Professor Derek Knottenbelt is a well known anti-ragwort campaigner. In one edition of Horse and Hound magazine (Horse and Hound 2005) he was quoted as saying. “Ragwort is a thug. It is the most hideous stuff in the world"

In 2000 he appeared at the Dodson and Horrell International Conference on feeding horses

Professor Knottenbelt had a paper entitled “Yellow Allert! [sic] - The latest on Ragwort” ( Knottenbelt 2000)

It starts off with an introduction saying:-
“Over the last 5 years there has been an explosion of ragwort in the UK. One can hardly avoid seeing it along road verges and in central reservations of motorways. However, animals seldom graze these areas and so many lay people driving along comment on how pretty the flowers are in high summer and how the fluffy seeds can be seen to blow away in the wind - giving the impression of a dandelion clock! The yellow peril is lurking and expanding its grip on the UK - it presents a serious hazard for horses and other grazing animals.”

There are no scientific references at all given in this paper, so it is hard to determine if there is a source for the idea that ragwort had exploded. However he was making essentially the same claim some years later See Professor Derek Knottenbelt Letter in the Yorkshire Post after a proper scientific survey by the government actually showed a substantial decrease in the amount of ragwort.

It would appear that the professor is the victim of a well known problem in science sometimes called Observational Selective Bias or the Recency Illusion Where people notice something new and then keep seeing it as if it has suddenly become common, when in fact it was there all along and it is only now they are noticing it, Ragwort has been brought to his attention and therefore he notices it more but the scientific survey shows that this is a misconception.

It is also worth noting that seed spread by the wind only usually takes place over very short distances

See Ragwort Seed Distribution

The paper continues.
“The potential danger of ragwort has been known for many years - indeed the plant was classified as a noxious weed in terms of the Act and as such every owner of land was required to control ragwort either by direct spraying or by lifting and burning it.” The Act referred to here would appear to be the The Weeds Act 1959 as it was at the time the only Act of parliament referring to Common Ragwort. It actually refers to Ragwort as an "Injurious Weed" i.e. one harmful to the interests to agriculture ( Several of the listed weeds are non-poisonous)

The professor is wrong about the law here. . Contrary to the impression apparently given here and apparently given elsewhere by the professor see Professor Derek Knottenbelt Friends of Bidston Hill Newsletter ., there is no automatic statutory requirement under this act for landowners to control ragwort and it would also appear that very little attention was paid to this act. There had been no prosecutions in the previous 5 years and quite possibly none ever and it had been considered for repeal in 1993

Later on the paper states the following :-
"Ingestion of the ragwort plant (in any state in any amount) will result in the absorption of the pyrrolizidine alkaloid (called jacobin[sic]) that passes to the liver in the portal blood vessels. On arrival the toxin damages the liver cells to an extent that is proportional to the concentration of the chemical.

It seems that again the professor is mistaken for it appears ,according to the scientific literature. that the statement "the pyrrolizidine alkaloid (called jacobin [sic])" is incorrect. Far from being a single pyrrolizidine alkaloid there are in fact fourteen of them which have been recorded as occurring in common ragwort of which Jacobine ( as it is correctly spelled) is only one. They are Senecivernine,Senecionine,Seneciphylline,Spartioidine,Integerrimine,Jacobine, Jacozine,Jacoline,Erucifoline,Jaconine,Adonifoline,Usaramine,Otosenine,Eruciflorine and Retrorsine. These are not necessarily present in every plant and vary in quantity. (Pelser et al.2005) and their level of toxicity may also vary depending how efficiently they are metabolised..

Indeed, it seems that the professor is doubly mistaken in stating Jacobine as the sole problem since research shows that it may only be present in small quantities. In the paper Variation in pyrrolizidine alkaloid patterns of Senecio jacobaea published in the journal Phytochemistry (Macel et al. 2004) the authors state that there are two different patterns of alkaloid occurrence

"Jacobine types were characterized by the PAs jacobine and jacozine and lacked erucifoline, while the erucifoline types contained erucifoline and acetylerucifoline but hardly any jacobine."

