Many plants that grow in Australia are potentially poisonous to horses. Such plants can cause considerable economic and welfare problems for horse owners. Under most circumstances, horses will avoid consuming toxic amounts of poisonous plants.
However, some poisonous plants are palatable to horses and some need only be consumed in very small amounts to cause poisoning. Horses may choose to eat poisonous plants when there is little alternative forage.
Also, seasonal conditions may influence the toxicity levels of certain plants, making them more deadly at various times of the year. An awareness of toxic plants growing in Australian and an understanding of the factors that influence the risk of poisoning will help horse owners keep their horses safe.
Pasture management decisions and landscaping decisions should be made with consideration to plants that are potentially poisonous to horses.
The risk to horses from poisonous plants:
Put simply, a poisonous plant is one that will have a detrimental effect on humans or animals if eaten. Plant toxins act as a chemical defence for the plant and most poisonous plants contain toxins that evolved to protect the plant against attack by insects. Some of these toxins are also poisonous to animals.
There are said to be around 1000 poisonous plants that grow in Australia.
However, not all of these plants are considered a risk to horses. Some plants contain toxins that are not thought to affect horse health, some are very unlikely to be found in areas where horses are kept and some need to be eaten in such large quantities to induce poisoning, or so unpalatable to horses, that in practical terms their risk to horses is virtually non-existent.
The plants described in this book are either known to be poisonous to horses because poisoning cases have been well documented, or they are suspected to be poisonous to horses and are commonly found in areas where horses are kept.
Horses are grazing animals that have evolved to selectively browse on pastures that contain a wide variety of plants and grasses. Domestication has altered the diet of horses, their daily habits and the environment in which they live compared to their ancestors. The way that horses and horse properties are managed can play a major role in reducing the risk of plant poisoning.
Risk factors for plant poisoning:
Type of toxin or toxins:
Different plant toxins have different effects on horses. Some plant toxins may have mild effects if ingested and some may be deadly. Plant toxins can affect body systems such as metabolism, the heart, brain, liver or the skin.
Stage of growth:
Some plants contain different amounts or types of toxins at different stages of growth.
Season:
Some plants are only poisonous at certain times of the year.
Environmental factors:
Environmental factors such as drought, flooding and fire can influence the growth of poisonous plants and the amount of toxins that they might contain.
Fertilisers:
The use of fertilisers can cause a flush in growth of potentially poisonous plants and it can increase the amount of toxins in some plants.
Herbicides:
The use of herbicides can make some poisonous plants more palatable to horses and these plants may be eaten when they would otherwise have been avoided. Wilted or dried poisonous plants are often just as toxic as fresh plants. Horses should ideally be removed from pastures that have been treated with herbicides until the toxic plants are no longer present.
Part of the plant eaten:
The different parts of some poisonous plants contain different amounts and/or types of toxins, for example the leaves, roots, bark, flowers and seeds of a plant may contain different levels of toxins and different types of toxins.
Availability of safe alternative feed:
Hungry horses are more likely to eat plants that they would otherwise avoid.
Boredom:
Horses that have low levels of activity or low levels of stimulation from their environment may indiscriminately graze on plants through boredom.
New environment:
Horses that have been moved to a new area may graze on unfamiliar plants. This factor should also be considered when horses are transported to events and are housed in unfamiliar surroundings, even if it’s only overnight.
Age and general health of the horse:
Young and old animals are often at an increased risk of poisoning if they eat toxic plant material. A healthy horse is better able to cope with the effects of a poisonous plant compared to a horse that is already unwell, is under- or overweight or has other health concerns.
Exposure to poisonous plants:
Most pastures contain some level of potentially poisonous plants. A few scattered poisonous plants in a healthy pasture are not generally a risk. Horses will either avoid these plants or they will not consume them in amounts that would cause poisoning.
Some plants however can be toxic if only small amounts are ingested and others are palatable to horses and may be grazed preferentially – these plants should always be controlled in horse pastures.
Poisonous plants should be controlled if a horse is kept on degraded or unhealthy pastures, in a very small paddock or in a yard. If a horse has little choice of alternative forage, the risk of poisoning is greatly increased.
Controlling poisonous plants:
It is good practice to take periodic walks around your property to check for the presence of potentially poisonous plants. This will not only reduce the chance that your horses may be exposed to poisonous plants but will allow you to keep any potential weed or pasture problems in check.
Many poisonous plants are also considered weeds. In the following section the term ‘weed’ refers to all invasive plant species, including ornamental plants, that grow as weeds in areas where horses are kept.
Avoid the introduction of weeds
If your property does not have poisonous weed species growing in the pasture it is best to try and keep it that way. There are some simple management techniques that will help keep your property free of invasive or poisonous weeds.
Make yourself aware of the weeds and plant species that can be invasive in pastures and/or poisonous to horses. Always be on the lookout for potentially invasive or poisonous weed species.
