Aminopyralid is a new systemic herbicide developed by Dow AgroSciences specifically for use on rangeland, pasture, Conservation Reserve Program acres, non-cropland, and natural areas. The herbicide is formulated as a liquid containing either, 240 g acid equivalent (ae)/L of aminopyralid as an amine salt or in a pre-mix with 2,4-D ( 40 g ae/L aminopyralid + 320 g ae/L 2,4-D). Aminopyralid has postemergence activity on established broadleaf plants and provides residual control of germinating seeds and emerging seedlings of susceptible plants. Field research was conducted to determine efficacy of various rates and timings of aminopyralid applied alone or in combination with 2,4-D for the control of broadleaf weeds in pastures and hayfields. Treatments were applied either postemergence to actively growing weeds or preemergence to evaluate residual weed control. Aminopyralid is effective at rates between 53 and 120 g ae/ha. These rates are about 1/4 to 1/20 less than use rates of currently registered rangeland and pasture herbicides with the same mode of action including, clopyralid, 2,4-D, dicamba, picloram, and triclopyr. Aminopyralid controls many annual, biennial, and perennial broadleaf weed species including Ambrosia artemisiifolia, Ambrosia psilostachya, Carduus acanthoides, Carduus nutans, Centaurea maculosa, Cirsium arvense, Cirsium vulgare, Rumex crispus, Solanum carolinense, Vernonia altissima, and Vernonia baldwinii. Applying aminopyralid with 2,4-D broadens the spectrum of weeds controlled including Dauca carota, Plantago lanceolata, Solidago canadensis, Solidago missouriensis, and Taraxacum officinale. Most warm- and cool-season rangeland and pasture grasses are tolerant to aminopyralid at 240 g ae/ha, which is twice the proposed maximum application rate.