It is questionable whether the claim that ragwort in any amount will result in the absorption of the alkaloids is correct, and the following statement that the toxin damages the liver cells in proportion would seem to overstate the simplicity of the process. In fact there are a number of factors which might prevent the alkaloids from being absorbed where there are only small quantities, bacterial destruction inside the digestive tract and then simple failure to be absorbed etc..

Then they are actually not poisonous in themselves until they have undergone conversion into other chemicals which are the actual toxins. Fu et al ( 2004) Each of these steps is unlikely to be 100% efficient and the resultant pyrrole compounds are highly reactive and can react with numerous substances in the cell. They will only result in a toxic effect when they attach themselves to DNA molecules.

In fact it is quite significant what is said in that work. Fu et al. in the paper Pyrrolizidine Alkaloids—Genotoxicity, Metabolism Enzymes, Metabolic Activation, and Mechanisms published in the journal Drug Metabolism Reviews Say-:

"Thus, metabolic formation of pyrrolic ester metabolites has been shown to be the primary metabolic activation responsible for the adverse toxicities .............Because of high reactivity, pyrrolic ester metabolites can also react readily with water and other endogenous constituents, such as glutathione, to form the detoxified products."

Water, would of course be very common constituent of the cells concerned. So it would seem that the fact that water in the cells could prevent the absorbed chemicals exerting a toxic effect would at least render small quantities of the alkaloids harmless even if they crossed all the other hurdles on the way.

Even then if damage occurs there are processes that enable the DNA to be repaired ( Mori et all 1985)

It would seem then that the professor is mistaken to conclude that "Ingestion of the ragwort plant (in any state in any amount)" will cause problems. There is a section in the paper on whether horses eat fresh ragwort that deserves detailed examination. First he says :-

"Do horses actually eat it? For many years it has been assumed that horses will not eat green growing ragwort plants but this is far from proven. " This would appear to be an example of a well known poor argument called an argument from ignorance, (where "ignorance" stands for: "lack of evidence to the contrary"), is a fallacy in informal logic. It asserts that a logical proposition is true because it has not yet been proven false. The standard answer to this is that famously given by the philosopher Bertrand Russell in 1952 Russell was talking primarily about the existence of a concrete entity but the same principle applies to the kind of concept about which the professor writes. The burden of proof is on the claimant to prove that something exists. You cannot in science say it exists because there is no evidence that it doesn't.

Bertrand Russell said:-
" If I were to suggest that between the Earth and Mars there is a china teapot revolving about the sun in an elliptical orbit, nobody would be able to disprove my assertion provided I were careful to add that the teapot is too small to be revealed even by our most powerful telescopes. But if I were to go on to say that, since my assertion cannot be disproved, it is intolerable presumption on the part of human reason to doubt it, I should rightly be thought to be talking nonsense."

To show this as fallacious we need to look at what the professor said next.

"Indeed it is only based on supposition that when horses are grazing in a field in mid summer the ragwort appears to be untouched. This is a very simplistic approach and it is impossible to know categorically that the horses are never even tempted to take a mouthful. The biggest difficulty lies in the palatability of the plant." Again the professor appears to repeat an argument from ignorance; a logical fallacy.

In fact there is plenty of evidence from the scientific literature which would explain why it is that horses don't normally eat fresh ragwort. We have already seen that small doses apparently contrary to the professor's assertions do not cause problems.

The evidence comes clearly from a major branch of modern biology. A large part of our understanding of ecology and the interactions of the natural world comes from the development of a simple idea. Organisms produce offspring that resemble them and in the struggle for existence only those which are most fit to survive in their environment and produce offspring. Nature shapes organisms through this phenomenon and the man who discovered it is credited with being possibly the greatest biologist in history, Charles Darwin. He is so respected in the UK.

Evolutionary biology predicts that if a toxin is a threat to an organism in its environment that that organism with evolve a resistance to it in some way or in a variety of ways. This is exactly what we find in nature with Pyrrolizidine alkaloids.

