Although many medicines and pharmaceutical products are based on plants, not all herbs – although ‘natural’ – are beneficial. Many can be toxic for both humans and animals. With respect to horses, pastures used to have a much wider variety and composition of herbs than is the case today. Keith Allison, in his book on this subject, points out that prior to the 1940’s, “a typical meadow may have contained well over a hundred different varieties of herbage, compared with perhaps ten species in a modern pasture”, (Allison, 1997, revised 2011). 

At the same time, as the number of different species of wild plants declined in the countryside, so has our general knowledge of their properties. Herbals in the 17th Century were created, amongst other things, as repositories of information about the nature of plants and their healing properties. Gerarde’s Generall Historie of Plantes (1593) and of course the more famous The English Physitian, later renamed as The Complete Herbal by Culpeper (1652) gave detailed information about what could be eaten, and which parts of a plant had medicinal uses. 

For example, of Apples, Culpeper tells the reader that: “They are very proper for hot and bilious stomachs, but not to the cold, moist, and flatulent. The more ripe ones eaten raw, move the belly a little; and unripe ones have the contrary ef ect. A poultice of roasted sweet apples, with powder of frankincense, removes pains of the side: and a poultice of the same apples boiled in plantain water to a pulp, then mixed with milk, and applied, take away fresh marks of gunpowder out of the skin…Roasted apples are good for the asthmatic; either raw, roasted or boiled, are good for the consumptive, in inflammations of the breasts or lungs…” Apples are said to clean the liver, cure constipation, and tone the gums. A half and half mixture of apple cider vinegar and water make a rinse to restore hair, scalp and skin.

As the diversity of meadow plants has decreased, so many potentially toxic plants disappeared, and so the need to be able to identify them became less and less pressing. However, with increasing interest in wilding, in natural pastures, organic farming along with changes in the way in which land is more generally cultivated, the diversity of species is in many areas on the increase. This naturally includes the return of potentially toxic varieties. For all these reasons it is increasingly important to be able to identify the species in one’s pasture, and assess the risks for horses. 

The most commonly toxic plants for horses are generally considered to be Ragwort, Laburnum, Bracken and Yew, and we will look at why these are toxic in a few weeks, in this series of blog-posts. There are, however, a large number of other plants that are toxic to varying degrees.

In his book, “A guide to plants poisonous to horses” Keith Allison has offered great service in guiding the horse carer through some 50 of the most common species which pose problems for equines. As many will know horses, (as indeed all animals), are capable of auto-medication through self selection (zoopharmacognozy). For the horse to do this spontaneously, however, depends upon having a pasture rich enough in benign and helpful varieties which the horses can self-select when they feel so-called. A horse-carer who wishes to enable their horses to self-select is required to maintain a healthy balance of beneficial varieties in the pasture. This, in itself, reduces the risk of horses ingesting toxic varieties. One of the highest risks lies in weed-infested, horse-sick paddocks, where the horses are driven to eat species that they would not normally ingest, even in minute quantities, or would avoid altogether. 

Another issue affecting potential plant toxicity, is water balance. Under drought conditions, the toxin concentrations of certain plants may increase, so that the plant becomes more toxic for a given mass ingested. Other species, conversely, can become more toxic in wet weather, so this is also something to look out for in very moist areas. Species which are highly invasive – such as docks, ragworts, and buttercups – will proliferate successfully  in highly grazed pastures. Docks, with their deep tap roots drain the soil of both water and nutrients, so reducing the capacity of the soil to support grasses. As the grass struggles, so these invasive varieties proliferate, also making it more likely that horses might eat them for want of other sources of nourishment. 

Whilst many plants are toxic, they are not all toxic in the same manner, and there are five main classes of toxins which may affect horses. These are the Alkaloids, the Glycosides, the Nitrates and Nitrites, the Oxalates, and a Photosensitive class. In my next article, I will look at these classes in some detail, and give examples of how each class of chemical acts, and some idea of representative species.

Dr Peter Jeffs is a Kinesiologist (Diploma in Kinesiology, 2016, Bristol School of Advanced Kinesiology), who specializes in working with horses. He holds an Honours degree in Botany from Durham University (UK), a PhD. from Cambridge, and is currently studying Permaculture with the Ecological Land Management Mastery Program, based in NORCAL, USA. He aims to bring his knowledge of Kinesiology to working with horses, herbs and land. If he can assist you with any of the above, please get in touch here, via the contact form. He is based in Wiltshire, UK, near Avebury.

A guide to Plants Poisonous to Horses
Allison, K.,(2011, first edition 1997) 
J.A. Allen, London.

The Herball or Generall Historie of Plantes
John Gerarde (1597), London.

The English Physitian, 
later renamed as The Complete Herbal
Nicholas Culpeper, (1652), London.