What is Metolachlor?

Metolachlor or S-metolachlor is a selective and systemic herbicide which control weeds by inhibiting the synthesis of long chain fatty acids. Metolachlor is one of the widely used herbicides on corn in Minnesota, and is also labeled for use on soybeans, potatoes, sugar beets, sunflowers, and tomatoes. It has been frequently detected in both surface water and groundwater in Minnesota. 

In 2016, it was applied to 21% of Minnesota corn acres, making it the fifth-most-used corn herbicide in the state in terms of acres covered, according to the most recent National Agricultural Statistics Service pesticide use survey.

Metolochlor

Pesticide Type Herbicide (Group 15)
Chemical Class Chloroacetamide herbicide
Common Trade Names* Acuron, Brawl, Dual II Magnum,
Matador, Prefix and Sequence
Registration Status EPA: Registered since 1976
MN: Registered
Structure Chemical structure diagram of metolachlor

*No endorsement is implied in the referencing of trade names.


General Information

Metolachlor controls weeds by inhibiting growth of seedling shoots. It proves effective on weeds when applied before germination; hence, it is typically applied before or immediately after planting of the crop. Application timing relative to the crop includes preplant, preplant incorporated, preemergence, and postemergence. Metolachlor provides preemergence control of many annual grasses and a number of small seeded broadleaf weeds. Its control of perennial weeds is limited to yellow nutsedge (Weed Control Guide, Ohio State University Extension, 2018 -  PDF). It is used in combination with other preemergence and postemergence herbicides to broaden the spectrum of weed control. Glyphosate and atrazine are the common herbicides used in combination with metolachlor.

Other preemergence corn herbicides which provide weed control similar to metolachlor include dimethenamid-P (Outlook, Slider), acetochlor (Harness, Warrant), pyroxasulfone (Zidua), pendimethalin (Prowl)*, and many others. (Trials comparing herbicide systems for weed control in field corn, University of Minnesota).

When metolachlor is manufactured, two forms (or isomers) are produced, S-metolachlor and R-metolachlor. S-metolachlor has greater herbicidal activity. Products differ in their content of these two forms of metolachlor.

Metolachlor is in the chloroacetamide herbicide family which also includes the herbicides alachlor, dimethenamid, and acetochlor. It is also known as a long-chain fatty acid inhibitor (Herbicide Classification Poster, Mode of Action, January 2017 - PDF). Metolachlor has been frequently detected in surface water and groundwater.

*No endorsement is implied in the referencing of trade names.


The following links explain the importance of metolachlor in Minnesota agriculture, its impact on Minnesota's environment, and steps being taken by the MDA to minimize the presence of metolachlor in water.