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The Kochia and Dicamba Story

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Herbicide Use History

Imagine there is a tool that enables you to complete a difficult job much quicker.  You end up with as good as, if not better, results when you use this tool compared to when you do not.  It is even pretty inexpensive when you compare it to other tools you could choose, and makes the work easier and safer.  Would you use this tool to help you complete your work?  Most of us would answer with a loud YES!  It’s human nature to find an easier and better way to solve a problem or to complete our work. 

This was the response of most farmers when glyphosate-resistant crops were commercialized beginning in 1996.  Weed control became much simpler.  Because glyphosate was an extremely potent herbicide, the farmer often had to only use glyphosate to control the weeds in the field, instead of a combination of two or three other products.

Figure 1: Soybean field under pivot irrigation. (L. Sandall, University of Nebraska-Lincoln)

Glyphosate could be applied to the crop after both the crop and weeds had emerged.  It was even effective at controlling large weeds if weather conditions prevented the farmer from applying the herbicide at the recommended time. In many fields, only one application of glyphosate per year was needed to provide excellent weed control. The crop was safe from herbicide damage, and there were no restrictions on which crops the farmer could plant the next year because glyphosate is not available to kill plants after it contacts the soil. Glyphosate was less toxic than some of the other pesticides available, and allowed more farmers to adopt a no-tillage approach, both of which were better for the environment.

However, because some farmers used only glyphosate to control their weeds, in the year 2000 the first glyphosate resistant weed in a soybean field was identified in Delaware.  This is not the first time herbicide resistant weeds have evolved.  The first report of a weed resistant to a herbicide was wild carrot to 2,4-D in 1957.  In the 1970’s and 1980’s populations of several different weeds were confirmed to be resistant to PSII-inhibiting herbicides like atrazine.  In the 1980’s and 1990’s many different species became resistant to ALS-inhibiting herbicides like Pursuit  (WeedScience.org).  Whenever the same herbicide is used by itself repeatedly in a system, the potential exists for weeds in that system to evolve resistance.   

Perhaps the main factor contributing to the emergence of herbicide resistance in weed populations is that herbicides worked well and simplified farming.  It is human nature that when things work well we do it the same way again next time – “if it isn’t broken, why fix it.”  These herbicides effectively controlled the weeds and solved the immediate problem of yield loss from weed competition.  What worked well in one crop was often duplicated in a second crop, which exposed a greater number of weed populations to the same herbicide.  For example, glyphosate resistant traits were commercialized first in soybean, and later in canola, corn, sugarbeets, and alfalfa. Many times, little thought is given to the long-term impact of widespread use of a single product until resistance occurs, and since 2000, new weed species have become resistant to glyphosate each year. 

Scientists and farmers can learn from history. Because of challenges caused by glyphosate-resistant weeds, new technologies are being developed to improve weed control.  One of these technologies is a trait which confers dicamba-resistance to soybean, which is naturally very susceptible to dicamba.  This will allow the use of a new herbicide in soybean that will help control some of the resistant weeds. Dicamba resistant soybeans are expected to be commercialized in 2014, pending regulatory approval (Moore, 2010; Toner, 2009).  However, scientists want this technology to be used in a way that avoids resistant weeds from developing and preserves it as a useful tool for farmers to manage weeds and protect crop yield.  But concern about resistance developing is sometimes not enough to convince farmers to think about the long-term.  A scientist’s best method to motivate change is to make a discovery and then tell the discovery story.  We will see in this research story a discovery that should motivate the appropriate use of dicamba resistance in crops to preserve it as a long-term tool for weed control. 

Watch this video, approximately 8 minutes. Seeds, Weeds and Disease

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