Dancing With Mr. D – New Troubles For An Old Herbicide

On Death and Dicamba.

About a year ago the recent controversy over the increased spraying of the herbicide Dicamba on midwest U.S. farms took a turn for the worse. Unfortunately, it involved alleged murder.

You can listen to an excellent account of the story here:

…and/or read about the story here and here.

Basically, this tragic story was the result of the illegal spaying of the herbicide Dicamba on Dicamba-tolerant GMO crops. Some of the Dicamba drifted through the air onto neighboring fields of non-GMO crops, which were damaged by this herbicide.

Dicamba drift has been a major issue ever since January 2015, when the USDA approved genetically-engineered, Dicamba-tolerant soybeans and cotton.

You can listen to a good story about Dicamba drift here:

…and/or read about it here and here.

What is Dicamba?

In the early 1940’s, it was recognized that some chemical compounds that have a structural resemblance to the naturally-occurring plant hormone indole-3-acetic acid (auxin) could, at very low concentrations, mimic the effects of auxin on plants. (These synthetic compounds are called auxin analogs.) It was later discovered that, at higher doses (still at relatively low concentrations), some of these synthetic auxin analogs actually killed plants, especially dicots (so-called “broadleaf” plants, as opposed to grasses).

Some say that this represents the birth of the commercial herbicide industry.

Dicamba (3,6-dichloro-2-methoxybenzoic acid) was registered in 1967 as a herbicide by BASF (Badische Anilin- und Soda-Fabrik, or, in English, Baden Aniline and Soda Factory). It’s been used, in various chemical formulations, by farmers and ranchers all around the world ever since, that is, for 50 years.

Dicamba is a member of the auxin-like herbicide family, which also includes the infamous herbicide Agent Orange (active ingredients = 2,4-D and 2,4,5-T). Though the chemical structure of Dicamba is distinct from the herbicides in Agent Orange, they all basically kill plants through mimicking auxin.

Why do such herbicides kill plants? The most common answer that you’ll likely encounter is that they interfere with plant growth regulation, leading to “abnormal” plants. It’s not uncommon to read that these herbicides “grow the plant to death”, whatever that means. In my opinion, this is not really a valid explanation.

The honest answer is that nobody really knows precisely how these herbicides kill plants. It most likely has to do with the fact that these auxin analogs have complex effects on auxin-sensitive plant physiology, including the stimulation of the production of other plant hormones such as ethylene and ABA. It’s quite plausible that these ancillary effects are what ultimately lead to plant death.

Anyway, I’ve previously blogged about this here, so I won’t bore you further. (It does, however, have relevance when it comes to explaining why some plants have become resistant to such herbicides.)

For a good review about Dicamba, please see Ref. 1 below.

Why Did Biotech Companies Make Dicamba-Tolerant GMO Crops?

As you likely have already gleaned from the stories above, it’s mainly because the herbicide glyphosate (commonly known as Roundup®) had lost its effectiveness against several pervasive noxious weeds, primarily, pigweed (Amaranthus palmeri).

For more about this, please see previous posts:

  • Why Herbicides Kill Plants (But Not You) – Part 2: Roundup®
  • Weed Control Using “Agent Orange Corn”: A Really Bad Idea?
  • The Last Roundup®?
  • Cause: Roundup Ready® Crops –> Effect: “Superweeds”
  • Dicamba Restrictions/Ban

    As a result of all this controversy, the use of Dicamba has been regulated by several midwestern states and also by the EPA. For more about this, please see links below:

  • Arkansas one step from dicamba ban – AgriNews (9/27/2017)
  • Ban of Herbicide Could Benefit Agriculture Prices– Bloomberg News (10/2/2017)
  • Indiana Mulls Rule Restricting Dicamba Use – AgWeb (10/4/2017)
  • Missouri Ag director lifts statewide ban on Dicamba – Daily Star-Journal (10/4/2017)
  • U.S. experts doubt EPA curbs on Monsanto, BASF herbicides will halt crop damage – Reuters (10/13/2017)
  • Monsanto

    A central figure in all of this is Monsanto, which has developed Dicamba-tolerant GMO soybeans and cotton, as well as proprietary technology that helps prevent Dicamba drift.

    Along with BASF, Monsanto has also been the target of a number of Dicamba-related lawsuits.

    Please see links below for more information:

  • Despite Problems, Monsanto Exec Says Dicamba Use Will Only Grow – WFYI (10/2/2017)
  • Monsanto Shrugs Off Herbicide Concerns With Surprise Profit – Bloomberg News (10/4/2017)
  • U.S. scientists to skip Monsanto dicamba summit – The Western Producer (10/5/2017)
  • Monsanto Holds Weed Scientist Meeting Discussing Dicamba – AGWeb (10/6/2017)
  • Dicamba lawsuits mounting – AGWeb (9/13/2017)


    1. Cox, C. (1994) Dicamba. Journal of Pesticide Reform, Vol 14., pp. 30-35. (Full Text – PDF)

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