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Weed Management Answers

March 12, 2011

A general knowledge about weed biology and understanding how herbicides work is important to preserving herbicide technology. The following questions and answers were developed with help from these weed scientists: Mark Bernards, University of Nebraska; Kevin Bradley, University of Missouri; Aaron Hager, University of Illinois; Robert Hartzler, Iowa State University; Peter Sikkema, University of Guelph; Christy Sprague, Michigan State University; Daniel Stephenson, LSU AgCenter; and Bryan Young, Southern Illinois University.

What is a weed?

The term generally refers to any plant growing where it is not wanted or intentionally sown. A plant is considered a weed when it interferes and has negative effects on a crop. A weed typically competes with crops for water, light, space and soil nutrients. Some weeds serve as hosts for crop disease and insects. Others, such as black nightshade or wild garlic, can impact crop quality.

How do herbicides kill weeds?

Herbicides work by interfering with essential processes necessary for a plant to live. The active ingredient within the herbicide formulation binds to a particular target site or sites within the plant, usually an enzyme or protein essential to plant growth and development. This causes toxic metabolic consequences that eventually lead to the death of the weed.

What factors impact herbicide performance?

Spray coverage, application method, herbicide rate, environmental conditions and weed size to name a few.

What is herbicide mode of action?

Mode of action (MOA) is how the herbicide controls the plant. It describes the metabolic or physiological process impaired or inhibited by the herbicide.

What is herbicide site of action?

Site of action is the specific location within the plant where the herbicide must bind to exert its mode of action. It is where the herbicide acts within the plant.

Why is the difference between mode and site of action important?

While understanding herbicide mode of action is important, classifying herbicides by site of action is a more useful way to describe herbicides as weed resistance issues surface. For example, the mode of action category "amino acid synthesis inhibitors" would place the herbicides Pursuit (imazethapyr) and Roundup (glyphosate) in the same family. Classification by site of action is more distinct and allows growers to more accurately rotate herbicides. The Weed Science Society of America groups products with similar sites of action together by group numbers from 1 to 28. This number is an easy way to identify products and premixes.

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FEATURED IN: Farm Journal - March 2011

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