Dicamba Drift – Part 1

Herbicide tolerant crops have undoubtedly made weed control easier for farmers, and have resulted in a small but notable reduction in herbicide toxicity in US agriculture. Adoption rates in the US have been astoundingly high, with farmers in the US planting genetically engineered (GE) varieties for over 90% of corn, cotton, and soybean acres, as shown in this chart from the US Department of Agriculture (USDA).
Herbicide tolerance traits are often created with genetic engineering, such as the infamous Roundup Ready crops that are tolerant to glyphosate, but can also be developed with traditional breeding or mutagenesis, such as Clearfield wheat that is tolerant to imazamox. Herbicide tolerant crops have been in the news in part due to the newest version on the market – dicamba tolerant crops. Specifically, there have been problems with dicamba drift, meaning the herbicide hasn’t consistently stayed in the fields where it was sprayed.
herbicide tolerant crops adoption USDA

What is dicamba?

According to the Environmental Protection Agency (EPA), the US agency that regulates pesticides:

Dicamba “is a widely used herbicide on agricultural crops, fallow land, pastures, turfgrass, and rangeland. It was first registered in the US in 1967. Historically, most dicamba applications occurred in late winter or early spring for removal of broadleaf weeds prior to planting crops. It is registered for use in agriculture on corn, wheat, cotton, soybeans, and other crops.”

Dicamba is a growth regulator that acts like natural plant hormones called auxins. The herbicide 2,4-D is also a growth regulator. Plants need natural auxins to grow in a normal shape in response to external stimuli like light and gravity, as in the sped-up Arabidopsis plant below.
Dicamba is sold as Banvel, Clarity, DiFlexx, Engenia, XtendiMax, and other name brands. Dicamba selectively controls broadleaf weeds (such as dandelion and ragweed), but grasses (including crops like corn and rice) are naturally resistant.
When a plant has too much growth regulator, it grows in unexpected ways and eventually dies. This could happen if it has a mutation in auxin-related genes or if it is sprayed with dicamba, for example. The cells divide too fast in the wrong directions, and they don’t respond to external stimuli as they should. Since plant growth regulators act specifically on plants, the effects on animals (and humans) are limited.

Human health and dicamba

Dicamba is a relatively safe pesticide, when used as intended. For example, a large study of pesticide applicators by the National Institutes of Health found “did not find clear evidence for an association between dicamba exposure and cancer risk”, and a later examination of the same data set by Health Canada similarly found that dicamba was not “associated with a significant increase in overall cancer incidence.”
People and animals would need to consume large amounts of dicamba for there to be health effects, and dicamba is excreted quickly. “Dicamba is moderately toxic by ingestion and slightly toxic by inhalation or dermal exposure.” However, fish may be more susceptible to harm. A summary of human health impacts of dicamba can be found on the National Library of Medicine’s TOXNET, but keep in mind that 1) studies on isolated cells or in animals are not necessarily predictive of human health impacts and 2) some of the information provided is for other pesticides.
As is typical for pesticides, there may be impacts to human health and the environment due to inactive ingredients, such as surfactants. Pesticides should always be applied according to the label – such as with proper protective gear and away from waterways.

Dicamba is drifting

First, what is drift? Pesticides are formulated and labels instruct application in ways that keep the pesticide where it was applied. Still, drift is always a possibility when using any pesticide, on the farm or at home. As described by the National Pesticide Information Center:

Pesticide drift is the airborne movement of pesticides from an area of application to any unintended site. Drift can happen during pesticide application, when droplets or dust travel away from the target site. It can also happen after the application, when some chemicals become vapors that can move off-site. Pesticide drift can cause accidental exposure to people, animals, plants and property.

