5.6 Myth: Roundup is a benign herbicide that makes life easier for farmers

Truth: Roundup causes soil and plant problems that negatively impact yield

GM Roundup Ready (RR) crops are marketed on the basis that Roundup is a safe herbicide that simplifies weed control and makes the farmer’s life easier. But recent studies show that Roundup and glyphosate can accumulate in plants, have negative effects on soil organisms, and harm the growth and health even of soy plants that are genetically engineered to tolerate it. These effects may be partly responsible for yield decline (see Myth 5.1) and disease outbreaks found in GM Roundup Ready soy and maize.

Glyphosate causes or exacerbates plant diseases

“When you spray glyphosate on a plant, it’s like giving it AIDS.”
– Michael McNeill, agronomist and farm consultant, Iowa¹

Manufacturers claim that glyphosate kills plants by inhibiting an enzyme necessary for plant growth. But research shows that glyphosate has another way of killing plants: it makes the plant more susceptible to disease, potentially leading to the plant’s death from the disease. Spraying glyphosate on a plant is, according to US agronomist Michael McNeill, “like giving it AIDS”.¹

One possible mechanism for this process is suggested in a study on GM RR soybeans. The study found that once glyphosate is applied to the plant, it accumulates in the plant tissues and then is released into soil through the roots. There, it stimulates the growth of certain fungi, notably Fusarium, a fungus that causes wilt disease and sudden death syndrome in soy plants.² Other studies confirm the link between glyphosate applications and increased infection of plants with Fusarium.^3,4,5,6,7,8

Fusarium is of especial concern because it does not only affect plants. It produces toxins that can enter the food chain and harm humans and livestock.⁹ In pigs, Fusarium-contaminated feed is a reproductive toxin¹⁰ and increases stillbirths.¹¹

Glyphosate has also been shown to increase the incidence and severity of other fungal diseases in plants, including take-all in wheat¹² and Corynespora root rot in soy.¹³

In an attempt to combat soil-borne diseases such as Fusarium, in 2011 Monsanto marketed its new Roundup Ready 2 Yield soy seed with a proprietary fungicide/insecticide coating.¹⁴ Such chemical treadmills are profitable for seed and chemical companies, but are costly to farmers and add to the toxic burden borne by humans, animals and the environment.

Glyphosate makes nutrients unavailable to plants

Glyphosate binds (chelates) vital nutrients such as iron, manganese, zinc, and boron in the soil, preventing plants from taking them up.^15,16,17,18 So GM soy plants treated with glyphosate have lower levels of essential nutrients and reduced growth, compared with GM and non-GM soy controls not treated with glyphosate.^19,20 Lower nutrient uptake may partly account for the increased susceptibility of GM soy to disease,² as well as its lower yield.³ It could also have implications for humans and animals that eat the crop, as it is less nutritious.

Glyphosate impairs nitrogen fixation

The yield decline in GM RR soy may be partly due to glyphosate’s negative impact on nitrogen fixation, a process that is vital to plant growth and depends on the beneficial relationship between the soy plants and nitrogen-fixing bacteria. In a greenhouse study of young RR soy plants, glyphosate delayed nitrogen fixation and reduced the growth of roots and sprouts, resulting in yield declines of up to 30%. In field studies, glyphosate had no such effect on GM RR soybean when there was adequate soil water throughout the growing season. However, glyphosate decreased biomass and seed yields in drought conditions.²¹ Other studies have also linked glyphosate with impaired nitrogen fixation in plants.^22,23

The mechanism of impaired nitrogen fixation in GM RR soybeans may be explained by a field study finding that glyphosate enters the root nodules and negatively affects the soil bacteria responsible for nitrogen fixation. Glyphosate inhibits root development, reducing root nodule biomass by up to 28%. It also reduces by up to 10% an oxygen-carrying protein, leghemoglobin, which helps bind nitrogen in soybean roots.²⁴

To counter such problems, GMO seed and agrochemical companies have begun to market a “techno-fix” in the form of nitrogen-fixing bacterial inoculants, which are either applied to soy seed before sale or to the soil after sowing. The companies claim that this will increase yield potential.²⁵ However, a soybean inoculant evaluation trial conducted in Iowa concluded, “none of the inoculants resulted in a significant yield increase over the non-inoculated plots”.²⁶ Inevitably, the costs of such treatments are borne by farmers.

Conclusion

Roundup and other glyphosate herbicides are not benign but have negative effects on soil and crops, some of which impact plant health and yield. Glyphosate’s link with Fusarium infection is especially serious as this fungus can harm humans and livestock. Seed and agrochemical companies are marketing various “techno-fixes” to address these problems, tying farmers to a chemical treadmill.

