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.

Follow Anastasia Bodnar:
Anastasia is Policy Director of Biology Fortified, Inc. and the Co-Executive Editor of the Biofortified Blog. She has a PhD in genetics with a minor in sustainable agriculture from Iowa State University. Her favorite produce is artichokes!
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