The exotic annual grass medusahead (Taeniatherum caput-medusae [L.] Nevski) has invaded millions of hectares in the western United States. The herbicide aminopyralid is an emerging tool for controlling medusahead. Aminopyralid reduces viability of grass seeds when applied just before seed production. In annual grasslands, desirable forage grasses generally begin seed production before medusahead, and applying aminopyralid when medusahead is still mostly vegetative but forage grasses have begun producing seed allows medusahead to be targeted somewhat selectively. In recent experiments, this approach reduced medusahead viability and cover to near zero and increased forage grasses. These experiments occurred in small plots, and results may differ in pastures if grazing or environmental heterogeneity causes plant stages to vary. Moreover, the results may prove highly sensitive to application timing. To address these information gaps, we factorially combined grazing and aminopyralid treatments in pastures and varied application timings in small plots. In pastures, aminopyralid reduced medusahead cover from about 45% to 20% one yr and from 60% to 20% another year. These reductions are smaller than in past research, but this doesn't seem attributed to the larger treatment area because reductions were similar in pastures and small plots treated the same time. Despite smaller reductions in medusahead cover, forage grasses increased as much or more than previously, perhaps because aminopyralid previously more sharply reduced forage grass seed viability. Seed viability did not vary with grazing, indicating grazing can occur during treatment, because aminopyralid does not have grazing restrictions. Careful application timing is critical: Just 12 d separated the least (jointing and boot stages) and most (boot to early heading) effective timings for controlling medusahead. A low rate of aminopyralid (22% of maximum labeled rate) is needed to reduce medusahead seed production, and the low cost of the low rate should encourage repeated applications when necessary.