summer prior to rains and seed germination in order to provide the best possibility of suppression or control. Grass control results will be poor if any of the winter annual grass seeds have germinated prior to application. Thus, aminopyralid has potential utility for suppressing medusahead and also cheatgrass, also called downy brome (Bromus tectorum), based on other studies. This may be a particularly effective management strategy if a target site is also infested with yellow starthistle or other problematic members of the Asteraceae (sunflower family), which are also highly susceptible to aminopyralid. However, the most effective rate (14 oz product/acre) is registered for use only as a spot application. In situations where this rate can be justifiably used, it would be expected to give season-long control of medusahead, as well as longer-term control of thistles and some perennial species. This treatment may be a useful management tool in situations requiring intensive management, such as small infestations and revegetation projects.
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