Pasture management is an important part of looking after your horse. You should regularly check your horse's grazing for poisonous plants. Many plants that are poisonous to horses cause neurological and liver damage which can be life-threatening.
How do I know if a plant is poisonous?
Unless you have a good general knowledge of poisonous plants it is unlikely that you will be able to easily identify which plants are poisonous to your horse or not. There are some plants that you will be familiar with, eg ragwort, but others that you may not be familiar with, eg charlock. It is useful to familiarize yourself with the most common plants poisonous to horses so that if you see some in your horse's field you will know that it needs removing.
Causes: depression, loss of appetite, colic, restlessness, incoordination, paralysis, head-pressing, permanent nervous injury.
Liver damage builds on itself over time.
Common cause of serious or fatal poisoning in horses.
Ragwort is bitter tasting so is usually avoided unless grazing is in short supply.
In the United Kingdom, Ragwort is classified as injurious by the 1959 Weed Act - it is an offence to allow it to spread.
Sycamore (UK)/Box elder (USA)
Causes: dark colored urine, muscle stiffness/weakness unrelated to exercise, muscle tremors, lethargy and quiet demeanor, some cases show colic-like symptoms, choke-like retching neck spasms, frenzied vocalization, vigorous head shaking/nodding/low head carriage, sweating, depression and signs of pain, lying down unwilling to stand but eating, rapid shallow breathing, complete collapse, death.
In the USA the disease is linked to the Box Elder tree (Acer negundo); whilst in the UK the European Sycamore tree (Acer pseudoplatanus) is thought to be the cause of the majority of cases.
The seeds from both trees contain the toxin Hypoglycin A which is capable of disrupting fat metabolism within the skeletal and cardiac muscle fibers.
Ingestion of sufficient quantities of either the seeds in autumn or the seed/seedlings in spring results in a dose of toxin capable of causing damage to the respiratory, postural and cardiac muscles which may result in death.
Causes: incoordination, tremors, rapid then weak pulse, excitability followed by collapse, acute heart failure.
In many cases there are no signs, acute heart failure leading to death within a few hours.
What should I do if I find a poisonous plant in my horse's field?
The most common plant poisoning in horses is caused by ragwort, yew, laburnum, oak and bracken fern. If you find any plants in your horse's field that you believe to be poisonous, they should be removed immediately. The plant should be completely removed from the roots to avoid it from re-growing. Avoid using chemicals or herbicides, unless you are able to keep your horse in another field while the other is rested for a few weeks. Fence off any trees, eg oak, if possible and collect up acorns if they fall within reach of your horse. Do not leave any dead plants where animals can get to them as some will still be poisonous once they are dead and have dried out. The easiest way to dispose of any plants you remove from your horse's field is to burn them.
What should I do if my horse eats a poisonous plant?
Many plants are toxic and if a horse is suspected of having ingested toxic herbage, veterinary help should be sought without delay. Horses may suffer mild toxic effects unnoticed by you or your veterinarian, however, reported cases of fatality by plant poisoning are rare; most toxic plants are too pungent or bitter for the horse to digest, unless he has a craving, in the case of oak or is exceptionally hungry. Fatalities are common where irreversible damage has been caused by ingestion of large quantities, or ingestion over a long period of time. If you are ever unsure or see any signs of poisoning in your horse, call your veterinarian immediately.
What else should I know?
Many poisonings occur when horses eat ornamental shrubs over a fence line. Be sure to talk to any neighbors and others who may unknowingly plant dangerous trees and shrubs too close to your pastures or who might even dump fresh cutting and clippings over the fence for your horse thinking they are providing a treat.
Be cautious immediately after heavy winds and storms. Trees and shrubs may get damaged and branches knocked down into your horse's field. Some plants are more toxic after they wilt, and your horse may eat the leaves from a dropped branch even though they have been in that pasture and never touched that particular tree previously.