Skip to main content

Competency Area 2: Weed Management

PO 18. Chemical - Familiarity with the ways herbicides are classified, i.e. how they are used and by herbicide family/site of action classification.

Chemical control, as we know it, had its’ start in the 1940s with development of 2,4-D. The rapid development of herbicides since then has led producers to think of herbicides as a panacea for their weed problems. They are not a panacea. They are a two-edged sword and should be used as part of an integrated weed management program.

How they are used - selective vs. nonselective
Foliar applied herbicides
   o Contact - act mainly by touching weeds rather than by being absorbed and moved to the site of action.
   o Translocated - moved from point of entry to site of action within the plant.
Soil applied herbicides

Site of action classification
Mode of action refers to the sequence of events from herbicide absorption into a plant to its’ final effect at the site of action. Herbicides with similar chemical structures and herbicidal activity are grouped into families. Two or more herbicide families may have the same site of action within plants and express similar herbicidal activity and injury symptoms. The Weed Science Society of America (WSSA) has approved a numbering system to classify herbicides by their site of action. A Group Number is given to all herbicides with the same site of action.