It is much easier to deal with these weeds when only a few small plants are in the pasture, rather than deal with them when they have spread to become large infestations. Some invasive weeds can spread from a few isolated plants to become the dominant species in a pasture in just one season! Check hay that is to be fed on the ground does not contain the seeds of potentially invasive weeds.
If you borrow or hire farm machinery such as mowers, slashers or cultivating equipment, ensure it is clean before it comes onto your property. The same applies to situations where you might lend your own machinery to be used on another property – ask that it be cleaned before it is returned. Weed seeds, root and stem fragments left on farm equipment is a major source of the spread of weeds.
Weed seeds, root or stem fragments can also be introduced onto properties in the mud that is left on the wheels of vehicles. It is a good idea to remove mud from your vehicle before driving onto your property. This is especially important if you have been in an area where particular weeds that you don’t have, or want, are commonly found.
When introducing new stock to your property, whether it is a horse or other livestock species, try to confine the animals in a separate small paddock for the first 10 days to 2 weeks after arrival. Weed seeds can still be viable when passed through an animal’s digestive tract.
This is especially so for horses as most weed seeds are unchanged from one end to the other when they pass through a horse. During this quarantine period manure can be collected and disposed of and the paddock can be watched for the emergence of weeds. Quarantining new animals on a property is also good practice to minimise the chance of a new horse spreading disease to other horses on the property.
Check newly arrived stock for weed seeds that might be present in their coats, manes, tails or in their hooves.
Controlling weeds:
Integrated Weed Management refers to strategies that use the greater combined effect of mechanical, chemical and biological control methods to reduce weed populations. The best way to control weeds in pastures and around horse properties is through an integrated weed management approach.
Pasture management:
The establishment of good pastures is one of the best ways to keep weeds and potentially poisonous plants to a minimum. Keeping down weeds and poisonous plants is of course only one benefit of maintaining good pastures. Good pastures will also supply the nutritional needs of most horses and will reduce the need for supplementary feeding.
There are many good books on pasture management available and one excellent resource for information on growing and maintaining pastures in Australia is the book on pasture management can also be sought from your local agronomist.
Mechanical control of weeds:
Small isolated plants can be pulled out by hand and this is a cheap and simple method of dealing with weeds before they spread or become too large to remove manually. It is always best to use gloves when hand-removing plants and this is especially important when handling plants that are poisonous or have irritant sap, prickles, burrs or stinging hairs.
Mowing or slashing pastures before weed species set seed can be an important management technique to control some particular weeds. It is important to get the timing and cutting height right to ensure the weed problem is reduced and not made worse! Sometimes mowing or slashing a paddock only serves to increase the spread of seed or stem fragments of weed species.
It is always best to consult a local agronomist or weed management agency before devising a mowing or slashing program for your property. Cultivation of paddocks is sometimes the only option when pastures are degraded and are dominated by highly invasive weeds. If you decide to ‘start over’ with your pasture it is important to plan ahead and gather as much information and professional advice as possible before embarking on a complete pasture renovation.
Grazing management to control poisonous weed species:
Sheep, cattle and goats are more tolerant of some poisonous weed species than to horses. In some circumstances it may be useful to intensely graze these animals on infested pastures to control the weeds. Advice should be sought from a local agronomist or weed management agency before implementing this weed control strategy.
The use of herbicides:
Herbicides are often an effective way to control individual weeds or infestations of weeds. It is advisable to contact a local agronomist or weed management agency before using herbicides as it is important to select the correct herbicide and spray program for the individual weed species and the conditions on your property.
It is also important to ensure that the use of herbicides on your property will not adversely affect desirable pasture species, native plants and animals, or the environment, as could occur when using herbicides along waterways.
Biological control of weeds
Biological control is the use of natural enemies to control weeds. Many invasive weeds in Australia have been introduced from other countries but their natural enemies do not exist here. Biological control of these weeds usually entails careful planning and research, culminating in the controlled release of a weed’s natural enemy.
Biological controls can be insects that specifically feed on the target weed species or they can be a fungus or other organisms that cause disease in only the target weed. In some cases, landowners are able to participate in biological control programs for certain weeds.
More information on the status of biological controls for particular weeds can be sought through your local agronomist, weed management agency or your state Department of Agriculture.
Disposing of weeds and poisonous plant materials Individual plants that are pulled from the ground can be composted if they do not have seeds present or nodes from which they may shoot new roots. Individual plants that do have seeds or nodes capable of shooting new roots should be placed in plastic bags before disposal.
Large amounts of weed material can be burnt, composted or taken to council green waste locations. Ensure that potentially poisonous weeds and the prunings of ornamental species are not dumped or composted in horse paddocks. Wilted plant material is often more palatable to horses than fresh plant material and the dumping of poisonous plants in horse paddocks is a significant risk for poisoning.
Information Sourced From:
