This section is from the book "A Guide To The Poisonous Plants And Weed Seeds Of Canada And The Northern United States", by Robert Boyd Thomson, H. B. Sifton. Also available from Amazon: A guide to the poisonous plants and weed seeds of Canada and the northern United States.
In ancient times the Horsetail family was one of the most important. The fossil record shows that its members formed a striking part of the coal forest swamps, their jointed stems, two feet or more in thickness, rising to heights of fifty to one hundred feet. To-day there are only a few isolated species of a single genus, composed of small plants ranging from a few inches to a few feet in height. Unable to keep pace with changing conditions the race has been falling behind for millions of years.
Animals of the coal forests may have found the Horsetails edible, but in their present form some at least are quite injurious. With regard to the nature of the poison, there is some diversity of opinion, although the old idea of the mechanical action of silica, a common and sometimes prominent constituent of the Horsetails, has been practically abandoned, in favour of a chemical basis. Aconitic acid was first found and suspected. Later work disclosed an alkaloidal nerve poison, called equisetin, which was proved to be present in one species at least, in sufficient quantity to be dangerous to animals. Whatever the nature of the poison, numerous instances of its harmful effect have been recorded. It is especially injurious to horses. In one much quoted case, nine horses, fed on good hay, were bedded with swale hay containing the Horsetail. They acquired the habit of eating their bedding and as a result all soon showed symptoms of Equi-setosis. The youngest, three years old, became unable to stand, and died in a few days. Others, somewhat older, became very ill, keeping on their feet with difficulty, but finally recovered. One mare, much older than the rest, showed only slight symptoms. Five other horses on the same farm, fed in the same way, but bedded with straw, remained in perfect health.
In this country the Field Horsetail, Equisetum arvense L., causes most trouble because it is often found in hay, especially from low lying fields. Animals may graze in pastures containing Horsetail without harm, either because of the laxative effect of the green forage or because they can pick their food better than when fed in the stall. Grain-fed work horses seem little liable to attack, while colts or young animals are chiefly affected. Cattle eat the Field Horsetail without apparent harm, although other species in Europe are said to be poisonous to them. The effect of Horsetail on sheep is doubtful.
A noteworthy indication of incipient Horsetail poisoning is the development of a depraved appetite, the animal often eating its bedding with Horsetail in it in preference to good hay or even grain. After the disease develops the symptoms are quite as marked.
Diarrhoea and rapid loss of flesh are first observed, followed by muscular incoordination. The animal sways and staggers, finally becoming unable to stand. The temperature is at first sub-normal, but after the animal falls its struggles produce a fever. The appetite usually remains good throughout, but weakness increases and death may follow from exhaustion due to the continued struggles to rise. The circulation is poor, as shown by coldness of the extremities and paleness of the delicate membranes of the mouth, nostrils and eyes. The disease can be distinguished from bracken poisoning which it somewhat resembles, by the constipation and redness of the eyes that accompany the latter. (See p. 33.)
The treatment is as follows. Remove all hay containing Horsetail and administer a purge. Rich and Jones recommend a pill of one ounce Barbados aloes, one or two drachms of ginger and enough soft soap to bind the powder together. If these are not available, a quart of linseed oil may be used as a substitute. A teaspoonful of nux vomica added to the grain ration three times daily helps to relieve the muscular trouble. It is important to avoid exciting the animal, and to keep him on his feet. If he seems likely to fall he should be supported in slings - arranged to press very lightly on the abdomen when he is standing but give support as soon as the least bending of the legs takes place.
The Field Horsetail thrives best in fairly light sandy soil, wet, or at least moist for a considerable part of the season. Other species live partially submerged in ponds or streams. There is a perennial root-stock - a horizontal stem running beneath the surface of the ground. This rootstock is jointed and branched, the branching providing an excellent means for the spread of the plant. The ordinary aerial stems are branched or simple, according to the species, and may be easily pulled apart at the joints, which are sheathed by the united bases of small pointed leaves. Stems and branches are characteristically ridged and furrowed, this and the ease with which the plant breaks at the joints, affording a definite means of identification. The Horsetails contain a large proportion of silica, which makes them harsh and rough to the touch. This has given rise to the popular name, Scouring-rush, for the Winter Horsetail, whose stout unbranched stem is sufficiently hard to scratch glass. The branched forms contain less silica, but still sufficient, it was formerly held, to account for Horsetail poisoning. Food manufacture is carried on by the green stems and branches, the scale-like leaves being useless in this connection. The spores are contained in cone-shaped structures at the tips of stems. In Equisetum arvense L. the stem that bears spores is brown and succulent and takes no part in food production. It grows and matures in early spring, living on food stored in the underground parts during the previous year. In other species the spores are borne on the ordinary green plant. The cone is composed of a number of peltate or umbrella-shaped hexagonal structures borne on short stalks. These are the sporangiophores, each of which bears a number of sporangia or spore-sacs, suspended from its under surface. The outer coat of the green spores contained in them is split into four ribbons which expand and contract with the moisture changes of the air and give to the spore mass an appearance of writhing life.
Fig. 3. - Field Horsetail - Equisetum arvense. a Ordinary, green, summer form, with whorls of simple branches, b Brown, spring torm, bearing cones and attached to an underground stem, with a food-tubercle. c A branch of the "bushy" type, with many secondary branches. d Branch segments, with pointed teeth (leaves) at the tip, and narrowed base. a, b and c about half, and d about twice natural size.
Four species have been experimented with and found to be harmful. One of these, Equisetum hyemale L., the Scouring-rush or Winter Horsetail, can be distinguished from the other three by the fact that it has an evergreen stem. This is usually un-branched and of considerable size, sometimes half an inch or more in diameter and several feet high. The other three species are all much branched. One, Equisetum sylvaticum L., is found almost exclusively in or at the border of rather shady woods. It is a slender form with characteristically leaning stem and gracefully drooping whorls of branches. Another, Equisetum palustre L., is practically confined to open wet places. It has rather stout 4 to 7-angled, hollow branches. The above species cause less trouble in this country than the fourth, which is known as the Field Horsetail, Equisetum arvense, because of its usual habitat. This form, as stated above, has separate fruiting stems which come up early in the spring and bear the spore cones at their tips. Later the green branched form appears, no fruiting cones being ever found on it. It is widely distributed and more abundant than any of the other forms. It can be distinguished from Equisetum pelustre by its solid 3 to 4-angled branches and by the larger cavity of its stem, this being one-half to two-thirds of the diameter in Equisetum arvense and about one-sixth in Equisetum palustre. From Equisetum sylvaticum it is ordinarily distinguishable by its much less abundant and less conspicuous secondary branching. Great numbers of secondary branches do occur, however, on the Field Horsetail when its main stem has been eaten or broken off. Then the whole aspect of the plant is changed, and it assumes a low bushy form that is even more distinct from the slender, graceful woodland species.