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Persistence of glyphosate in the environment

What is Glyphosate?


Glyphosate is a widely used and financially successful herbicide that has steered controversy between scientists, activists, and governments, due to its link to environmental and human health implications. But where does Glyphosate come from, and how does it work?


Originally, glyphosate was synthesized and patented as a metal binding agent by Swiss chemist Henry Martin in 1950. Although not initially used as a herbicide, it quickly became evident that glyphosate binds to manganese, which is essential to an enzyme necessary to the ‘shikimate biosynthetic pathway’. The inhibition of this pathway blocks the production of aromatic amino acids, which leads to several metabolic disturbances. In other words, glyphosate kills plants, and it is great at it!


In 1974, glyphosate was introduced into the market by Monsanto, and was used as a non-selective weed killer, including elimination of unwanted plants around power lines and train tracks, in fruit production for elimination of weeds between rows in orchards, and following crop harvest. During the 1990s, Monsanto introduced glyphosate-resistant "Roundup ready" crops, which meant that farmers could also apply the herbicide during crop growth. Because of their broad-spectrum weed control, these products quickly became the standard practice among farmers, and since the 90s are used in cropping systems throughout the world. To put this into perspective, in 2014, an estimated 747 million kg of agricultural applications of glyphosate were applied on the total of 1.4 billion hectares of cropland that exist worldwide. Accordingly, if this volume of glyphosate had been applied evenly, about 0.53 kg of glyphosate could have been sprayed on every hectare of cropland on the planet [1].


What is the problem then?


1) Herbicide resistant weeds

Because of its high effectiveness, farmers have increasingly relied on glyphosate spraying, which has led to the appearance of herbicide resistant weeds. In simple words, although glyphosate is effective on most weed species, there is always a minority of robust weed species that survive the herbicide sprayings and subsequently grow and spread, whereas sensitive weed species disappear [1]. This means that every year, harder and harder weed strains occupy the field, with higher difficulty of managing them.


Herbicide resistant weeds are not a new thing, and have been observed since the early years of synthetic herbicide development in the 1950’s and 1960’s. However, compared to then, the number of herbicide resistance cases has risen significantly (Fig. 1). Thirty-eight weed species have now evolved resistance to glyphosate, distributed across 37 countries and in 34 different crops [3]. Lolium rigidum is mentioned as the world’s worst herbicide-resistant weed (Fig. 2), present at 12 countries, within 9 cropping regimes that cover more than 2 million hectares; followed by Amaranthus palmeri, Conyza canadensis, Avena fatua, Amaranthus tuberculatus, and Echinochloa crusgalli [4]. In order to combat these stronger weeds, farmers tend to increase the frequency and intensity of glyphosate application, or to mix with other types of herbicides such as ALS (that inhibit acetolactate synthase), and paraquat. Sadly, the only thing achieved by intensifying the herbicide application, was to generate weed resistance for ALS, and paraquat [5].




Figure 1. Glyphosate-Resistant Italian Ryegrass infesting a cornfield at Louisiana, USA.




Figure 2. The increase in the number of herbicide resistant species in United States, Australia, Argentina, Brazil, and Canada, between the years 1995 - 2015. Graph taken from Heap and Duke (2018).



2) Human health and exposure to glyphosate

A multitude of studies have shown that exposure to glyphosate may be harmful to humans [6]. Exposure to glyphosate may lead to endocrine disruption, respiratory problems, severe birth defects among other issues. The concern has deepened since the World Health Organization’s International Agency for Research on Cancer (IARC) re-classified glyphosate as “probably carcinogenic to humans” [7].


One might expect that only farmers are affected by glyphosate, especially since they are the ones that use it. This is far from true. Individuals may be exposed to glyphosate through various routes such as food and drinking water, both in the occupational and environmental settings [8]. For example, the UK-Food Standard Agency residue testing conducted in October 2012 found glyphosate residues at, or above 0.2 mg/kg in 27 out of 109 samples of bread [9]. Testing by the US Department of Agriculture in 2011 revealed residues of glyphosate in 90.3% of 300 soybean samples [10].



3) Glyphosate spreads far and remains for a long time

Recent studies have found that 45% of Europe’s top soil contains glyphosate residues [11]. A report from the US Geological Survey that examined water and soil samples from 38 states collected from 2001–2010 found glyphosate residues to be widespread in the environment, especially in sediments, soils, precipitation, ditches, drains, rivers, and streams [12]. This phenomenon is due to the fact that glyphosate can transport in various ways (Fig. 3), and is very persistent in the environment.