Drift often happens when there is an unexpected weather change during or shortly after pesticide application. Farmers generally follow pesticide labels (the Label is the Law) and use pesticides in a way that minimizes drift (drift = wasted pesticide = wasted money), so drift happens at a fairly constant, low level.
Dicamba has been in use since its registration in 1967. Farmers used dicamba with minimal incidents all the way up to 2016. Then, in 2016, 2017, and 2018, there were manymany incidents of reported dicamba drift onto non-tolerant crops, and the problem is likely much larger than reported. In Nebraska for example, a survey of farmers found that 51% of respondents had dicamba injury on non-dicamba tolerant soybean, but only 7% of those filed an official complaint with the Nebraska Department of Agriculture.
The damage isn’t just to soybean. Drift from dicamba is impacting wild plants, too. Beekeepers in some areas have reported that dicamba reduced pollen and nectar sources for their bees. Dicamba has also caused damage to small farmers growing speciality crops and to homeowner’s landscaping. People may not report damage because they want to stay friendly with neighbors. But drift keeps happening, harming relationships in addition to the plants, including one dicamba-motivated murder.
The financial damage to soybean at least may be limited. A recent meta-analysis looked at soybean response to dicamba drift and found that while visible damage could be caused by very low amounts of dicamba, visible damage isn’t necessarily an indicator of yield loss. Dicamba injury is easy to see in soybean; it causes characteristic cupping of leaves, as shown in the image below.
When there is financial damage, farmers have a few options. Initially, crop insurance did not cover dicamba drift, but rules have changed: farmers should check with their insurance agent immediately upon discovering damage. Farmers might use liability insurance to help recover losses, but insurance companies will not cover off-label (illegal) applications. Neighbors also have the option of negotiating with each other out of court, or can pursue a civil lawsuit, all typical options for chemical trespass. Of course, the first step is to document the damage.

soybean leaves affected by dicamba drift
Healthy soybean plants (left) have broad, flat leaves. Soybeans injured by dicamba drift (right) have characteristic cupped leaves. Healthy soybean image by United Soybean Board, injured soybean image by the University of Arkansas, both via Flickr.

Why is dicamba drifting more since 2016 compared to previous years? Coincidentally, the USDA approved dicamba tolerant soybean and cotton in 2015, and farmers began planting in 2016. The story is far more complicated than that, though.
For a timeline of how dicamba drift became a problem starting in 2015 through 2018, and for a preview of what might happen in 2019, see Dicamba Drift – Part 2. In Part 3, I’ll discuss some of the specific issues associated with applying dicamba and what might be contributing to drift. Part 4 wraps up the series by considering the response of the dicamba manufacturers and potential social impacts this whole debacle might have moving forward.

Written by Anastasia Bodnar

Anastasia Bodnar is a science communicator and science policy expert with a PhD in plant genetics and sustainable agriculture from Iowa State University. Anastasia has had various risk analysis roles in US government and military service. She serves as BFI's Director of Policy and as Co-Executive Editor of the Biofortified Blog.

14 comments

    1. Hi Ron, I understand your concern. No one wants pesticides on their food. Thankfully, the data shows we generally don’t need to be concerned about dicamba and our food, or generally about pesticide residues at all. The USDA and FDA’s results are reassuring – I invite you to read their results for yourself, and feel free to ask here if you have any questions.
      That said, there could be additional diamba residues from drift as described in this post. Those plants might have noticeable damage, depending on what the growing stage was when they were exposed to dicamba, but they might not. So there could be some produce going to markets or being used in people’s homes from kitchen gardens with additional dicamba residues. That’s not ok, and it is an additional consequence of the drift problem that I honestly hadn’t thought about – so thank you for making me think about it. I might add this to Part 2.
      While pesticide drift is never a good thing, at least we know from the toxicity data (again that’s summarized at TOXNET https://toxnet.nlm.nih.gov/cgi-bin/sis/search2/r?dbs+hsdb:@term+@DOCNO+311) that someone has to consume fairly large amounts of dicamba for there to be any harm. It seems unlikely that there will be unintended health impacts resulting from people consuming produce that had been exposed to dicamba drift, but I would like to look into this further.
      Note: FDA does not approve pesticides, EPA does.

