We are fortunate here in New Zealand that we don’t have any real nasties like snakes or scorpions lurking in the grass; however, there are plenty of other potential problems in our paddocks.
Of course, horses are very well known for finding something to injure themselves on even in the safest-looking paddock, but that’s not the only potential danger.
Plants and trees that are toxic to horses, ponies and donkeys may be present either in the pasture itself, within reach in the garden over a fence, or by way of seeds/fruit blown into their paddocks.
Many of these will not be eaten unless there is a shortage of feed (for example if the horse is on a diet, or in a drought situation) but it is important to be aware of the potential they have to cause problems.
Some of the plants and trees (this list is not comprehensive) to be aware of are:
Plants: black bryony, black nightshade, bracken, buttercup, celandine, charlock, columbine, cowbane, deadly nightshade, foxglove, flatweed (false dandelion) ground ivy, hellebore, hemlock, water-dropwort, holly berries, horsetail (mare’s tail), ivy, Iceland poppy, larkspur, lily of the valley, linseed, meadow saffron, monkshood, Mexican devil weed, oleander, onions, potato, privet, ragwort, rhododendron, St John’s wort, spurge, white bryony and woody nightshade.
Trees: avocado, black walnut, elder, horse chestnut, laburnum, laurel, laurel cherry, leylandii, ngaio, oak, plum, peach, red maple, sycamore, tutu, wild cherry and yew
Now we will look in more detail at some of these plants, and the problems they cause.
Ragwort is prevalent in many areas; you often see it growing in areas of wasteland and it may be seen growing in paddocks. It is very noticeable when the distinctive yellow flowers are present. Most of us are aware of the dangers of ragwort; if you have any growing in your paddocks you should get rid of it. It is one of the poisonous plants that horses will usually avoid eating when it is growing (unless there is not much grass) but once it is made into hay it becomes more palatable – so definitely get rid of it from your hay paddocks. The toxins from ragwort build up, and result in liver disease. How quickly the signs are seen depends on how much the horse has eaten, and it can take from four weeks to 12 months after eating the plant for signs to be noticeable. Early signs are the horse going off his food, weight loss and possibly jaundice (a yellowing of the normally pink areas of the gums and the conjunctiva around the eye). Later signs can be diarrhoea, swelling under the chest and of the legs, increased thirst and even being very itchy. Blood samples will show up liver disease. To get a definitive diagnosis, a liver biopsy needs to be taken, but often a diagnosis may be made if there is a history of eating ragwort, and the clinical signs and blood tests show liver disease. Unfortunately, there is no specific antidote, so only general supportive care can be given until liver failure occurs. It’s clearly much better to prevent this by getting rid of any ragwort plants you see!
Yew is commonly seen growing in gardens, and so if your horse’s paddock borders your garden you should check for any yew. Most animals will not graze from a growing tree, but wilted cuttings are palatable, and so they will eat them. It is an important one to look out for as just 100-200g of yew leaves will kill a horse, by causing heart failure. Only a very small amount of the leaves are needed to cause sudden death, so please be very aware of it.
Sycamore seeds are responsible for the awful, fatal disease called atypical myopathy. This is the result of a toxin (called hypoglycin A) which is found in the seeds of both the sycamore and box elder trees. We usually see this disease in the autumn when the seeds are on the ground, but there have been reports of it happening in the spring, which is thought to be from horses eating the newly grown seedling of the tree. Signs may start gradually; often there are a couple of days where the horse is a little depressed before the more severe signs are apparent. These signs are weakness, stiffness, trembling, lying down, sweating, irregular breathing and a fast heart rate but with a normal body temperature. A very noticeable sign is an unusual coloured dark red/brown urine. Call your veterinarian urgently if you see these signs. Two horses in North Canterbury died from atypical myopathy from eating sycamore seeds last autumn.
Treatment is really only supportive, and ideally needs to be in an equine hospital. Survival rates at present are only around 26%, so not good. There are some additional risk factors; young or very old horses seem to be affected more often, as do horses with no access to a salt block. There is still a lot more research to be done to understand this disease, but it is important to be aware of it, as sycamore trees are fairly common.
Stringhalt
The condition stringhalt is where horses have a very strange action with their hind legs. As they move, one or both hind legs are quickly snatched up and held up really high. There can be different causes, but one type of stringhalt occurs after grazing a certain type of pasture, affects both hind legs and is associated with nerve damage. The condition begins suddenly, usually in late summer or autumn, and in severe cases may affect the horse so badly that it is unable to move.
It is thought that it is due to eating large amounts of flatweed (hypochaeris radicata), otherwise known as false dandelion or catsear, under certain conditions. More research needs to be done to find out exactly what is the cause. Fortunately, most horses recover after being taken off the suspect pasture; however, it can take many years, and some still have lingering signs. Horses can be so severely affected that they have difficulty walking so it is important to call your vet if you see your horse developing an unusual gait. There are some treatments that may help.
Ryegrass staggers
Ryegrass staggers (or to give it the proper term, perennial ryegrass staggers) is caused by mycotoxins from an endophyte (fungus) that grows in a certain kind of perennial ryegrass.
The most well-known toxin produced by this fungus is Lolitrem-B. The toxin can cause problems in horses (and other animals) in late spring, summer and autumn – it all depends on the conditions. It is usually when horses are eating shorter, well-grazed grass, as the toxin concentrates lower down in the grass. When horses are affected by it, they show signs of muscle trembling and behavioural changes and they often stagger around – and even fall over in very severe cases. Call your veterinarian if you see these dramatic signs, as they will need to first make sure it isn’t anything else that may need to be treated differently. Then, your horse will need to be moved off the grass to a safe pasture. This can be either endophyte-free ryegrass, a non-ryegrass pasture, off pasture altogether or to a paddock with long grass, as this will reduce the exposure to the toxin. Your veterinarian will advise if any other treatment is needed, but your horse should make a fairly rapid recovery.
Anti-coccidials
One other potential source of toxins for horses that is not from the plants, but may be found in the paddock, is in calf feed. Most of this type of feed (if not all) has an additive to prevent coccidiosis, an infectious diarrhoea in calves. Monensin and Lasalocid are two kinds of anti-coccidials. Horses should not have access to calf feed containing anticoccidials, as they are very sensitive to toxicity from these drugs; they only need to eat a relatively small amount to be affected.
The bags carry a warning, but accidents do happen and we vets do see cases of toxicity from horses getting access to calf feed from time to time. These can range from an extremely severe toxicity with shock and death within a few hours, to severe colic with watery diarrhoea, sweating, muscle weakness and wobbling, fast heart rate and low blood pressure. This can last for up to four days before the poor horse deteriorates and dies (or more likely, humane euthanasia will have taken place). There are some horses who have less severe effects, and recover with supportive treatment, but they may still show long-term effects such as reduced athletic performance, unthriftiness and heart failure.
**There is unfortunately no specific antidote, so avoiding calf feed is vital. Don’t feed any calves with meal in the same paddock as your horses and ponies (especially greedy ones) and keep calf meal in a safe place**.
Summary
There are lots of potential problems in the paddock to look out for, but most of the time our horses will be safe. Get rid of any obvious problems like ragwort and yew, and avoid grazing near sycamore trees. Don’t feed calf meal in a paddock with horses, and be careful if grazing a horse or pony next to a garden as they may have access to a lot of different plants. There is information readily available on the internet about which plants may be toxic, so do some research yourself. Look out for any changes in your horse’s behaviour or health as always, and ring your veterinarian with any concerns.
