Glyphosate Herbicide

Glyphosate

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Glyphosate is the most widely used herbicide active ingredients in the United States and throughout the world. It is the active ingredient in Roundup^® herbicide and is also sold under other product and premix names such as Rodeo^®, Durango^®, and Accord^®.

Glyphosate controls a variety of annual, biennial, and perennial broadleaf and grass weeds in agriculture, forestry, rights-of-way, non-crop areas, parks, and other sites. It is also popular among homeowners for controlling weeds in landscaping, patios, driveways, around trees, and along fences. Additionally, select products can be applied in aquatic environments to control unwanted plants on the edges of ponds and streams. Glyphosate is a foliar herbicide and lacks soil activity.

Over 20 million pounds of glyphosate were sold in Minnesota in 2017.¹ Agriculture accounts for the bulk of glyphosate use in Minnesota, particularly applications on glyphosate-tolerant (e.g., Roundup-Ready^®) corn and soybean. According to a 2013 USDA:NASS Survey, glyphosate was applied to 85% of corn acres, and 92% of soybean acres in the state.² It is also used in wheat, hay, and sugar beet production.

Mode of Action

Glyphosate controls weeds by inhibiting the function of the 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase enzyme, which is involved in the production of certain amino acids needed for protein production. Because EPSP synthase is found in all plants, glyphosate has the potential to control most actively growing broadleaf and grass plants including crops. For this reason, early glyphosate use was limited to application for total vegetation control prior to crop emergence, directed foliar treatments where desired plants could be avoided, and noncrop areas.

Spray adjuvants are commonly included in glyphosate spray solutions to improve its activity. Nonionic surfactants can be added to reduce spray droplet surface tension and allow them to spread across the leaf surface, thereby improving absorption. Ammonium sulfate is commonly added to glyphosate spray solutions to overcome the impact of hard water. When mixed with hard water (i.e., water high in dissolved minerals such as calcium and magnesium), glyphosate can bind with certain cations and lose its efficacy.

Glyphosate Tolerant Crops

In 1996, glyphosate tolerant crops developed using genetic modification techniques were introduced. This allowed glyphosate to be applied over the crop to selectively control weeds without injuring the crop. This resulted in a large increase in glyphosate use in the state. Minnesota crops that have glyphosate-tolerant varieties include corn, soybean, canola, sugar beet, and alfalfa.

Weed Resistance

Herbicide resistance is the inherited ability of a plant, such as a weed, to survive an herbicide application that the original population was susceptible to. The development of resistance to herbicides such as glyphosate is a growing concern for weed management because it can lead to the loss of herbicide options, which can have important economic and environmental consequences.³

Over time, the widespread and repeated use of glyphosate without diversified weed management has increased the selection for resistant weed species.⁴ In Minnesota, weed species with confirmed glyphosate resistance include common ragweed, giant ragweed, and tall waterhemp. Glyphosate-resistant kochia, palmer amaranth, and horseweed have also been found in neighboring states or provinces.⁵ To prevent the development of resistant weeds, practices such as rotating herbicides with different sites-of-action and using mechanical weed control are promoted.³

Glyphosate Movement in the Environment

Glyphosate binds tightly to soil, which limits its mobility in the environment. Glyphosate is not expected to reach groundwater; however, it can reach surface water via spray drift during application or through the transport of residues bound to eroded soil particles. It may also enter surface waters from direct application to control aquatic weeds. In soil, microbes readily break down glyphosate; however microbial degradation is limited under anaerobic conditions (i.e., when no oxygen is present).⁶

Detection in Minnesota Waters

The MDA routinely monitors surface water and groundwater for glyphosate and its main breakdown product, aminomethylphosphonic acid (AMPA). In Minnesota, glyphosate has been detected in surface water, but not in groundwater; AMPA has not been detected in either. Method reporting limits (MRLs) for glyphosate and AMPA are 1,020 ng/L and 5,090 ng/L, respectively.

In 2019, glyphosate was detected in 15% of storm flow river and stream samples (n=54), and 3% of base flow samples (n=38)7. The maximum glyphosate concentration (5,880 ng/L) detected was less than 1% of the lowest water quality reference value (11,900,000 ng/L). Neither glyphosate nor AMPA were detected in any of the routine groundwater samples (n=180) or rainfall samples (n=5) during the 2019 monitoring season.

Glyphosate and Non-target Organisms

Aquatic: Glyphosate toxicity to fish and aquatic invertebrates is low; however, the presence of other ingredients in some products formulated for terrestrial use can increase their toxicity compared to pure glyphosate. Only glyphosate products labeled and specially formulated for aquatic systems can be used in these environments. Glyphosate is effective only on shoreline, emergent, and rooted floating plants. It is less effective on floating mat vegetation and does not control submerged plants.⁴

Terrestrial: Glyphosate is slightly acutely toxic to birds and mammals and is practically non-toxic to honeybees.⁶

Glyphosate and Human Health

Glyphosate has a relatively low acute toxicity by oral, dermal, and inhalation exposure routes, and the EPA has classified it as “not likely to be carcinogenic to humans.”⁸ In 2015, the International Agency for Research on Cancer (IARC) classified glyphosate as “a probably carcinogen;” however, the EPA and Minnesota Department of Health (MDH) do not consider glyphosate to be carcinogenic at concentrations people are likely to be exposed to in the environment. Furthermore, MDH non-cancer guidance values are considered to be protective if it were to be carcinogenic at very high doses.⁹ The MDH guidance value for glyphosate in drinking water is 500,000 ng/L.¹⁰, and the EPA Maximum Contaminant Level for glyphosate in drinking water is set at 700,000 ng/L.¹⁰ Additional information about glyphosate’s health effects can be found on the MDH website.

References

¹ Minnesota Department of Agriculture. 2017. Pesticide Sales Database.

² Minnesota Department of Agriculture, USDA: NASS Minnesota Field Office, 2016. 2013 Pesticide Usage on Four Major Crops in Minnesota. Accessed May 7, 2019.

³ University of Minnesota Extension, 2018. Herbicide-Resistant Weeds. Accessed March 31, 2020.

⁴ Boerboom, C., Owen, M. Facts About Glyphosate Resistant Weeds. Purdue Extension. Accessed March 31, 2020.

⁵ Heap, I.  The International Herbicide-Resistant Weed Database. Accessed March 30, 2020.

⁶ US Environmental Protection Agency, 2015. Registration Review- Preliminary Ecological Risk Assessment for Glyphosate and Its Salts (EPA-HQ-OPP-2009-0361-0077).  Accessed May 7, 2019.

⁷ Minnesota Department of Agriculture, 2020. 2019 Water Quality Monitoring Report. Accessed October 30, 2020.

⁸ US Environmental Protection Agency, 2017. Glyphosate Draft Human Health Risk Assessment for Registration Review (EPA-HQ-OPP-2009-0361-0068). Accessed May 7, 2019.

⁹ Minnesota Department of Health, 2017. Glyphosate and Drinking Water. Accessed May 7, 2019.

¹⁰ Minnesota Department of Health, 2020. Comparison of State Water Guidance and Federal Drinking Water Standards. Accessed October 30, 2020.