Be aware of what is growing in and around your horse's pasture. It will take time but your reward will be the knowledge that you have done your best to keep your horses safe and healthy. Walk around to check fencing for security and safety. Posts work loose with time and nails/screws work loose too so to prevent accidents it’s a good idea to verify the state of your fencing and gates - as if you are moving your horses to a new field.
Remember that Tetanus is deadly and the bacteria that causes it enter mainly through the smallest puncture wound so make sure your horse’s vaccinations are up to date and that there are no rusty nails or screws on the ground or sticking out of fencing. Water troughs need attention too. Check that they are clean and that fixed plumbing is leak free and working properly.
Many common British plants contain chemical compounds that are poisonous to horses. Generally horses won’t eat poisonous plants as they are often bitter or unappetising, but hungry horses may be tempted, so it makes sense to be able to identify problem plants and get rid of them. Inspect pastures regularly, on both sides of the fence! As a horse owner you will know that the grass is always greener on the other side of the fence so make sure that nothing that is harmful for your horse can be reached from the paddock.
Poisoning can range from mild to deadly depending on what is eaten and the quantity eaten. It is often difficult to diagnose as the symptoms are general and in some cases only supportive treatment can be offered by your vet. The best way to avoid it is to remove plants and fence off or cut back problem trees. Many of the plants that are poisonous to horses are very successful at colonising pasture and can drift in from adjoining properties. Some thrive on overgrazed or badly drained pasture where they can out-compete grasses. Remember that many are poisonous when fed in hay and some are more palatable when dry (especially Ragwort) so although your horse may avoid them when grazing it may eat them in hay. Check your hayfields too and only buy hay from a reputable source.
Common Ragwort (Senecio jacobea) is quite easy to recognise from its 3 foot high bunches of flowers mainly in September, though this is the second year's growth and in the first year it is a small flat leafy plant, flush with the grass. It can be mixed up with mixed up with St John’s Wort which is also poisonous but less so. Ragwort flowers between July and September but it is best to pull it out before it flowers to avoid seeds for the next seven years. Learn to recognise the rosette leaves of the first year plant and pull them up along with the root and burn them. Special Ragwort forks are available to help you get the whole plant out. The plant grows to form a small rossette, close to the ground in the first year. This plant then grows in the second year to around a metre high and forms a large head of small yellow flowers. Seeds from these are tiny and are blown by the wind on fluffy spikes, a bit like dandelion seeds. The plant is bitter and will usually not be grazed by horses, but when grass is short in a dry year they will ingest the young plant along with the cropped grass and the symptoms are delayed, usually showing over the winter months. Ragwort contains alkaloids which cause liver damage when ingested over a prolonged period. These chemicals cause cumulative damage and clinical symptoms only appear when liver function has been seriously impaired. Symptoms include: weight loss, diarrhoea, constipation, abdominal pain, photosensitivity, behavioural changes, blindness and jaundice. Treatment is rarely successful as, by the time the symptoms appear, the chronic damage has been done. (a bit like a chronic alcoholic) Euthanasia is usually the only humane course of action.
All parts of the Bracken plant (Pteridium aquilinum) are toxic but horses need to eat a large amount for it to pose a problem. Bracken contains the enzyme Thiaminase which breaks down vitamin B1 so horses that have eaten a lot of Bracken are likely to be suffering from Vitamin B1 deficiency. Symptoms include depression, muscle tremors, ataxia (coordination problems) and a slow heart beat. Treatment involves administering intravenous thiamine, to restore the levels of vitamin B1 and severe cases may need nursing care and antimicrobials. 
Buttercups (Ranunculus) look very pretty in the field but are a sign of poor drainage and horse sick pastures so owners should try to reduce their numbers by improving pasture management. This year’s wet spring means that we are experiencing a bumper crop. Horses usually avoid eating them as they are bitter and their pollen is irritating to some horses. They lose their toxicity when dried so their presence in hay is not problematic, unlike Ragwort, which is worse when dried, or preserves in hailage.

Some trees, or parts of them, are dangerous to horses so you need to check that there areno potentially toxic trees growing in the field or overhanging the pasture. Fencing them off or cutting them back is the best way to manage the situation and protect your horse’s health.

Yew (Taxus) is an evergreen shrub or tree; all parts of it are poisonous. Even if eaten insmall amounts the effects of the alkaloids in the tree often proves fatal. Symptoms include trembling, muscle weakness, convulsions and death. Signs are acute and the horse often dies before treatment can be initiated. That's why it is usually only seen in cemeteries, so that animals have no access to it. Animals can be found dead with the first mouthful still in their mouth!

Oak poisoning tends to be a seasonal problem. In the spring when the buds and leaves are tender and in autumn when the tree drops its acorns. You may have to fence off areas overhanging your horse's pasture. Acorns contain high levels of tannins and some horses will gorge on them. Oak poisoning causes gastro-intestinal problems and kidney damage. Symptoms include reduced appetite, constipation or diarrhoea, abdominal pain, depression and blood in the urine. There is no antidote and so supportive therapy will be prescribed, but it is very serious. 

Some ornamental garden plants are highly toxic. Oleander (Nerium oleander) is one such plant. It is, however, very unpalatable so horses don’t normally eat it but just a small amount is deadly so it is wise to check your garden and avoid planting such plants if you keep your horse at home. Laburnum (Laburnum anagyroides) is a high risk garden tree containing alkaloids and it ranks second only to the Yew in terms of danger to horses. The bark and seeds are more toxic than the leaves. Symptoms include diarrhoea, salivation, ataxia (lack of co-ordination), colic and convulsions. There is no antidote so supportive therapy will be offered. Rhododendrons look nice in flower but are toxic to all animals. Make sure none are in reach.

If you suspect your horse is suffering from poisoning call your veterinary practice immediately. Quick diagnosis and treatment is essential but better still, check your pastures regularly, get to know the problem plants and get rid of them. There is  old poisonous plant legislation that states you must take active measures to remove poisonous plants from a pasture, so if you have  neighbour who ignores this, you can report them to the local authority, in contravention of these acts. Read more HERE