Marthélize Tredoux: The science behind herbicides and pesticides

By , 10 March 2017



This week I want everyone to buckle up for a quick science lesson. In the conventional versus organic farming debate, I keep seeing basic terms used, abused and misappropriated – more often than not to push an agenda. So for the sake of all that is accurate (and by extension to me as a scientist, holy), let us get a few things straight.

Disclaimer: this is not me debating the pros and cons of organic / traditional / GMO. That requires an entire series of lengthy tomes, which I will one day get around to writing, as soon as my doctor prescribes me the Prozac I’ve been begging for. This is a clarification of simple misconceptions.

1) Toxin / Toxic / Toxicity
I get it – words that sound the same are interchangeable. Except they are not. A toxin is “a poisonous substance produced within living cells or organisms – synthetic toxicants are thus excluded”. Snake venom is a toxin. Botox is a toxin.

The word you’re thinking of is “toxic” or toxicity” which is “the degree to which a substance can damage an organism”. Here’s the kicker: ANY substance you can think of is toxic. But the dose makes the poison. Even water is toxic if you drink enough of it (water poisoning or hyperhydration is a real and dangerous thing). Most of us happily ingest our favourite toxic carcinogens daily: coffee and alcohol, but more on those later

2) Glyphosate
RoundupRemember your new mantra: the dose makes the poison. Now, let’s look at scary, scary glyphosate – the big, bad herbicide found in Roundup, which is evil agriculture company Monsanto’s evil product that goes around doing evil things like… erm… making prank calls and rubbing kittens’ fur the wrong way.

We just learned about toxicity, and there is a method of determining the relative (acute) toxicity of substances. It’s called the LD50 and it refers to the mg/kg dose of a substance that will kill 50% of a test sample if ingested (LD = Lethal Dose).

Glyphosate has an LD50 of 5600mg/kg. This means an 80kg individual will need to ingest at least 448 grams of pure glyphosate to suffer from acute toxicity and possibly die. Caffeine has an LD50 of 192mg/kg. So the same individual only needs to ingest 15.36 grams of caffeine for similar effects. Table salt has an LD50 of 3000mg/kg.

To oversimplify: the higher the LD50 value, the less toxic the substance is, at least compared to the typical amounts you would ingest it in. An example on the very dangerous end of the scale: the LD50 for highly toxic snake venom is 0.013mg/kg.

Let’s get back to glyphosate. A quick search for a Roundup package insert prescribes the general use of the preparation as 10ml of Roundup per litre of water, spraying 2-3 litres per hectare. So that’s 20-30ml of Roundup concentrate per hectare of crop. The active constituent in the concentrate is 360g/L, so 0.36g/ml. That roughly calculates to a range of 7.2 – 10.8g of glyphosate per hectare of crop. And that factors out to one person needing to ingest the equivalent of 41.2 hectares worth of crops sprayed with glyphosate before it’s acutely toxic.

What’s the LD50 of copper sulfate – a widely used organic pesticide? I’m so glad you asked. 30mg/kg. So roughly 187 times more toxic. And typically applied at higher concentrations (according to a US EPA Copper Sulfate package insert, which recommends 4lbs per acre of copper sulfate, which works out to 4.48kg per hectare for application to grapes in treatment of powdery mildew).

3) Carcinogens / carcinogenic
This is a tricky one. The IARC (International Agency for Research on Cancer) classifies compounds, agents and exposure methods by how likely they are (based on all the available evidence, both in humans and experimental animals) to cause cancer. Class 1 is definite carcinogens, Class 2A is probable carcinogens and Class 2B is possible carcinogens (e.g. caffeine).

Glyphosate is Class 2A – probable carcinogen. In the same group, you will also find high-temperature frying, household combustion of biomass fuel (i.e. wood-burning fires or braais), red meat and “very hot beverages”.

You know what’s a Class 1 (definite) carcinogen? Ethanol. Yes – yummy, delicious alcohol.

I am not saying herbicides and pesticides in any shape or form are healthy and should be chugged like soda. Realistically though, they are still necessary. My point is that you can enjoy your organic wine, possibly loaded with copper sulfate, while flipping your carcinogenic steak on the carcinogenic fire, while fashionably complaining about the big bad pesticides made by “Big Agri” while stroking your trendy beard.

Or you can take a minute, learn the difference between facts and fear mongering, and move the conversation around sustainable agriculture forward, instead of backwards.

  • Marthélize Tredoux is the co-owner and editor at Incogvino. By day, she helps SA wineries sell their wine in the USA. She won the Veritas Young Wine Writers Competition in 2013.


9 comment(s)

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    Anton | 16 November 2020

    Bull article.

    Watch ‘Secret Ingredients’ the movie by Jeffrey Smith and
    ‘Kiss the Ground’ on Netflix.

    Nothing can justify the use of Glyphosate / Roundup if you truly know its impact on humans, animals and the environment! It has to be banned!

    Kim Larson | 29 March 2017

    Fantastic article! Thank you for educating with the science–much appreciated.

    Benjamin | 17 March 2017

    Greetings. If we’re discussing semantics then there is no need to incorrectly parse ‘herbicides’ and ‘pesticides’. All herbicides are pesticides. For some reason the general public doesn’t think of field bindweed or star-thistle as pests (they should get out more).
    The most damning information on RoundUp, however, is not glyphosate, but the myriad of other coformulants that Monsanto adds to the mix that they can classify as ‘inert ingredients’, which therefore do not have to undergo the scrutiny that glyphosate had to when passing EPA and other regulatory bodies. These proprietary chemicals do change the LD50 of the product, and you will find lower values published for ‘RoundUp’ than for glyphosate alone. What chemicals are these? The public, even industry professionals, have no way to know.

    Dr Meenakshi Noll | 17 March 2017

    I agree with Marthelize, hopefully people would understand from your article. thanks

    Marthelize | 16 March 2017

    FYI I posted that reply three times. I wanted to share the link to the post of the guy who did a whole bunch of other calculations but the comment section isn’t allowing links. It just cans your comment…

    Kevin | 16 March 2017

    I’m afraid you got the maths wrong on the Roundup. 10 ml / liter is a 1 % solution. Yo assumed that 2-3 liters of this solution would be sprayed per hectare. What would be used is 2-3 liters of .Roundup. So the actual glysophate dosage would be 720 – 1080 grams / ha or 100 times more tan your calculations.

      Marthelize | 16 March 2017

      I just spent 15 minutes replying to your comment, Kevin (but from my phone) and it disappeared. Thanks, Monsanto.

      Right. I checked and it’s not the calculations that are incorrect but my grasp of how one sprays crops. An important lesson to be learnt here: admit when you are wrong.

      However, even being out by a factor of 100 with my example calculations, the principle still stands. The relative toxicity of the substances is correct. I also found someone who did similar examples but who is better at maths than I am. but basically he calculated that:

      If you assume the minimum threshold for harmful ingestion of glyphosate is 2mg/kg per day (and that’s “worst case” – so it probably won’t have any effect but they’re working on the far edges of the margins here), you would still need to consume 62lbs or 28kgs of produce per day for an extended period of time before feeling any effects.

      Thanks for pointing out the mistake – sucks to be wrong but any scientist knows admitting it is part of the job.

      I’m still standing by the principle though. Of all the pesticides (and yes, by definition they are all harmful to something – it’s supposed to be a poison, after all), Roundup is hardly the one to be feared most.

    Bachus | 13 March 2017

    What a powerful, concise and thought provoking article. More along these lines please Marthelize!

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