The toxins (pyrrolizidine alkaloids) in Ragwort (sometimes known as Stagger Weed) can cause liver poisoning. It is a cumulative poison that eventually leads to the rapid onset of symptoms before death, however the symptoms are variable and resemble those of a number of other diseases. Furthermore the diagnosis can only be confirmed by dissection of the liver. The lethal volume of Ragwort is around 7% of body weight for horses and cattle are also prone. Sheep are thought to be less prone to poisoning although it is difficult to find solid evidence of any fatal effects on livestock other than horses.
|Gatekeeper butterflies on ragwort © Raz|
In 1990, MAFF (the Ministry of Agriculture, Fisheries and Food) published figures showing that only 10 horses died that year from Ragwort poisoning however in 2002 the British Horse Society (BHS) and the British Equestrian Veterinary Association (BEVA) reported figures of 6500 per year! Although these figures have now been removed from their websites, they are sometimes still quoted as evidence for an increase in numbers of poisonings. At the time the Wildlife and Countryside Link (a consortium of most of the main voluntary conservation and countryside organisations) countered that the statistical basis of this analysis was inaccurate as only 4% of BEVA members responded saying on average they had seen 3.37 suspected cases. This was then multiplied by the membership of BEVA (1,945) to give a total of 6,553 cases. It is highly unlikely that this is anywhere near the true figure because;
1) The data is not based on confirmed cases, but on the suspicions of vets.
2) Vets who did not encounter any Ragwort poisoning would be less likely to respond to a survey about Ragwort poisoning, so extrapolation of results is not possible.
A more realistic reporting of the survey results would be that up to 283 horses were suspected of dying of Ragwort poisoning in 2002.
Clearly any Ragwort poisoning is a tragedy to both horse and owner and there is no doubt that poisoning incidents do genuinely occur in Britain, but the figure of 6500 is improbably high. Horses are susceptible to Ragwort poisoning via two main routes;
1) Grazing in fields containing Ragwort. Ragwort is not a preferred food plant for horses however, problems arise where paddocks and fields are over-grazed and animals have no choice but to eat toxic plants. Moreover, over-grazing opens up the turf to reveal bare ground which is ideal for Ragwort seed germination. To this extent, horse owners can generate the very problem that they wish to avoid.
2) The presence of Ragwort in stored food such as silage or hay. The plant is still toxic when dead, but horses are less able to detect and avoid it. For this reason, hay from fields with Ragwort should not be fed to stock or horses.
The most appropriate solutions to the problem of Ragwort poisoning are therefore;
1) The proper implementation of Agriculture Act 1970 and Feeding Stuffs Regulations 2000, in which regulation 14 makes it an offence to sell feeding stuff contaminated with dangerous material.
2) The promotion of improved management of horse pastures, using established techniques (e.g. stock rotation and reduced grazing pressure) to minimise Ragwort content.
A further worry is seed dispersal from land adjacent to horse fields or those used for hay. Though the seed is wind dispersed, a study showed that only 0.5% became airborne and that only a tiny fraction reached 40m from the parent plant (cited in Harper & Wood, 1957). Whilst it is possible that seed could travel more than 40m, there is no need for extra wide buffer zones.