Poisonous Plants for Horses

It is a common misconception that all horses have an innate instinct to avoid eating plants that may be harmful to them. Whilst this may have some semblance of truth for wild horses, who have grazing access over vast areas and a variety of forage types to choose from, the same sadly cannot always be said for the domesticated horse.

In particular, horses on overgrazed pastures or those on restricted diets may end up consuming poisonous plants if there is a lack of other forage types available, with recent research suggesting that plant poisoning is most often associated with a limited availability of other food (Aboling, 2023). Keeping on top of poisonous plant removal is recommended for all horses, but for those who are overweight or laminitis-prone on restricted grazing, it can also be helpful to speak to a nutritionist for advice on how much additional forage is required and how to keep them trickle feeding for longer.

Some poisonous plants appear to have a taste or smell which discourages horses from eating them fresh, but they can sometimes lose this once they have been dried. For example, a 2021 study found that hay contaminated with a low level of dried meadow saffron was readily consumed by horses over multiple feeding periods, even when forage provision was plentiful (Mueller et al, 2021). Care should therefore be taken when harvesting or selecting conserved forages like hay or haylage to make sure they do not contain any poisonous plants or weeds.

Whilst many horse owners are aware of the dangers of ragwort and are adept at recognising it, less attention is often paid to ornamental plants like yew trees or deadly nightshade that can pose just as much of a threat. For horses who are kept adjacent to gardens or public parks, it is particularly important to ensure they are not able to eat anything they shouldn’t over the fence, and that signs are displayed to discourage well-meaning passers-by from feeding them.

How does toxicity occur?

The toxic effects of poisonous plants and their resulting symptoms can vary greatly and may be affected by the type of plant and the amount consumed, as well as how sensitive the horse is as an individual. Some toxins may have an impact very quickly, within a few minutes or hours, whereas others may not cause any visible problems initially but could have a cumulative effect within the horse’s body as more is consumed over time, with symptoms only being seen after months or years of consumption.

Clinical signs

Usually, the digestive system is where the problems start, but the horse’s other organs and nervous system may also be affected. Poisoning can therefore lead to a wide range of clinical signs, including:

• Loose droppings
• Colic or abdominal pain
• Weight loss
• Behavioural changes
• Tremors or convulsions
• Reduced appetite
• Constipation
• Jaundice (a yellow tinge in the eyes and the gums)
• Increased sweating
• Changes to the heart rate, breathing rate and temperature
• Red or blistering mucus membranes
• Differences in the skin, e.g., photosensitivity
• Difficulties with balance and coordination
• Lethargy and depression
• Death

Since most of these can also be associated with other diseases or health problems, it can sometimes be difficult for vets to diagnose the issue, unless the horse has actually been caught in the act of eating something known to be toxic. But what are the main plants that typically pose a risk to horses in the UK and how do we know what to look out for?

Common poisonous plants for horses

The list below shows some of the more common plants in the UK which can have toxic effects in horses. Using plant ID mobile apps or your search engine’s AI technology can be helpful to identify unknown plants around your horse’s grazing area and ensure that none are poisonous.

Bracken (Pteridium aquilinum)

• Contains an enzyme that destroys vitamin B1, leading to deficiency. Initially causes weight loss and ill thrift and later muscle weakness and ‘bracken staggers’, followed by death if not treated in the early stages.
• Remains toxic even when cut and dried. Bracken is mostly found on hill and moorland. May be consumed for 1-2 months before clinical signs are seen. The roots (rhizomes) are five times more toxic.

Buttercups (Ranunculus spp)

• Contain a compound that causes inflammation and ulceration in the mouth. Can also cause colic if consumed in very large quantities.
• Loses toxicity when cut and dried.

Common Privet (Ligustrum spp)

• Causes digestive upset, paralysis and convulsions. Death can occur 4-48 hours after consumption.
• Most common as garden hedges and paddock boundaries. Prunings and clippings remain toxic.

Cowbane (Cicuta virosa)

• The roots are particularly toxic. Causes excessive salivation and spasmodic colic with even small amounts potentially being fatal.
• Most likely to grow in damp locations and may be more of a risk when farmers clear ditches and expose roots.

Foxglove (Digitalis purpurea)

• Causes diarrhoea, abdominal pain, tremors and convulsions, irregular heartbeat or heart failure. Can be fatal.
• Grows on woodland edges and close to hedges. Remains toxic when cut and dried. Usually only eaten if found in hay.

Hemlock (Conium maculatum)

• Causes paralysis and convulsions. Can be life threatening as it causes death by respiratory paralysis.
• Grows beside streams/ditches. All parts of the plant are poisonous with the seeds containing the highest levels of toxins.

Horsetail (Equisetum spp)

• Causes similar toxicity as bracken.
• Remains toxic even when cut and dried.

Laburnum (Laburnum anagyroides)

• Contains toxic alkaloid in all parts of plant especially the seed. Life threatening if consumed in large quantities. Causes salivation, colic and convulsions.
• Ornamental tree mostly found in gardens but can escape to the wild. Has long, thin, grey seed pods with hard black seeds over winter.

Meadow Saffron (Colchicum autumnale L.)

• Contains a substance that inhibits cell division which results in a range of clinical signs including difficulty swallowing, loose droppings and colic. Can be fatal.
• All parts of the plant are toxic. Becomes more palatable to horses when cut and dried in hay but still retains its toxicity.

