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Why Herbicide Modes and Sites of Action Differ

Weed scientists and agronomists frequently intersperse the
terms of pesticide modes of action (MOA) and sites of action (SOA). What do
they really mean?

Although the two are used interchangeably, they mean differ

MOA refers to how a pesticide kills a pest. For example,
Select, a soybean herbicide, curtails lipid synthesis in targeted weeds. Thus,
its mode of action is a lipid synthesis inhibitor. It works by binding to the
enzyme. Thus its site of action is as an ACCase inhibitor. Herbicides can also
bind to specific proteins in a weed, disrupting plant processes. Most
herbicides bind to a single protein.

 “When a
herbicide bins to a single protein, that protein no longer functions,” says Bob
Hartzler, Iowa State University Extension weeds specialist. “When it doesn’t
function, the biological process doesn’t function, and it (weed) dies.”

Group Number

Herbicides are also assigned a group number so that applicators
can know which different sites of action are being used in a herbicide mix. This
helps farmers and applicators diversify herbicide lineups to ensure they are
using several different ways to kill weeds. Weed scientists and agronomists note
mixing up sites of action is a good way to forestall herbicide resistance in

A chart showing mode and sites of action is at




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