Weed Management Answers

March 11, 2011 08:25 PM

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.

What is herbicide resistance?

It’s the naturally occurring inherited ability of some weed biotypes to survive a herbicide treatment that should, when properly used under normal conditions, effectively control a weed population. Changes in weed populations begin when a small number of plants within a species, called a biotype, have a distinct genetic makeup that makes them less susceptible to a herbicide. It’s possible for multiple weed biotypes to exist in a single field. When a farmer continues to apply a particular herbicide or fails to change other cultural practices, the resistant biotype continues to survive, mature and produce seed. Globally, there are 352 herbicide-resistant weed biotypes; 130 of those are in the U.S.

Is resistance different than tolerance?

Yes. Some weeds are naturally tolerant to herbicides. For example, most grass species have some tolerance to 2,4-D herbicide.

Are some weeds more likely to become resistant?

There are some characteristics that seem to lead to the development of herbicide resistance. They include: large amounts of seed produced per plant; high levels of germination of those seeds; capable of cross pollination; several weed flushes per season and a high frequency of resistance genes. Weed seed dispersal also play a role. A weed such as kochia can tumble for miles spreading seed.

What is cross resistance?

Weeds can become resistant to herbicides with the same site of action. The site of action often is structurally changed to prevent herbicides binding within that group. For example, weeds resistant to atrazine can develop resistance to simazine—both are photosynthesis inhibitors.

What is multiple resistance?

A weed is able to survive treatment from more than one herbicide group. Waterhemp biotypes, for example, that are resistant to four herbicide families occur in some states. However, that does not mean every resistant weed is resistant to more than one herbicide product at the same time.

What are the first symptoms of resistance?

Because weed control is rarely 100% effective, herbicide resistant populations can easily go undetected. As the ratio of resistant to susceptible weeds increase, irregular patches of a single weed species begin to appear and spread in subsequent years. Surviving weeds of the problem species may be in close proximity to dying plants of the same species.

What should I do if I notice small weed escapes?

Reducing weed seed production is always a good idea. Consider spot herbicide applications, row wicking, cultivation or hand removal.



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