Glyphosate can move large distances, due to: leaching into the groundwater table, aerial transport via droplets during spay application, transport of soil particles by wind-erosion or water-erosion, and soluble transport by runoff [11,13,14]. It all starts shortly after application. Glyphosate quickly sticks to the soil and degrades very slowly. Depending on physicochemical characteristics and climate, the average half-live of glyphosate in the soil ranges from 2 to 197 days [13]. Heavy leaching can occur under heavy rain, or when soils are sandy. After adsorption, these soil particles may further act as a vehicle, transporting the adsorbed glyphosate even further, via runoff, or via wind-erosion [11,13]. Eventually, these soil particles reach water (streams, lakes, the sea), where average half-life ranges from a few days to 91 days [15].



Figure 3. Glyphosate can disperse in a variety of ways, including via wind and water. Image taken and adapted from Silva et al. (2018).


Where do we go from here?


Green PRAXIS investigates alternatives to manage vegetation, including what is considered weeds, as well as ways of remediating surfaces that have been contaminated with glyphosate. Stay in touch for the next blog posts, which will talk about glyphosate and biodiversity, financial implications of glyphosate use, and alternatives for weed control.



References


1. Benbrook, C. M. Trends in glyphosate herbicide use in the United States and globally. Environ. Sci. Eur. 28, 1–15 (2016).

2. Schütte, G. et al. Herbicide resistance and biodiversity: agronomic and environmental aspects of genetically modified herbicide-resistant plants. Environmental Sciences Europe vol. 29 1–12 (2017).

3. Heap, I. & Duke, S. O. Overview of glyphosate-resistant weeds worldwide. Pest Management Science vol. 74 1040–1049 (2018).

4. Heap, I. Herbicide resistant weeds. in Integrated Pest Management: Pesticide Problems, Vol.3 281–301 (Springer, Dordrecht, 2014). doi:10.1007/978-94-007-7796-5_12.

5. Peterson, M. A., Collavo, A., Ovejero, R., Shivrain, V. & Walsh, M. J. The challenge of herbicide resistance around the world: a current summary. Pest Manag. Sci. 74, 2246–2259 (2018).

6. Myers, J. P. et al. Concerns over use of glyphosate-based herbicides and risks associated with exposures: A consensus statement. Environmental Health: A Global Access Science Source vol. 15 1–13 (2016).

7. Guyton, K. Z. et al. Carcinogenicity of tetrachlorvinphos, parathion, malathion, diazinon, and glyphosate. Lancet Oncol. 16, 490–491 (2015).

8. Gillezeau, C. et al. The evidence of human exposure to glyphosate: A review. Environmental Health: A Global Access Science Source vol. 18 1–14 (2019).

9. UDECoPRiF. (PRiF) UDECoPRiF: Monitoring program. http://www.food.gov.uk/business-industry/farmingfood/pesticides. https://www.food.gov.uk/business-guidance/pesticides-in-food.

10. USDA. Agricultural Marketing Service . Appendix C Distribution of Residues in Soybean by Pesticide. Washington, D.C: U.S. Department of Agriculture; 2013. Pesticide data program annual summary, program year 2011. (2013).

11. Silva, V. et al. Distribution of glyphosate and aminomethylphosphonic acid (AMPA) in agricultural topsoils of the European Union. Sci. Total Environ. 621, 1352–1359 (2018).

12. Battaglin, W. A., Meyer, M. T., Kuivila, K. M. & Dietze, J. E. Glyphosate and its degradation product AMPA occur frequently and widely in U.S. soils, surface water, groundwater, and precipitation. J. Am. Water Resour. Assoc. 50, 275–290 (2014).

13. Bento, C. P. M. et al. Glyphosate and AMPA distribution in wind-eroded sediment derived from loess soil. Environ. Pollut. 220, 1079–1089 (2017).

14. Vereecken, H. Mobility and leaching of glyphosate: A review. Pest Management Science vol. 61 1139–1151 (2005).

15. Rueppel, M. L., Brightwell, B. B., Schaefer, J. & Marvel, J. T. Metabolism and Degradation of Glyphosate in Soil and Water. J. Agric. Food Chem.25, 517–528 (1977).

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