You may have heard, or maybe not, that the EU are debating the continued licensing of glyphosate weed killers.  There are conflicting studies and arguments between various authorities about the safety of glyphosate based weed killers, the best known being Roundup by Monsanto.  Monsanto developed Roundup in the 70’s and until 2000 had a patent.  Now of course there are many similar formulations on the market.  Regardless of the arguments between scientists advising the World Health Organisation (WHO) and European Food Safety Authority (EFSA) about whether or not glyphosate can be classed as possibly carcinogenic in humans, I was concerned about ANY use of weed killers around produce I have to eat.

At recent General Committee meetings, I had reason to be discussing the affects of using Roundup on spare allotments to control weeds and the future use of the plots by tenants who wished to grow organically.  The mantra that was chorused by most of the committee was the usual.  ‘..it is perfectly safe, as soon as it touches the soil it breaks down and is harmless…’

When it was reported that a prospective tenant did NOT want their new plot sprayed, there was a condescending aside that ‘….people should learn their chemistry…’

Not wanting to let this go and realising that most allotmenteers do not have the time to trawl through scientific papers and studies to find out the details (assuming of course that they can understand the underlying terminology, science and analysis, required) I decided to investigate a bit and summarise some of the information.

…And it turns out that the ‘slash and burn’ style gardeners that promote the use of glyphosate are sort of right……but are also, at the same time, very wrong, are badly informed and guilty of parroting the same line probably given out originally by Monsanto et al.

Glyphosate works by interfering with a key enzyme (EPSPS) which in turn prevents synthesis of some key amino acids, effectively shutting down a plants growth and killing it.  It is absorbed mainly through the leaves and growing parts above ground and also minimally through the roots.  EPSPS is only found in plants and some microbes.

When it gets into the soil (as it will, due to the method of spraying and run-off , etc) it is indeed broken down by microbial action.  But the simplistic statement that as soon as it touches the soil it breaks down and is safe is at best a fundamental misunderstanding of the process and at worst deliberate disinformation.

Firstly, the rate at which it breaks down is very variable.  It depends on both the soil type and climate.(and the availability of sufficient microbes).  The persistence in soil is given as a ‘half-life’.  This is the length of time taken for half the glyphosate present in the soil to be degraded.  The half-life can vary from 2 up to 200 days.  An assumed average in the field is around 45 days.  (Note that this is not the length of time to degrade all the glyphosate.  It could persist for months in significant amounts)  Glyphosate bonds readily to soil and this makes it difficult for microbes to get at it, but also means it could later be reabsorbed by plants.  Tests published in a fact sheet by the National Pesticide Information Centre in the USA showed residues of glyphosate in lettuces, carrots and barley up to one year after treatment of the soil with glyphosate weed killer.  And another point.  The committee were discussing Roundup as a control until a plot was taken.  This could involve multiple applications over the same ground over a one to two year period.

Glyphosate on it’s own has been shown to have little harmful effect on humans at low levels of exposure.  There are arguments over links to non-Hodgkin lymphoma and other cancers among operators exposed to higher doses.  This led to the recent classification of glyphosate by the World Health Organisation as ‘probably carcinogenic in humans’.

The glyphosate in soil is broken down by microbial action into aminomethylphosphonic acid (AMPA)  This can persist in the soil for long periods of time (longer than glyphosate).  AMPA is then broken down over time into final products such as formaldehyde, ammonium (the cation NH4+ , which is itself toxic to plants) phosphate and CO2.  You may be pleased to know that AMPA is considered to be no more toxic than glyphosate itself (interpret that as you wish).

In other studies glyphosate does affect microbes that use the enzyme EPSPS, including some beneficial gut flora normally found in humans.  It has also been shown to affect the growth of some species of earthworms and some amphibians.

And don’t forget the bees.  Published scientific studies have shown detrimental affects on honey bee navigation and learning after exposure to sub-lethal doses of glyphosate on farms.  As bees do make it back to the hive they then carry trace amounts back to the colony which are fed to the next generation (and put in the honey that is then fed to humans).  The overall effect could be to degrade the ability of the colony to forage and threatening colony survival.

Note that the majority of studies on toxicity, etc are just for glyphosate alone.  Commercially it is formulated with many other ingredients, some or all of which could have an effect on the user, the environment or the consumer.  Most notable is the use of various surfactants (think washing up liquid) to help the product wet the foliage and increase absorption.

Interestingly, Roundup is formulated with a surfactant called polyethoxylated tallow amine (POEA) presumed to be made from tallow (a mixture of fats from the fatty tissue of cattle and sheep) and so is not exactly good news for vegetarians and vegans.

So while it is an invaluable product to use to control weeds in the urban environment or eradicate some very bad invasive species, totally safe on your vegetable plot it is not.  Unless you wish to deliberately feed your children traces of various chemicals as a long term experiment.

Maybe some members of the committee need to read up a little more on chemistry ….and possibly biology.