Clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) is a selective herbicide used for control of broadleaf weeds, especially thistles and clovers. For control of Canada Thistle, Cirsum arvense, a noxious, perennial weed, clopyralid is the only effective herbicide available.

Clopyralid is in the pyridine family of herbicides, which also includes picloram, triclopyr, and several less common herbicides. It is particularly active on members of the aster family, and legumes. It does not affect grasses.

CAS No.: 1702-17-6 C₆H₃C_l2NO₂

Clopyralid is notorious for its ability to persist in dead plants and compost, and has accumulated to phytotoxic levels in finished compost in a few highly publicized cases. In Seattle, USA, clopyralid was widely used for weed control in lawns until prohibited in 1999. There, a city-mandated curbside grass clipping collection and composting program produced compost with measurable levels of clopyralid. Subsequently, DowAgro, the manufacturer of clopyralid, voluntarily deregistered it for lawn uses.

The deregistration was viewed by some as a triumph of grass-roots activism in the face of corporate and governmental lies. Others saw it as an exaggerated, politically correct response to isolated incidents with no basis in science.