High Quality Content by WIKIPEDIA articles! 2,4-Dichlorophenoxyacetic acid (usually referred to by its abbreviation, 2,4-D) is a common systemic pesticide/herbicide used in the control of broadleaf weeds. It is one of the most widely used herbicide in the world, and the third most commonly used in North America. 2,4-D is a synthetic auxin (plant hormone), and as such it is often used in laboratories for plant research and as a supplement in plant cell culture media such as MS medium. It was a major ingredient in Agent Orange alongside its chemically similar relative, 2,4,5-T (2,4,5-trichlorophenoxyacetic acid).