2,4-D: Back to the Past

So, having just spent a decade breeding mutant superweeds by pouuring pesticides on crops, what’s the recommended future of weed science?

Pour pesticides on crops until they breed more mutant superweeds.

So what is our old friend 2,4-D, which used to be commonly used back in the 1980s?

2,4-D is a WHO Class II ‘moderately hazardous’ pesticide. This places it in the same class as endosulfan, lindane, paraquat and toxaphene. It has an LD50 of 375 mg/kg in the rat with evidence suggesting a similar level of toxicity in humans(9).

Occupational exposure to 2,4-D has produced serious eye and skin irritation. Other symptoms of 2,4-D poisoning include nausea, weakness and fatigue, and in some cases neurotoxic effects including inflammation of nerve endings(10). Some medical reports from practitioners who have treated victims of acute exposure to 2,4-D mention severe and sometimes long lasting or even permanent symptoms. These include, as well as those listed above, diarrhoea, temporary loss of vision, respiratory tract irritation, confusion, numbness and tingling, bleeding and chemical hypersensitivity(11).

Oh, hey, nothing to worry about.

Well, except for chronic exposure;

It seems that long term exposure to 2,4-D can affect different animals in a wide variety of ways. Rats for example were found to be largely unaffected when fed moderately large amounts in their diet over long periods, although signs of kidney pathology were demonstrated. Dogs however died when fed smaller amounts over shorter periods. A human fed 16.3 grammes over 32 days showed severe symptoms of intoxication(13).
And it may not cause cancer:
Phenoxy acid herbicides have been linked with soft tissue sarcomas, but the UK ACP has concluded that ‘the data do not suggest a positive link with 2,4-D’14 as have the Canadian authorities(15). However, the International Agency for Research on Cancer (IARC) has classified 2,4-D among the phenoxy acid herbicides MCPA and 2,4,5-T as a class 2B carcinogen-possibly carcinogenic to humans(16) (concluding that there was limited evidence in humans, inadequate evidence in animals).

The US authorities have also been reluctant to declare 2,4-D as a potential human carcinogen, but the US courts decided that a forestry worker contracted cancer and died as a direct result of his exposure to 2,4-D during the course of his work(17).

Well, at least it doesn’t get mixed with 2,4,5-T anymore.

One concern about 2,4-D has related to dioxin contamination. 2,4-D was in the past frequently co-formulated with the herbicide 2,4,5-T. Production of 2,4,5-T was contaminated with the carcinogenic dioxin TCDD. Those who were exposed to the mixed formulations might therefore have been exposed to TCDD. The most notorious mixed formulation was Agent Orange, used first by the UK military in Malaysia and later extensively by the US military to defoliate jungle regions in Vietnam. In the UK, 2,4-D + 2,4,5-T formulations were in use until 1994(18). 2,4-D has been produced with contaminant dioxins, but not the harmful TCDD(19).
Oh, just other contaminant dioxins. I’m sure they’re not harmful.

And they wouldn’t get on you, would they?

Er, unless they drift. Or you’re the farmer applying them. Or you wear the cotton or eat the peanuts they were sprayed on.

2,4-D has low soil sorbtion and a high potential for leachability(23). Indeed 2,4-D residues have been recorded many times both in water company monitoring programmes and by the UK Department of the Environment(24,25). It has also been detected in groundwater supplies in a number of US States and in Canada(26). In 1994, 3% of groundwater samples, and in 1995, 4% of surface water samples in England and Wales exceeded the EU standard(27).

Oh, right: or unless you drink the water. Well, that couldn’t really be a problem, could it?

Administration of drinking water dosed with moderate levels of 2,4- D (about 50 mg/kg) to pregnant rats did not result in any adverse effects on birth weights, or litter size. Rats fed higher levels (188 mg/kg) had fetuses with abdominal cavity bleeding and increased mortality. DNA synthesis in the testes was significantly inhibited when mice were fed large amounts (200 mg/kg) of 2,4-D (8). While there is some conflicting evidence about the reproductive effects of the compound in animals, most of the evidence suggests that 2,4-D causes reproductive effects at moderate doses in animals. This indicates that humans may be at risk with 2,4-D exposure though no direct evidence of reproductive problems associated with 2,4-D exposure exists.

Well, “no direct evidence” is reassuring. No other problems, right?

Teratogenic Effects

2,4-D has a very limited ability to cause birth defects. However, rats fed 150 mg/kg on days 6-15 of pregnancy had an increase in skeletal abnormalities such as delayed bone development and wavy ribs (10) which are a function of general toxicity. The same conclusions may be drawn for 2,4-D’s potential to cause teratogenic effects in humans as was noted above.

Mutagenic Effects

2,4-D has been very extensively tested for mutagenicity and found to be non-mutagenic in most systems. However, significant increases of damage occurred in chromosomes in cultured human cells at low exposure levels (17). 2,4-D did not damage DNA in human lung cells. The evidence is too equivocal to draw any conclusions.

You know, 2,4-D was only developed during WW II, and only commercially released in 1946. Maybe we should round it up and not let it out again. It’s 64 years old. Time for it to retire.