  1. I checked the USDA’s Pesticide Data Program to see how much diamba residues are found on food. For all program years (194-2016) there were 12,894 tests for dicamba residues on different fruits, vegetables, grains, and milk. All had a result of 0. Search here: https://www.ams.usda.gov/datasets/pdp
    The FDA also has a pesticide residue monitoring program. It’s harder to search across years so I just looked at the 2016 data (which is relevant because it might tell us if the 2016 drift had an impact on residues). Of the 2,670 food samples tested, 11 had dicamba residues, and all were below the established tolerance. Find that information here: https://www.fda.gov/Food/FoodborneIllnessContaminants/Pesticides/ucm2006797.htm
    The USDA did have 11 samples of water (5 untreated, 3 ground, and 3 finished) with positive dicamba tests. Those water samples were part of a 2001-2013 drinking water project. Learn more here: https://www.ams.usda.gov/datasets/pdp/pdp-drinking-water-project

    1. The problem with dicamba drift is not that it is likely to leave residues in food. The amount drifting will mostly be too low to leave detectable residues.
      The real issue is that a lot of crop species are very sensitive to dicamba and the symptoms are very obvious, even if there is no yield loss.
      There are a number of factors that have contributed to the problem and in my opinion they were all forseeable.
      1. The crop was sold to farmers before a herbicide was registered for use. This meant that farmers used dicamba off label in the first year. While Monsanto has been correct that this is illegal and farmers should not have done it, it was easily predictable that farmers would do it.
      2. Farmers often understand that you should not spray pesticides when it is too windy, what many don’t fully grasp is that spraying under still conditions can be problematic due to inversion drift. Because there has been so much focus on wind drift, farmers (and indeed others in the industry) tend to think it is OK to spray in still wind conditions.
      3. Dicamba formulations volatilize. Monsanto and BASF did a lot of good work in developing low volatility dicamba formulations. The trouble has arisen that while immediately volatility is reduced, formulations cannot change the fact that the dicamba acid is volatile. With only a few users in the environment, the volatility of the dicamba acid may not have mattered too much, but when you have lots of users the amount volatilizing into the atmosphere can become a problem.
      4. Location and environment where users adopted the technology. Dicamba resistance was developed primarily for the management of glyphosate resistant Palmer amaranth and waterhemp. The mid-south was clearly going to be dicamba central. Volatilization is going to be highest in warm and humid conditions, which are common in these states during the summer. The trouble with the acid volatilizing is that conditions for spraying can be ideal on the day, but later inversion conditions can develop that will trap the volatilized herbicide on the inversion layer.
      This has resulted in an almost perfect storm. Farmers became used to using off label dicamba formulations that were more volatile, and I would be confident some of this is still occuring. Large areas were sown where Palmer amaranth and waterhemp were glyphosate resistant and environmental conditions during summer are ideal for maximizing volatility. Monsanto has tended to blame farmers for the problem, but in my view, Monsanto should share the blame as these problems were foreseeable.

      1. Hi Chris, these are all great points. I address all of these and more in Dicamba Drift – Part 2, as I mentioned in the last paragraph of this post. I had it all in one post but it ended up being more than 1500 words. Better to have a shorter post for those who just want the basics, and then a detailed post for those who want all the gory details.
        While it is unlikely that dicamba drift presents a human health risk in the form of residues on food, it’s reasonable that consumers might worry, as Ron here demonstrated. Therefore, it is worth looking into and providing data for.

      2. Been following this a bit. Do you think the manufacturers have an legal/financial liability in this? I don’t have an opinion at this point. You are clearly more educated on this topic and I would like your opinion.

        1. Hi Tom, after all the research I’ve done for this series, I do think the manufacturers bear responsibility for the problems that occured. They were warned, and all of this was preventable. Their response (which I discuss in part 4, coming soon) has poor to say the least. As for legal and financial responsibility, that’s for the courts and insurance companies to sort out. So far, I haven’t found examples of successful suits or insurance claims against the manufacturers.

