Marthélize Tredoux: The science behind herbicides and pesticides
By Christian Eedes, 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
Remember 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.
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