References

1. Dodge J. Expert: GMOs to blame for problems in plants, animals. Boulder Weekly. http://www.boulderweekly.com/article-6211-expert-gmos-to-blame-for-problems-in-plants-animals.html. Published August 11, 2011.
2. Kremer RJ. Glyphosate affects soybean root exudation and rhizosphere microorganisms. Int J Anal Environ Chem. 2005;85:1165–1174.
3. Johal GS. Glyphosate effects on diseases of plants. Eur J Agron. 2009;31:144–152.
4. Johal GS, Rahe JE. Effect of soilborne plant-pathogenic fungi on the herbicidal action of glyphosate on bean seedlings. Phytopathology. 1984;74:950-955.
5. Sanogo S, Yang XB, Scherm H. Effects of herbicides on Fusarium solani f. sp. glycines and development of sudden death syndrome in glyphosate-tolerant soybean. Phytopathology. 2000;90:57-66. doi:10.1094/PHYTO.2000.90.1.57.
6. University of Missouri. MU researchers find fungi buildup in glyphosate-treated soybean field. 2000. Available at: http://web.archive.org/web/20130625073438/http://www.biotech-info.net/fungi_buildup.html.
7. Kremer RJ, Means NE. Glyphosate and glyphosate-resistant crop interactions with rhizosphere microorganisms. Eur J Agron. 2009;31:153–161.
8. Fernandez MR, Zentner RP, Basnyat P, Gehl D, Selles F, Huber D. Glyphosate associations with cereal diseases caused by Fusarium spp. in the Canadian prairies. Eur J Agron. 2009;31:133–143.
9. Food Standards Agency (UK). About mycotoxins. 2013. Available at: http://www.food.gov.uk/policy-advice/mycotoxins/about/#Fusariumtoxins.
10. Alm H, Brussow KP, Torner H, et al. Influence of Fusarium-toxin contaminated feed on initial quality and meiotic competence of gilt oocytes. Reprod Toxicol. 2006;22(1):44-50. doi:10.1016/j.reprotox.2005.11.008.
11. Diaz-Llano G, Smith TK. Effects of feeding grains naturally contaminated with Fusarium mycotoxins with and without a polymeric glucomannan mycotoxin adsorbent on reproductive performance and serum chemistry of pregnant gilts. J Anim Sci. 2006;84(9):2361-6. doi:10.2527/jas.2005-699.
12. Huber DM. Managing nutrition to control plant disease. Landbauforsch Volkenrode. 2007;57(4):313–322.
13. Huber DM. Association of severe Corynespora root rot of soybean with glyphosate-killed giant ragweed. Phytopathology. 2005;95:S45.
14. Monsanto. Get soybean and corn crops off to a good start in 2011 with Acceleron® seed treatment products. http://bit.ly/1ndlWoq. Published February 7, 2011.
15. Cakmak I, Yazici A, Tutus Y, Ozturk L. Glyphosate reduced seed and leaf concentrations of calcium, manganese, magnesium, and iron in non-glyphosate resistant soybean. Eur J Agron. 2009;31:114–119.
16. Neumann G, Kohls S, Landsberg E, Stock-Oliveira Souza K, Yamada T, Romheld V. Relevance of glyphosate transfer to non-target plants via the rhizosphere. J Plant Dis Prot. 2006;20:963–969.
17. Huber DM. What about glyphosate-induced manganese deficiency? Fluid J. 2007:20–22.
18. Bott S, Tesfamariam T, Candan H, Cakmak I, Römheld V, Neumann G. Glyphosate-induced impairment of plant growth and micronutrient status in glyphosate-resistant soybean (Glycine max L.). Plant Soil. 2008;312(1-2):185-194. doi:10.1007/s11104-008-9760-8.
19. Zobiole LH, de Oliveira RS, Visentainer JV, Kremer RJ, Bellaloui N, Yamada T. Glyphosate affects seed composition in glyphosate-resistant soybean. J Agric Food Chem. 2010;58:4517-22. doi:10.1021/jf904342t.
20. Zobiole LHS, de Oliveira RS, Huber DM, et al. Glyphosate reduces shoot concentrations of mineral nutrients in glyphosate-resistant soybeans. Plant Soil. 2010;328:57–69.
21. King CA, Purcell LC, Vories ED. Plant growth and nitrogenase activity of glyphosate-tolerant soybean in response to foliar glyphosate applications. Agron J. 2001;93:179–186.
22. De María N, Becerril JM, García-Plazaola JI, Hernandez A, De Felipe MR, Fernandez-Pascual M. New insights on glyphosate mode of action in nodular metabolism: Role of shikimate accumulation. J Agric Food Chem. 2006;54(7):2621-2628. doi:10.1021/jf058166c.
23. Bellaloui N, Reddy KN, Zablotowicz RM, Mengistu A. Simulated glyphosate drift influences nitrate assimilation and nitrogen fixation in non-glyphosate-resistant soybean. J Agric Food Chem. 2006;54(9):3357-3364. doi:10.1021/jf053198l.
24. Reddy KN, Zablotowicz RM. Glyphosate-resistant soybean response to various salts of glyphosate and glyphosate accumulation in soybean nodules. Weed Sci. 2003;51:496–502.
25. Dekalb. Increase soybean yield potential with inoculants, protect with seed treatments. 2010. Available at: http://www.dekalb.ca/_uploads/documents/Agronomic_Information/SoybeanArchive/increase_soybean_yield.pdf.
26. Iowa State University Soybean Extension and Research Program. Seed inoculation. 2007. Available at: http://extension.agron.iastate.edu/soybean/production_seedinoc.html.