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Medusahead (Taeniatherum caput-medusae [L.] Nevski), the most problematic invasive grass on many California rangelands, 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 aminopyralid, normally used for control of certain broadleaf species such as thistles, suppresses some annual grasses when applied pre-or early postemergence. In 2009-2010, we tested the efficacy of aminopyralid for medusahead control in preemergence applications at three foothill rangeland sites in northern California. We compared a rate series of aminopyralid (53, 88, 123, and 245 g Á ha À1 acid equivalent [ae]) with rimsulfuron (18 and 35 g Á ha À1 active ingredient) and imazapic (140 g Á ha À1 ae). Plots were 339 m with four replications at each site. Treatments were applied in early fall 2009. In May 2010, we took visual cover estimates and biomass/seedhead samples in three quadrats per plot. In regression analysis, medusahead cover was found to decrease consistently with increasing rates of aminopyralid. Medusahead control at the highest rates of aminopyralid was consistent across the three sites, averaging 89% 6 3 standard deviation (SD) with 245 g Á ha À1 ae and 59% 6 10 SD with 123 g Á ha À1 ae. Aminopyralid at lower rates, rimsulfuron, and imazapic were less consistent. Cover of other annual grasses increased in plots treated with aminopyralid at all sites. Aminopyralid has potential utility for suppressing medusahead, particularly in sites also infested with invasive members of the Asteraceae. However, the most effective rate (245 g Á ha À1 ae) 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 other susceptible species. Resumen Medusahead (Taeniatherum caput-medusae [L.] Nevski), la especie de pasto invasiva má s problemá tica en muchas á reas de California, es difícil de controlar selectivamente en los pastizales. Se ha probado el fuego prescrito, el pastoreo, y los herbicidas con algo deéxito pero no son prá cticos en todas las situaciones. El herbicida selectivo aminopyralid, normalmente usado para el control de ciertas especies de hoja ancha como cardo, suprime algunos pastos anuales cuando se aplica durante la etapa de pre o al inicio de pos-emergencia. In 2009-2010 se evaluó la eficiencia de aminopyralid para controlar medusahead en aplicaciones pre-emergentes en tres sitios localizados en pastizales del norte de California. Se compararon una serie de rangos de aplicación de aminopyralid (53, 88, 123, y 245 g Á ha À1 acido equivalente [ae]) con rimsulfuron (18 y 35 g Á ha À1 de ingrediente activo) e imazapic (140 g Á ha À1 ae). Las parcelas fueron de 3 m por 9 m con cuatro repeticiones en cada sitio. Los tratamientos se aplicaron durante el inicio del verano de 2009. En mayo de 2010 se tomo una estimación visual de la cobert...

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Exotic annual grasses such as medusahead [Taeniatherum caput-medusae (L.) Nevski] and downy brome (Bromus tectorum L.) dominate millions of hectares of grasslands in the western United States. Applying picloram, aminopyralid, and other growth regulator herbicides at late growth stages reduces seed production of most exotic annual grasses. In this study, we applied aminopyralid to T. caput-medusae to determine how reducing seed production in the current growing season influenced cover in the subsequent growing season. At eight annual grassland sites, we applied aminopyralid at 55, 123, and 245 g ae ha −1 in spring just before T. caput-medusae heading. The two higher rates were also applied pre-emergence (PRE) in fall to allow comparisons with this previously tested timing. When applied in spring during the roughly 10-d period between the flag leaf and inflorescence first becoming visible, just 55 g ae ha −1 of aminopyralid greatly limited seed production and subsequently reduced T. caput-medusae cover to nearly zero. Fall aminopyralid applications were less effective against T. caput-medusae, even at a rate of 245 g ae ha −1 . The growing season of application, fall treatments, but not spring treatments, sometimes reduced cover of desirable winter annual forage grasses. The growing season after application, both spring and fall treatments tended to increase forage grasses, though spring treatments generally caused larger increases. Compared with other herbicide treatment options, preheading aminopyralid treatments are a relatively inexpensive, effective approach for controlling T. caput-medusae and increasing forage production.