Nightshade (Atropa belladonna)

• Causes dilated pupils, drowsiness, inflamed mucus membranes and the inability to stand. Can be fatal.
• Grows on woodland edges, close to hedges and on wasteland. All parts are poisonous.

Oak Trees (Quercus spp)

• All varieties of oak contain toxic tannins. Can cause colic and liver damage. Can be life threatening.
• Some horses will actively seek out acorns and leaves to eat. Any acorns found on grazing land should be collected.

Ragwort (Senecio spp)

• Causes damage to the liver over time. Symptoms do not show until the liver is severely damaged by which stage it may be too late.
• Remains toxic even when cut and dried. Very bitter plant so is generally avoided in the field. Becomes more palatable when cut and dried. A comprehensive guide on controlling and removing ragwort can be found on the BHS website.

St. John’s Wort (Hypericum perforatum)

• Contains a pigment that causes photosensitisation – skin becomes sunlight sensitive and lesions/scabs form.
• Grows in dry grassland, meadows and open woods.

Sycamore (Acer pseudoplatanus)

• Causes Atypical Myopathy. Sycamore seeds and possibly the leaves contain the toxin hypoglycin A that prevents energy being produced within the horse’s muscle cells. The disease results in muscle damage, and particularly affects the muscles that enable the horse to stand, the muscles that allow breathing and the heart muscle. Can be fatal if not treated immediately.
• More common in autumn (peak October) when the ‘helicopter’ seeds are falling.

Yew Tree (Taxus baccata)

• Very toxic plant, no cure and very quick to kill as it causes heart failure.
• All parts of the plant are toxic and remain so even when cut and dried.

More information on common poisonous plants and advice on their management and removal is available from the following organisations:

• The British Horse Society
• The Blue Cross
• World Horse Welfare

What should I do if I suspect my horse has eaten something poisonous?

As a first priority in this situation, the vet needs to be called, especially if your horse is showing any clinical signs. The horse should be prevented from consuming any more of the plant or substance and be provided with free access to plenty of their normal water supply. If they are becoming unwell, try and move your horse to a safe area, such as a sand pen, smaller paddock (with no access to other poisonous plants) or a large, well-bedded stable, whilst still ensuring they have drinking water freely available.

The next course of action is to try and identify what your horse may have eaten. If it is very apparent or if you witnessed them in the act, collect a sample of the plant in a small bag and take a photo to show your vet. However, if you are unsure, consider the following:

• Are there any other horses on the premises who are showing similar symptoms?
• Check the feed, bedding and forage for any signs of contamination – remember that many poisonous plants will retain their toxicity even when dried and may be readily consumed by the horse if mixed in with the hay.
• Check the field and surrounding trees and hedges for any unknown or poisonous plants – has your horse recently moved to a new field?
• Is it possible that your horse may have consumed something unbeknownst to you? For example, could they have had a quick mouthful of vegetation while out hacking, or could a well-intentioned neighbour or passing walker have fed them over the fence or stable door?
• Remember that poisoning can be caused by other substances and not just plants – could your horse have accidentally consumed anything like rodent poison, insecticides, herbicides or any other non-food substance?
• Is your horse currently receiving any medication? Clinical signs similar to those of poisoning can also occur as a result of an unexpected side effect or potential overdose.

Your vet may take samples of urine, blood or possibly gastric contents to analyse for the presence of any plant residue or other chemicals, but laboratory processing times could mean the results are slow to arrive. Doing your own detective work to try and assess the cause may therefore give you a quicker indication of what has happened and take steps accordingly to ensure that no other horses on the premises become affected.

Once the vet has assessed the horse, they will be able to advise on the next steps and the appropriate course of treatment. As with all health problems though, prevention is better than cure, so it is advisable to be vigilant and do what you can to reduce the likelihood of your horse consuming a poisonous plant in the first place.

Top tips for reducing the risk of plant poisoning

• Familiarise yourself with common poisonous plants for horses
• Inspect your horse’s field and surrounding plants, hedges and trees regularly and take measures to fence off or remove anything that could be poisonous
• Keep on top of your grassland management with appropriate use of fertiliser to help support grass growth which can then push back undesirable plants
• In sparse or overgrazed fields, ensure that sufficient supplementary hay or alternative forages are provided and that your horse is not being ‘bullied’ away from accessing this by their field mates
• Don’t allow your horse to consume any vegetation when out hacking if you cannot clearly identify it as being safe
• Check bales of hay and haylage before feeding for any signs of foreign plants or weeds and ensure that any present can be identified as safe
• If applicable, place signs around your horse’s field to warn members of the public about the dangers of feeding horses
• Be vigilant and know what is normal for your horse to help you identify problems sooner

References

• Aboling, S. (2023) Do poisonous plants in pastures communicate their toxicity? Meta-study and evaluation of poisoning cases in Central Europe. Animals, 13(24), p.3795.
• Mueller, C., Sroka, L., Hass, M.L., Aboling, S., These, A. and Vervuert, I. (2022) Rejection behaviour of horses for hay contaminated with meadow saffron (Colchicum autumnale L.). Journal of Animal Physiology and Animal Nutrition, 106(2), pp.327-334.