New Weed Resistance

By James Hoorman, Hoorman Soil Health Services, and Kevin Bradley, University of Missouri

Farmer have a new type of weed resistance to worry about.  Farmers have been using herbicide or chemical weed control for many years. Weeds have learned several ways to get around herbicides in order to survive.  While it is recommended that farmers rotate different groups of herbicides and use full rates, sometimes that does not happen and weeds become herbicide resistant. That has been the main way weeds like pigweed, water hemp, and marestail have become weed resistant. Some weeds have become resistant to one or several groups of herbicides and now some weeds have even learned to become resistant to almost any herbicide.  That is a scary proposition!

Often farmers use several passes of herbicides with different modes of action (MOA’s) to control weeds.  The goal is to reduce the weed population down to zero so that no survivors pass on any genes that are resistant.  The problem is that there are millions of weed seeds in an acre of land, stored over many years.  So weed seed is always present somewhere.  With good weed control, the weed seed population can be reduced greatly. 

Most weeds develop target site weed resistance which is based on herbicide MOA.  Herbicides disrupt different pathways in a plant so the plant cannot grow. These usually involve disrupting different proteins and enzymes.  When a plant can not grow, it usually dies out from starvation.  Most herbicides target certain enzymes or growth processes. Here are some examples of different MOA’s and different herbicide groups.

Group 2 or ALS herbicides like Pursuit, Scepter disrupt certain amino acids.  Group 4 herbicides like 2-4D and dicamba (Banvel) disrupt cell membranes and growth.  Group 9 herbicides like glyphosate (Roundup) also disrupt amino acids like the ALS herbicides but at different sites.  Group 10 herbicides like Liberty disrupts a key enzyme to put ammonium or nitrogen into amino acids to make proteins.  Liberty has been a good herbicide to control many weeds. 

Group 14 herbicides are called PPO inhibitors which causes cell wall damage.  Herbicides like Sharpen, Resolve, Valor products and Cobra are good examples.  The Group 15 herbicides inhibit cell wall division (Mitosis) and include herbicides like Harness, Dual II Magnum (Metochlor) and Zidua. 

Typically, weeds will become resistant to one MOA then over time they might become resistant to another MOA or group of herbicides.  Farmers keep rotating herbicides until they run out of options.  However, now weeds have developed a new way to get around MOA’s.  They have developed something called metabolic resistance.  With target resistance or traditional weed resistance, the plants developed an enzyme that would target almost all the herbicides using a certain MOA or a certain group of herbicides.  With metabolic weed resistance, the weed can destroy any herbicide.  It simply uses natural enhanced biological processes to break up any type of herbicide, even if it has never been used before.  Plants have several hundred enzymes that can perform this function to detoxify unnatural foreign substances within the plant. 

For example, in corn which is a grass plant; atrazine can be used because the corn plant has a natural defense to break down the atrazine before it causes any harm.  Soybeans are a broadleaf, so they do not tolerate atrazine the same as many other broadleaf weeds. In corn, this is called metabolic resistance.  This is not anything really new, however, it is troubling because some weedy plants are learning or eveloping genes to take out almost any herbicide.  Metabolic weed resistance versus target weed resistance is becoming more common.

For example, a relative of barnyard grass has been found that is resistant to 16 different herbicides and 6 different MOA’s or groups of herbicides even though none of the 16 herbicides had ever been applied to this plant!  Water hemp can cause up to 70% decline in corn yields.  Some areas now have water hemp that is resistant to seven different MOA’s or seven groups of herbicides.  Illinois researchers are now investigating about 60 different gene regions or two small area on weeds where weed resistance occurs.  Researchers want to develop a simple test or bioassay to determine which herbicides a plant is resistant to in that plant.  Farmers, in the near future, may need to do a bioassay before they spray so they can determine which herbicides will work.

Farmers have been battling weeds for centuries and it looks like the battle will continue. Many herbicides are still valid but farmers can also use crop rotation, mulches, cover crops, and mechanical means to control weeds.  The source for this information came from the Kevin Bradley,  University of Missouri and No-till Farmer. 

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