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Resistant Weed Control: Things Every Grower Should Know


Resistant weeds are here to stay--but there are ways to start dealing with them. In fact, they must be dealt with for growers to meet their production goals. Today, herbicide-resistant weeds may be costing U.S. growers as much as $2 billion a year in decreased yields, increased input costs, and decreased land values, according to University of Wisconsin extension weed scientist Vince Davis. With margins tight, that’s a cost that must be addressed.

How Does Herbicide Resistance Spread?

Even the remaining farms that do not have resistant weeds should design herbicide programs as if resistant weeds were present. Resistant weeds can move onto farms in many ways, making even these farms susceptible.

Wind can carry resistant pollen to cross-pollinate with susceptible weeds or roll weeds across the soil surface, spreading resistant weed seed to nearby farms. Rain run off can carry weed seed into adjacent farms. Wildlife, domestic livestock, and humans can transfer resistant seed from farm to farm. Machinery such as combines and tillage tools, if not properly cleaned, commonly spread resistant seed between farms. These many avenues open up almost any farm to resistance.


Understanding Herbicides for Weed Resistance

In order to chemically control resistant weeds, a basic understanding of herbicides is needed. You’ll need knowledge of things like Mode of Action (MOA) and Site of Action (SOA) to make decisions about how to reduce the probability of creating new resistance.  

The herbicide Mode of Action (MOA) describes how the herbicide works. The herbicide Site of Action (SOA) describes where the herbicide works. A herbicide is given a “Group Number” based on its SOA. For example, Cobra®, a Group 14 herbicide, and Gramaxone®, a Group 22 herbicide, both have same MOA as Cell Membrane Disrupters.  Cobra, however, is a PPO Inhibitor (Polyphenol oxidase) SOA, which is classified as Group 14, while Gramaxone’s SOA is as a PED inhibitor (Photosystem electron diverter), part of Group 22.

There are only 19 SOA herbicide groups.  Many weeds have resistance to several SOA groups.  Common Waterhemp has shown resistance to Groups 2, 4, 5, 9, 14, and 27 in Nebraska and 13 other states.


The Evolution and Prevention of Resistance

Resistance evolves as a natural selection process caused by using the same herbicide on the same farm for several years in a row. This makes it is critical to use several different MOA’s and SOA’s on each farm to reduce the probability of creating new weed resistance while achieving acceptable weed control. Other ways to control resistant weeds are to plant cover crops, no-till farming, and conventional tillage.


Tips for prevention of Herbicide Resistance

  • Starting with a weed-free seed bed at planting to keep the planted crop from competing for soil moisture, nutrients, and sunlight with weeds that may be present.

  • Choose herbicides that provide residual control for target weeds. Weeds are most reliably controlled before they emerge.

  • Overlap an additional residual herbicide as a post-application to ensure the best weed control of most targeted weeds. When crop canopy is reached, additional weeds cannot emerge.

Know your weed resistance issues.  Diversify herbicides used by selecting several MOA’s and SOA’s.  This will result in greater weed control and aid in the prevention of herbicide resistance on target weeds on your farms.

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