Auxins are a class of plant hormones (or plant growth regulators) derived from tryptophan. Auxins are found in the tips of shoots and roots and promote cell division, stem and root growth. They can also have a radical effect on plant orientation by promoting cell division on one side of the plant in response to sunlight and gravity, called phototropism. 

Auxinic herbicides are organic compounds that are used as active ingredients in many herbicide preparations. These are the oldest synthetic herbicides.

These herbicides are synthetic auxins and can also be described as herbicidally active growth regulators or herbicides with growth regulating action.

Dicamba, also known as 3,6-dichloro-o-ananisic acid, is one of the herbicides that acts as a phytohormone. In minute doses, it can stimulate or interrupt the growth of broadleaf weeds. Dicamba can also act as a selection agent for plant cells expressing the DMO gene.