Enlist Duo: Dow Walks Into Monsanto’s Footsteps

In October of 2014, the U.S. Environmental Protection agency announced that they would register Dow Chemical’s Enlist Duo, a herbicide treatment system consisting of 2,4-D and glyphosate along with seeds of crops that were genetically modified to resist the pair of herbicides. It was a controversial decision which prompted a lawsuit. Slipping under the radar is that a year earlier, Health Canada also gave the product its approval without any of EPA’s restrictions. Adding more nuance to the issue is that since the EPA decision, of one of Enlist’s ingredients has been reclassified by the International Organization for Research on Cancer.

1. What is Glyphosate?

Roundup’s use soared after introduction of foods genetically modified to resist glyphosate

Although it had been made 20 years earlier, glyphosate was rediscovered by Monsanto in 1970.



The compound kills plants because it interferes with a plant enzyme and prevents the synthesis of certain amino acids. Its use skyrocketed after Monsanto, the manufacturer of Roundup (commercial name of glyphosate), also genetically engineered corn to be resistant to the herbicide. In fact, mostly due to its application in corn and soy fields, its use went up by more than a factor of 10 in about 2 decades (see graph).

In a current example of natural selection, by the mid 2000s, morning glory, amaranthus species, lambs quarters, giant and common ragweeds, horseweed, and velvet leaf all developed resistance against glyphosate. The natural variants or perhaps mutants that were not killed by Roundup went on to reproduce, and eventually their genes became those of the majority. The resistant ” superweeds” are now spread over 70 million acres in the US alone. To combat the resistance, Dow Agrosciences used genetic modification to protect crops against both glyphosate and 2, 4-D, which they include in their new Enlist Duo weed-killing mix.

2. What is 2,4-D and How Has Glyphosate Been Classified?

2,4-D, short for 2,4-Dichlorophenoxyacetic acid, is a synthetic auxin, a plant hormone which overstimulates and especially kills broadleaf plants. Critics of Dow point out that 2,4-D was an ingredient in the health hazard known as Agent Orange, a notorious defoliant used in the Vietnam War. Dow defends itself by pointing out that Agent Orange also had 2, 4, 5-T, which at the time was contaminated with dioxins. Certain dioxins are in the same class as tobacco smoke and benzene; they are proven human carcinogens. However, 2,4-D is not in that category of compounds. It is a possible carcinogen, classified as 2B, safer than class 2A, which includes probable carcinogens. But  as reported  in the Lancet in March, 2015, the problem is that Enlist‘s other ingredient, glyphosate, has just been placed in class 2A by WHO’s cancer research division.


3. EPA’s Restrictions and is the Duo Approach Ecological?

The EPA correctly points out that the variety of 2,4-D is an ester of lower volatility (it’s a quaternary ammonium salt called 2,4-D choline),

Choline quaternary ammonium salt  of 2,4-D, formed easily by mixing choline  hydroxide with 2,4-D in water
Choline quaternary ammonium salt of 2,4-D, formed easily by mixing choline hydroxide with 2,4-D in water

lowering the amount that will end up in groundwater. They ordered  a 30-foot in-field “no spray” buffer zone around application areas. It has also banned use when wind speeds are over 15 miles per hour.  EPA will also apparently monitor the herbicide for resistance and reevaluate the product after 6 years, instead of granting the usual 15-year time frame. But since their decision was made before the WHO’s reclassification of glyphosate, they have given Dow the authority to toss more of a questionable compound into an already stressed ecological stew. The whole idea of having the same company market a package of herbicide and GMO-compatible seeds seems to be more about short-term economic gains and less about using science to feed the world in an ecological manner.


In 2017, EPA amended the registration to allow use of Enlist Duo on genetically engineered cotton in those 15 states and extend the use of the herbicide on GE corn, soybean and cotton to an additional 19 states. They also stipulate that Enlist Duo cannot be applied when wind speed is over 15 mph. One cannot help wonder how often that recommendation is violated, given that 15 mph is not much of a wind, just 4 on the Beaufort scale, just enough to move small branches on a tree.

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