Many of the herbicide modes of action kill the plant by blocking a particular biochemical pathway.

For example, mode of action groups 1, 2 & 9 reduce the ability of the plant to produce substrates required for growth. Typically, these modes of action are slower to show herbicidal symptoms.

Groups 5, 6, 10, 14 & 22 result in a rapid build up of toxic compounds created when the pathway is blocked, which often expresses as rapid cell damage.

Synthetic auxin herbicides, often called growth regulators (Group 4) are thought to kill plants through causing undifferentiated growth by disrupting the normally tightly regulated hormone systems within a plant.