To Spray or Not to Spray – Part V

Herbicides

by M.J. Harvey March 2005

Ever since the Neolithic Revolution when tribes in at least four regions of the globe started to cultivate plants, as opposed to merely gathering what grew naturally (although the change-over was a very complex process), weeds – plants in the wrong place – have been a problem.

It is easy to control weeds either by hand or by hoe if the area is small and if that is all you do. But cultivation led to settlement and this in turn led to job specialization where not everyone was available for weeding. This led to the invention of ploughs and eventually to herbicides. This sequence of hang-hoe-herbicide we call ‘civilization’.

Herbicides and Targets.

To kill a weed with a herbicide there has to be a ‘target’ – some organelle or chemical – in the cell which is disabled by the herbicide. The history of herbicides is the history of these targets. Herbicides have got increasingly sophisticated as our knowledge of cell structure and chemistry has increased. Plant cells, it turns out, are incredibly complicated, much more complicated, people are surprised to hear, than animal cells.

Plant and animal cells have certain structures in common: membranes, mitochondria, nuclei, DNA, for instance. Herbicides that target these structures and the chemicals inside them are liable to be toxic to both plants and animals. In this case the handling and method of application become very important. Such chemicals comprise some of the early herbicides.

On the other hand plants have certain structures (plastids) and many hormones and enzymes that do not occur in animal cells. Targeting these will damage plants but not affect animals. This is what some of the more recent herbicides do. “If you’re not green you won’t lean.” Let me illustrate some of the above sequence of herbicides with my personal history of avoiding hard work.

1) Gasoline

I had a couple of uncles who were winders at Denaby main Colliery in Yorkshire. One of them, Uncle John, had two acres on which he grew vegetables, roses and one of those perfect English lawns. Except that the lawn wasn’t quite perfect because dandelions and daisies kept appearing in it, which irritated him. Being an engineer he took one of those old metal oil cans which had a long spout and a lever which when pressed caused oil to squirt out of the spout. This can he filled with petrol and went round the lawn giving each weed a few drops in the center of each rosette. I was allowed to do this at times. It was effective, the dandelions turned brown and died in a few days.

Petrol (gasoline) is a good grease solvent that readily flows through the cellulose wall of plant cells and dissolves the lipid from the cell membrane causing it to leak. Cell death follows rapidly. On the human skin gasoline degreases the surface making it rough but fails to penetrate in sufficient quantity to kill the underlying skin cells. So gasoline is a fairly safe and effective herbicide but its application is labour intensive so it is only a little step up from the daisy grubber.

2) Sodium chlorate

This is an early herbicide that is effective and cheap since it is produced by the electrolysis of brine. It was especially favoured by the railroad companies that sprayed a dilute solution either alone or mixed with some paraquat or 2.4-dimethylamine along the tracks each year. As an eleven-year-old budding chemist I was of course interested in its use in fireworks and would buy it by the pound loose from the local seed merchant until they started to mix in sodium carbonate (soda), which ruined it.

Chlorate is an oxidizing agent attacking several systems in plant cells. It is probably not very toxic to animals since I have never heard of any problems.

3) 2.4-D

I dealt with this in Part I of this series. It has been used in enormous quantities in agriculture, golf greens and home lawns as a selective broadleaf weedkiller. I gather that the EU has recently restored its permit for domestic use. 2,4-D acts on a plant hormone system which is absent from the animal kingdom and its toxicity to humans and other animals, with precautions, is very low, although I personally intensely dislike its smell.

4) 2,4,5-T

Derived from the above and non-selective, it was used as brush-kill. I am not sure of its present status but I am pretty sure it is banned for toxicity problems. [Ed. Note: 2,4,5-T became better known as ‘Agent Orange’, accursed for its application by the US as a defoliant of Viet Nam forests in that country’s war. Though judged non-toxic in its earlier forms, the ‘Agent Orange’ variety was said to contain free dioxin, a deadly poison.]