The resistance shaped by biology takes two forms. Being able to detect and avoid the poisons and then being able to cope with the exposure. Those animals that are most exposed are most resistant to the effect.

Pyrrolizidine alkaloids do not taste bitter by accident. Nature has shaped and created the receptors on the tongues of animals to detect alkaloids because a lot of them have some kind of poisonous effect. Those animals with better receptors produced more offspring

In the paper Is the Bitter Rejection Response always adaptive? (Glendening 1994) Published in the journal Physiology and Behavior, the author states:-
"Animals with a relatively high occurrence of bitter and potentially toxic compounds in their diet (e.g., browsing herbivores) were predicted to have evolved a high bitter taste threshold and tolerance to dietary poisons. Such an adaptation would be necessary because a browser cannot “afford” to reject all foods that are bitter and potentially toxic without unduly restricting its dietary options. At the other extreme, animals that rarely encounter bitter and potentially toxic compounds in their diet (e.g., carnivores) were predicted to have evolved a low bitter threshold. Carnivores could “afford” to utilize such a stringent rejection mechanism because foods containing bitter and potentially toxic compounds constitute a small portion of their diet. Since the low bitter threshold would reduce substantially the risk of ingesting anything poisonous, carnivores were also expected to have a relatively low tolerance to dietary poisons. This hypothesis was supported by a comparison involving 30 mammal species, in which a suggestive relationship was found between quinine hydrochloride sensitivity and trophic group, with carnivores > omnivores > grazers > browsers. Further support for the hypothesis was provided by a comparison across browsers and grazers in terms of the production of tannin-binding salivary proteins, which probably represent an adaptation for reducing the bitterness and astringency of tannins. Tannins are a widespread class of plant compound that are abundant in the diet of browsers (i.e., forbs, shrubs and trees) but rare in that of grazers (i.e., grasses). Whereas browsers produce large quantities of tannin-binding proteins, grazers produce none. Finally, it was found that species with a high bitter threshold (i.e., herbivores) were generally more tolerant to toxic compounds. This latter adaptation would minimize the risk of toxicity after unwittingly ingesting foods laced with nonbitter poisons"

We find this right across the literature. Firstly animal taste systems are shaped to enable them to chose food properly. Pandas which are vegetarians that have evolved from carnivores have lost the ability to taste meat properly ( Huabin Zhao et al 2010) Cats have lost the ability to taste sugar. ( Li et all 2005) Indeed it has been shown that the taste systems of carnivorous animals have been changed significantly by evolution to adapt to their diets (Jianga et al 2012)

Then the toxicity levels of the alkaloids change with the likelihood that animals are going to encounter them. Animals that are more likely to eat plants with the alkaloids in them like sheep and goats, rabbits and guinea pigs are very much more resistant.

References

Peihua Jianga, Jesusa Josuea, Xia Lia, Dieter Glaserb, Weihua Lia, Joseph G. Branda, Robert F. Margolskeea, Danielle R. Reeda, and Gary K. (2012) Beauchampa Major taste loss in carnivorous mammals PNAS vol. 109 no. 13
Li X, Li W, Wang H, Cao J, Maehashi K, et al. (2005) Pseudogenization of a Sweet-Receptor Gene Accounts for Cats' Indifference toward Sugar. PLoS Genet 1(1): e3. doi:10.1371/journal.pgen.0010003
Huabin Zhao,Jian-Rong Yang,Huailiang Xu and Jianzhi Zhang,Pseudogenization of the Umami Taste Receptor Gene Tas1r1 in the Giant Panda Coincided with its Dietary Switch to Bamboo Mol Biol Evol (2010) 27 (12): 2669-2673
Pieter B. Pelser, Helene de Vos , Claudine Theuring , Till Beuerle , Klaas Vrieling Frequent gain and loss of pyrrolizidine alkaloids in the evolution of Senecio section Jacobaea (Asteraceae) Thomas Hartmann Phytochemistry 66 (2005) 1285–1295