Coming to the two German reports, it is stated by Weber (1903) that _E. palustre_ contains a specific poison for cattle and other ruminants, but sheep and goats are able, owing to their fine muzzles, to separate it in fodder, and hence suffer less. Horses and pigs, he says, seem to suffer very little. Young animals and stock, from districts where the species does not occur, suffer more than those from places where it occurs—the latter appearing to learn early to avoid it.
Lohmann conducted feeding experiments with guinea-pigs with _E. arvense_, _E. palustre_, _E. pratense_, _E. sylvaticum_, _E. maximum_, and _E. heleocharis_ (not British). He also fed _E. arvense_ and _E. palustre_ to horses, cattle, sheep, pigs, and geese in considerable quantities for many days on end; and made experiments with aconitic acid on guinea-pigs and horses. The feeding experiments with guinea-pigs showed that of the species named only _E. palustre_ and to a less extent _E. sylvaticum_ are poisonous plants (to guinea-pigs). With the large domestic animals the experiments showed _E. arvense_ to be a harmless plant, and _E. palustre_ to be really injurious to cattle but avoided by other stock. Lohmann considers that the many statements in the literature agree in part with this result, and that the divergent observations may be traced to various causes, among which perhaps an abnormal chemical composition of the weed fed plays a principal part.
In this connection, however, the American results must be carefully borne in mind, and _E. arvense_ must not too hastily be regarded as blameless.
_Toxic Principle._ It was for some years believed that the apparent toxic symptoms induced by Horsetails were due to Silica or to Aconitic acid—the latter a substance found by Matz and Ludwig. The conclusion come to by Weber, however, was that the poisoning by _E. palustre_ is of an organic character, not due to silica. The young shoots, which contain little silica, were found in general to be much more poisonous than the old plants, which contain much silica. It was long since found by Wiggers that dried plants of _E. palustre_ contained 8·88 per cent. of silica, but all species contain this substance in greater or less degree, and it varies considerably in amount, even in the same species. The feeding experiments conducted by Lohmann, with certain species of _Equisetum_, and the observed symptoms of illness after the consumption of some of them, particularly _E. palustre_, serve to show that the ill effects are neither to be attributed to greater or less digestibility, nor to the silica present. Neither are the aconitic acid and other organic substances, in part found in previous investigations, responsible for the poisoning. However, an active compound named _Equisetine_, a substance belonging to the alkaloid group, was isolated; this occurs usually, perhaps only, in _E. palustre_, at any rate in sufficient quantity to be dangerous to animals. Lohmann then, following up the investigations of Paucerzynski, Matz, Meyer, Weber and others, ascertained definitely that _E. palustre_ contains an alkaloidal nerve poison, to which the name _Equisetine_ was given, and the experiments were held to decide that this is the poisonous substance in this species. (As stated above, _E. arvense_ was held to be harmless.)