5) Paraquat

A truly horrible chemical. It was first synthesized in 1882 but its herbicidal properties were not discovered until 1955 at the ICI Laboratories in England (since split off as Zeneca Corporation). Commercial production started in 1961 and at peak production the Huddersfield plant turned out 8000 tonnes per year. It is now manufactured in several countries.

Paraquat is marketed under many proprietary names including Gammexane, Weedol and Pathclear and is useful as a rapid-acting contact herbicide that is not translocated very far in the plant. I have used it on a brick path in the past and it sometimes fails to completely kill dandelions and other plants with deep rhizomes. It was used in the Maritimes on early potatoes to brown off the haulms so that the tops did not clog up the harvesting machinery.

It was and still is widely used in the tropics and in orchards since it does not penetrate bark and so keeps weeds down in coffee, cocoa, oil palm, orange, bananas and other plantations. When I was in Crete in 1984 it was in use by the increasingly wealthy landowners to clear herbs under the olive trees including, to my horror, the rare, native Paeonia clusii. Ah, the ill effects of affluence – even goats didn’t do that much damage.

The chemical nature of Paraquat – a bipyridyl – is such that it is able to reduce a single component (NADPH) in the electron transport chain thus bypassing the production of energy in the form of ATP. This shuts down the cell. The problems arise when it is ingested by animals. It is a stable compound that accumulates in the lungs leading to pulmonary oedema, kidney and liver damage. It became a favourite vehicle of suicide in the UK for a time and the manufacturer had to add an emetic and a stench agent for the domestic market. There is no antidote. Accidental poisoning is common in the Third World where users may not be able to read the precautions about mixing and application. Its effectiveness, rapid action and low price still do not hide the fact that this is a herbicide from hell. Most western nations have banned its sale and where it is still used it is restricted in application to licensed operators.

6) Surflan

This is used as a pre-emergence herbicide. In other words it attacks weed seeds in the soil just as they are germinating. Its mode of action is aimed at yet another aspect of plant biology: cell division. Surflan disrupts the assembly of the protein units that form the spindle of dividing cells. The spindle pulls apart the divided chromosomes to form two nuclei where one existed previously. No cell division – no growth – plant dies. (Surflan is a colchicine mimic and as such has its own use in inducing polyploidy in plants and I have a sample for that use).

I don’t have animal poisoning data but animal cells pull the chromosomes apart slightly differently and are not so susceptible (which was a disappointment for cancer treatment). Surflan is only available to commercial operators since this form of weed control is not attractive to home gardeners and I have not heard of any poisonings.

Santa Baby

I am writing this before Christmas (although it will not get printed until the New Year), listening to that old recording by Earth Kitt parodying greed and consumerism (and this was the 1950’s!). I particularly like the blackmail: “trim my tree, with trinkets bought at Tiffany. I believe in you – let’s see if you – believe in me.” My wife tells me I like her voice because it combines tremulo and vibrato. This got me thinking of asking Santa for the ideal herbicide – my own fantasy gift. I’m sure one of the elves must have a degree in biology. Let’s see what Santa can do for me!

The Ideal Herbicide

Santa Dearie,

I want a can of herbicide which is completely non-toxic – except to plants of course – one I can chug-a-lug with impunity, one that will not harm my cast, or the neighbour’s dog, or fish, or even spiders! And once applied it must decompose to harmless chemicals in a few days. Oh, and Santa, nothing with chlorine combined in it; those chemicals tend to persist. I want it to be economical to use so it has to work in a very dilute form – I’m sure you don’t want me to be continually asking you for more.

Your good little believer,

Joe

Now I happen to know that my letter will get passed on to a university-educated elf who will sit down and work out how to make this fantasy herbicide. One obvious way would be for him (or her, I don’t want to be sexist) to target something in a plant chloroplast, something which is present in extremely minute amount, but is still absolutely essential to its function – some enzyme or co-enzyme, say. Anyway I’ll let you know what, if anything, I found under the tree, give it a trial, and print the results in the next newsletter.