Ragwort produces alkaloid toxins which cause neurological and liver damage, blindness, paralysis and death in livestock (especially horses and to a lesser extent cattle). However, when alive it is unpalatable and animals usually avoid it unless they are starving. It is dangerous mainly when cut and dried in hay, when it loses its bitter taste and is more likely to be eaten. This is why ragwort was one of the five plants listed as "injurious" in the Weeds Act 1959, which required landowners to prevent it spreading. That Act has been updated by the Ragwort Control Act 2004, under which Defra has produced a code of practice for ragwort control.
Ragwort is a biennial with a high production of wind-blown seeds, and a very successful colonist of bare ground. On the commons we have large areas of bare soils including where the runways and roads have been removed, and the bioremediation areas (where soil contaminated by aviation fuel is being "aired" to allow bacteria to break down the pollutants). WBC attempt to control it by spraying (but there's a limit to what can be sprayed in SSSI areas) and by mowing it before its sets seed. Cutting ragwort down doesn't kill it - in fact it provokes the plant into becoming a perennial, but at least it stops the current year's seeds. The best method of control is to dig or pull it out by the roots - which the GCCV do on one or two tasks each summer. It always seems to be a really hot day, and it's hard work, so we reward ourselves with an evening barbecue after the June or July task.
However, this is a native species, a legitimate part of the ecosystem and one of our most valuable plants for wildlife. It is colonised by 14 fungi, its leaves and stems feed at least 77 insects and its nectar at least 177. These include five "red data book" species and eight classified as "nationally scarce". Some are generalists, but at least 30 invertebrate species depend totally on ragwort (including the caterpillars of the cinnabar moth, pictured here). Even if it were possible (dream on!) to eradicate ragwort from the commons, we would not want to do so. We just want to keep it down to a level which doesn't pose a threat to the cattle.