        2. I guess there will be some lawsuits and I am by no means an expert on the vagaries of US civil cases. I expect that Monsanto/Bayer will offer the following arguments:
          1. They developed and specifically registered a low volatility dicamba product for use with their crops.
          2. They cannot be responsible for farmers using off-label products.
          3. They cannot be responsible be responsible for farmers applying their products under unsuitable conditions when this is prohibited on the labels.
          So finding them legally or financially liable is going to be hard.
          I don’t think that absolves them from the moral responsibility of not managing the introduction of the technology better and having better systems in place to deal with the predictable problems. It certainly hasn’t stopped me holding their feet to the fire over this. If this problem continues, I see a probable outcome of the de-registration of dicamba, which would be very unfortunate for growers who were using it for other purposes.
          However, more than anything else, this will be just one more emptying of the limited well of social license to use new technologies in agriculture. The world simply cannot afford this to taint the potential introduction of new agricultural technologies.

        3. I really don’t see a successful class action suit out of this. The product manufacturers obtained proper EPA registration and the testing data for the dicamba formulation looked to be sound. The dicamba formulations have had their initial registrations extended for 2 more years with some additional application restrictions.
          That doesn’t mean some lowlife law firm won’t try, though.

      3. “Dicamba formulations volatilize. Monsanto and BASF did a lot of good
        work in developing low volatility dicamba formulations. The trouble has
        arisen that while immediately volatility is reduced, formulations cannot
        change the fact that the dicamba acid is volatile.”
        This is an important point. I think everyone involved was not sufficiently cognizant of the potential for re-volatilization of the newer dicamba formulations.
        “Monsanto has tended to blame farmers for the problem, but in my view,
        Monsanto should share the blame as these problems were foreseeable.”
        That was a predictable response from Monsanto which did bear some element of truth, but what I found disturbing was Monsanto’s treatment of several university weed scientists like Dr. Baldwin of U. of Arkansas and Dr. Norsworthy of U. of Arkansas – that was really rotten behavior and significantly damaged Monsanto’s reputation (deservedly so, in my opinion) among university weed scientists and heads-up farmers.

  2. They should ban Dicamba until they figure out a way to keep it from volatilizing. I don’t see how a chemical gets approved when it can cause so much damage to surrounding crops. Some of the most damage I’ve seen is coming from Status sprayed on corn. 2 years ago there was a field of corn that was sprayed and it cupped the leaves of 5 bean fields around it. I’ve been told if I don’t want my beans cupped I should buy extend beans. Isn’t that extortion? Why should I have to buy protection from a product I don’t use? There were a lot of beans in my area that didn’t come out of the ground very well. They found out that the seed fields from the previous year had Dicamba damage and it carried through to the seed. The farmers and seed companies suffered financial damage because of it. I had a field this year that was drifted from all four sides from corn and bean products. The plants stayed stunted and didn’t canopy. It makes it hard to control weeds when they don’t canopy. I saw damage in vineyards, trees, and gardens. One vineyard lost most of his flowers and didn’t get much of a crop. One field of corn broke off 85% in a windstorm shortly after it was sprayed with Status. Farmers give me the excuse that they have to use Dicamba because nothing else controls their weeds. And you shouldn’t worry about drift because thats just part of farming. I’m sorry but I don’t think I should be damaged because of someones else’s problems. Everyone ran to RR years ago as the savior to weeds, until the weeds became resistant. Now we are doing the same with Dicamba, and we are already seeing resistant weeds. I guess the next answer is 2-4D resistant crops. We know where that will end up, and we don’t need another volatile product used. Dicamba use is exponential. This is heading for a huge train wreck. I had fields drifted with 2-4D and the coops insurance company said that I would have to prove damage in court. I suppose that they are getting so many law suits that they figure having to take them to court would weed some out. It use to be that a coop would calculate the damage and pay you for it. Crop rotation and cultivation do a lot more to control weeds then relying on GMO’s and chemicals.

  3. Dicamba is sold as Banvel, Clarity, DiFlexx, Engenia, XtendiMax, and other name brands. On commoditag it is sold as Dicamba Max. Dicamba selectively controls broadleaf weeds (such as dandelion and ragweed), but grasses (including crops like corn and rice) are naturally resistant.

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