Glyphosate damage to creeping oxalis.
The Loam Ranger – Glyphosate
Dear Loam Ranger,
If I think someone’s been spraying glyphosate around my property, how can I prove it?
Glyphosate is the most common herbicide that we at SESL see used to kill trees, and often features in neighbourhood disputes. SESL offers a service for the detection of glyphosate in soil, plant tissues and water. This article provides a background to glyphosate and explains how to collect samples for analysis.
How it works
Glyphosate, the most widely used herbicide these days, was discovered in 1970 at Monsanto. It is derived from the amino acid glycine, and has the chemical name N-(phosphonomethyl) glycine, from which its common name is derived (gly + phos). It is quickly absorbed through plant tissues and, being highly soluble in water (12 g/L), is quickly translocated through the plant to the growing points, where it blocks the synthesis of three amino acids, phenylalanine, tyrosine and tryptophan. Without these critical amino acids, the plant cannot grow or conduct a number of essential biochemical processes.
Glyphosate is a broad-spectrum, non-selective systemic herbicide used for the control of most types of plants. Here is Australia it is most familiar under the trade names Roundup and Zero.
Glyphosate works only when applied to the green parts of plants; the presence of bark blocks its absorption. It works only when plants are actively growing – typically during warmer weather – as it has to be translocated to the actively growing tissues.
Toxicology
When used according to the label instructions, glyphosate poses little risk of toxicity to humans and other animals. It is necessary to swallow over 85 mL of the concentrate before toxic effects are seen. Numerous tests and studies have shown that glyphosate is safe when used correctly. No long-term effects have been seen, and glyphosate does not accumulate in the body. However, some formulations are toxic to fish on account of the surfactants used in them.
Animals (including humans) don't use the same biochemical pathway as plants, which is why glyphosate is much less toxic to us.
Breakdown in soil
Glyphosate has an average half-life of 47 days in soil, meaning that it can take 6 months for >99% of it to break down. However, it is strongly adsorbed to most soils, and thus does not leach or run off appreciably. Soil microorganisms then break it down. It has a comparable fate in water.
Plant resistance
As with any chemical, repeated use can promote resistance. The first known resistance appeared in an orchard in Orange, NSW, in 1996, in a population of ryegrass that had been sprayed two or three times a year for 15 years. Since then, resistance has appeared in other species around the world. Lessons learned from other examples of resistance, such as bacterial resistance to antibiotics, show that it is necessary to rotate or combine unrelated chemicals to prevent the appearance of resistance to glyphosate.
Genetic engineering for crop resistance
Genes for resistance, taken from the soil bacterium Agrobacterium tumefaciens, have been deliberately introduced into crop cultivars to make them resistant to glyphosate so growers can spray their crops safely. This allows selective growth of the crop, decreased use of herbicides, improved soil quality and therefore better crop rotations.
These genes are patented and the cultivars are protected by plant variety rights, and growers are forbidden to save seed from the previous crop. Although this suits many growers, plants don’t respect boundaries, and the resistance genes can and do move into neighbouring crops in pollen and internationally in grain shipments. Percy Schmeiser, a canola farmer in Canada, learned this to his cost when he was prosecuted for growing glyphosate-resistant canola because the resistance genes drifted onto his farm in pollen from a neighbour’s crop. Japanese researchers have found glyphosate-resistant canola growing in ports and along the sides of highways leaving the ports, yet this canola has never been grown in Japan.
Signs of glyphosate spraying
Within the first week after application, plants generally show no signs of anything, although growth stops immediately. Then within the second week, leaves start to yellow or bleach and distort. By the end of the second week, plants are dead.
Collecting samples for analysis
If you suspect that glyphosate has been used without your permission, don’t delay. Because glyphosate is quickly adsorbed onto soil, the best samples are leaves. Simply collect leaves that are showing signs of stress but are still alive; dead leaves are not suitable for analysis. Collect 200 g of leaves and place them in a paper bag to limit decay during transport to the lab. If the grass or groundcover plants around the targeted plant are also showing signs of decline and these signs have been picked up quickly, then soil analysis may also be appropriate. Provide 250 g of soil in an air-tight plastic bag or container.
If you suspect that water has been contaminated as well, send us 200 mL in a clean sealed jar.
Keep all samples in the fridge (4 °C) until they are sent to the lab. Because of the complexities of the test, allow 10 to 14 days for the results.
Further reading
Extoxnet: http://extoxnet.orst.edu/pips/glyphosa.htm
Wikipedia: http://en.wikipedia.org/wiki/Glyphosate
Monsanto vs Schmeiser: http://www.percyschmeiser.com/
Saji H. et al. 2005. Monitoring the escape of transgenic oilseed rape around Japanese ports and roadsides. Environ. Biosafety Res. 4: 217–222.