The environmental impact of the world’s most famous weed killer

[This post was originally published in Envirobites]

Weed killers are sprayed (see figures 1 and 2) to control land weeds on agricultural fields, residential lawns and golf courses. They are also sprayed on water bodies to control aquatic weeds. One of these weed killers is an herbicide called glyphosate. An ecological assessment of glyphosate, the active ingredient in RoundUp and the world’s most heavily used agrochemical, was carried out by the Environmental Protection Agency (EPA). As spraying can cause a compound to drift and move off-site, it is possible for multiple different species (that is, any animals or plants that are in the surrounding areas) to get exposed to glyphosate.

USDAaerialSM
Figure 1: Example of how an aerial pesticide application is done. Aerial applications cause greater off-site drift of pesticide compared to ground applications (see figure 2). Photo source
tda_pesticide
Figure 2: Example of how a ground pesticide application is done. Photo source

Glyphosate acts by interfering with an enzyme that is necessary to make some amino acids. This leads to cell death in plants. Agricultural crops are often made glyphosate-resistant, meaning the enzyme is genetically modified so that the herbicide cannot act on it.

In reports recently released, EPA evaluated the effect of glyphosate on several plants and animals, including humans. They used data from published papers and those that were provided by the glyphosate manufacturer. They also relied on computer models to make predictions on how the herbicide would spread out and move in the environment. They looked at the active ingredient alone (glyphosate) as well as different formulated products (glyphosate plus surfactants). Exposure to glyphosate can occur when it touches your skin, when you eat plants that had been sprayed and when you inhale vapors of the product. Multiple toxic effects were studied, particularly lethality and reduced growth.

Risk to an organism is measured by finding the concentration of glyphosate or the formulated product that the organism is getting exposed to and comparing it with the concentration that causes any toxic effect on the organism. An organism’s risk to glyphosate is considered low when the exposure concentration is lesser than the toxicity concentration. An organism is considered affected if the exposure concentration is similar or higher to the toxicity concentration. Think of it like household cleaning products. You know that you may end up in the hospital if you drink a bottle of the product, but most people will be fine if they breathe in some vapors upon entering a freshly cleaned bathroom. The typical exposure (breathing in some vapors) is lower than the toxic amount (drinking a bottle of solution).

Impact on aquatic organisms

The EPA researchers studied whether plants and animals that live in the water are impacted. Glyphosate can enter water bodies either from direct application or due to spray drifts and runoffs from land applications. The risks to fish (freshwater and marine) and aquatic invertebrates from short-term and long-term glyphosate exposure is low.

While spray drifts and runoffs are unlikely to pose a risk to submerged aquatic plants and mosses, plants with leaves and stems rising above water could be adversely affected. Also mosses could be killed from direct aquatic application of a formulated product.

Impact on terrestrial organisms (excluding humans)

The impact on species living on land was also studied. Risks to birds from short-term exposure to glyphosate alone is low. Long-term exposure could decrease body weight of birds, but the eggs and the chicks inside were not harmed. However a study showed that one type of formulated product could affect the survival of birds.

Terrestrial invertebrates, including honeybees, were generally unaffected at lower glyphosate application rates but it is unknown if they would be affected at higher application rates as no toxicity studies were carried out at higher doses. However, a study showed that survival of predatory mites could be affected.

Generally, risks from exposure are low for large mammals but small mammals consuming short grass could be affected. One glyphosate containing product was shown to impact how successfully mammals reproduce.

Terrestrial plants adjacent to treated fields could be adversely affected (i.e., there could be a decrease in survival and/or growth). This means that an organic farm next to one that sprays could have fewer or smaller crops to sell.

There is no evidence that glyphosate interacts with hormones in mammals or non-mammalian wildlife.

Impact on humans

In order to assess the impact on humans, studies on animals like rats, mice and rabbits were done and the results were extrapolated. Safety factors to account for differences between species were added where necessary. And while exposure to the herbicide through skin contact and breathing are also likely, risks from these routes were not assessed. Since humans often eat crops that have been sprayed with glyphosate, the exposure from eating it were studied.

A dietary exposure study showed reduction in body weight and also eye, liver and kidney toxicity at or above the limit dose (defined as the maximum dose of pesticide that is toxicologically acceptable for humans). No toxicity effects were observed on the nervous and immune systems.

Impacts on pregnant mothers and fetuses were only seen at or above the limit dose. Also, no effects were seen on children who had been exposed while they were in the womb. Glyphosate was not present in the milk samples analyzed from a few mothers.

The risk from combined exposure to glyphosate present in food, drinking water and housing is low.

Impact on threatened and endangered species

There are potential risks to some species that are already on the endangered species list, including birds, mammals, bugs and plants. These are incomplete assessments because the concentrations that cause toxicity to the listed species are not known yet.

Conclusion

There are a lot of unknowns and uncertainties surrounding the impacts of glyphosate. EPA will follow up with more complete assessments in the future. In the meantime, this provides us with a good indication of how glyphosate affects different organisms.

Source: EPA draft ecological and human health risk assessments for glyphosate

https://www.epa.gov/sites/production/files/2017-12/documents/registration_review_preliminary_ecological_risk_assessment_for_glyphosate.pdf

https://www.epa.gov/sites/production/files/2017-12/documents/glyphosate_draft_human_health_risk_assessment_for_registration_review.pdf

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