How toxic are herbicides?

Although herbicides are chemicals and all chemicals should be treated with respect there needs to be some perspective given  as there is a mountain of misinformation being spread by people that really don’t know what they are talking about.  I hear it all the time on talkback and it makes my blood boil.

Below is a table which shows the relative toxicity of the active ingredients in a number of common herbicides.  To provide some reference I have also included some commonly known products such as salt, caffeine and nicotine as well as some well-known actual poisons such as strychnine, arsenic and 1080 poison.

The LD50 toxicity is based on the mg of the product (active ingredient)  needed per kilogram of body weight to kill half the rats tested. 

 Table 1 - courtesy of Massey University - Understanding Herbicides Course notes

Let’s boil this down and use the currently very topical glyphosate or Roundup as our example.  The figures in the table are for the active ingredient, so 5,400mg of active ingredient per kg of bodyweight.  Assume that glyphosate has the same effect on a humans as on rats.

Allow for a reasonably conservative 60kg human.  To kill one of two 60kg humans they would need to consume 324,000mg of the active ingredient in a reasonably quick time.  This would mean consuming 900,000mg of the neat Glyphosate 360.  This is nearly 1 litre of Glyphosate 360 that would need to be drunk.  This would be unlikely but there have been cases where people put chemicals into household containers like water bottles and then grab them by mistake.  Even under these circumstances you might get sick but would be unlikely to consume enough to kill you.

Once diluted to be used for spraying the numbers get even more silly.  Even caffeine which is much more toxic than glyphosate you would need to consume more than 200 cans of Coke or 75 cups of coffee to get poisoned by the caffeine. 

So let’s now dilute Glyphosate ready to spray. Even at the high blackberry rate you are spraying at around 10 litres of Glyphosate 360 ha.  Let’s assume your spray tank is 400L and this does one hectare.    So if drinking the made up tank spray you would need to drink around 36L of tank mix.  I couldn’t do it with beer so pretty unlikely I could stomach 36 litres of Glyphosate tank mix!

Take it one step further.  Assume you let stock onto the sprayed pasture by mistake.  Let’s use sheep and assume they also weight 60kg to keep the math simple.  To get a toxic does they would need to eat 900m2 of just treated pasture.  Not all the herbicide gets onto the grass and what does is absorbed quite quickly and starts to deactivate in the plant as it starts to kill.  Glyphosate is deactivated in the soil as well.  So even if all the glyphosate was sitting on top of the pasture the 900m2 of pasture would have to be consumed within a few hours.  Just not possible.

Many plants and weeds produce more toxins and in higher levels than the chemicals being applied.  Docks are a perfect example of this.

In an ideal world we would love to not have to use any chemical, herbicides, insecticide, pesticides etc. but the environmental goals that we all share I think need to be balanced with practicality and common sense. 

So in these days of environmental virtue signaling and PC correctness I hope this adds some sanity and perspective to what is not a simple issue.