Plants poisonous to livestock

Plant poisonings in livestock most commonly occur when animals are introduced to unfamiliar surroundings containing toxic plants or when other feed is scarce, and animals go in search of the green pick.

When grazed in small amounts, many of these potentially toxic plants may be tolerated well by livestock. However, under certain conditions, or if grazed to excess, poisonings can occur. Cattle, sheep, and horses are vulnerable to plant poisonings that can result in death or permanent damage to organs such as liver or kidneys.

Plants may be toxic in small amounts, or have a cumulative effect on animal health over time as plant toxins cause damage to internal organs.

Good farm management includes learning to recognise plants in your area that are toxic to livestock, as well as how to remove poisonous plants from pastures or control the conditions under which poisonings are more common.

Visit WeedWise to learn how to identify plants that may be toxic.

Plants toxic to livestock

Examples of potentially toxic plants grazed by livestock in Australia include:

• Pimelea
• Flatweed
• St John's Wort
• Common and blue heliotrope
• Paterson's curse
• Caltrop and Yellow Vine
• Panicum grasses
• Green cestrum
• Mother-of-millions

Common and blue heliotrope

Heliotrope can be found in paddocks with minimal competition and can quickly spread across a paddock. Although called blue, the tubular flowers are actually purple or lilac with yellow centres. These plants often flower in spring and autumn and this is the preferred growth stage to target this weed with herbicides, as control is often quite difficult.

Stock will not target heliotrope species for grazing when there are more palatable species available, however they will when there is little other choice. Both plants contain pyrrolizidine alkaloids which causes chronic liver damage in stock. The extent to which the liver is damaged depends on the quantity ingested and which species consumed it. While cattle and horses are more susceptible to pyrrolizidine alkaloids, sheep (in particular cross bred sheep) are more often affected due to being more likely to consume blue and common heliotrope.

Symptoms will not appear initially and are often seen after the stock has been removed from paddocks containing the weed. In sheep the damaged liver accumulates copper. When stock experience a stressful event later in the season, this triggers the release of copper from the liver, causing copper toxicity. This breaks down red blood cells, and impacted stock can be observed to wander off alone one day and dead the next. The colour of their skin is generally a bright yellow at this point.

Pictured above: Heliotrope.

Caltrop and Yellow Vine (Tribulus sp.)

Caltrop (commonly known as Cat Head) is an introduced annual summer broadleaf with a small yellow flower and spine covered fruit. Yellow Vine is an annual summer native species and has a larger yellow flower with spineless fruit. Dusting is a major issue when attempting to control these weeds with herbicides, due to their semi-erect to prostrate growth habits. Dust can gather on leaves and create a physical barrier, reducing the efficacy of herbicide applications. In particular, glyphosate readily binds to soil particles present as dust or in dirty water, causing poor control.

Tribulus sp. can pose a serious risk to animal health. They are capable of causing severe acute liver disease and secondary photosensitisation. Saponin is the primary toxin which causes crystals to form in the liver. The secondary toxin is currently unknown and could potentially explain the sporadic nature of the disease. Generally, risk is highest for 10 days after small rainfall events as Tribulus sp. often provide the first green pick. Goats and juvenile sheep appear to be the most susceptible.

Pictured above: Caltrop.

Flatweed

Flatweed (Hypochaeris radicata) is also known as catsear and false dandelion. Flatweed is a perennial plant originally from Europe, and because it has yellow daisy-like flowers, it is often confused with Dandelion (Taraxacum officinale) and Hawkbit (Leontodon taraxacoides).

Flatweed is a problem in horse pastures because it can cause stringhalt when it is grazed by horses. Australian Stringhalt is a condition of horses resulting in the hindlegs jerking upwards as the horse walks, with the legs reaching up and sometimes making contact with the belly. The abnormal gait is a result of involuntary control of the hindlimbs, which is suspected to be caused from a neurotoxin that affects the long nerves in a horse’s body.

Veterinary attention should be sought if you suspect stringhalt in a horse.

On smaller areas, flatweed can be controlled by removing the entire plant and taproot, while in larger areas, a broadleaf weed herbicide maybe appropriate. Best practice should also include establishing a competitive pasture, so that pasture grasses and legumes can out compete the weeds.

Pictured above: Flatweed.

Green cestrum

Green cestrum is toxic to animals including cattle, sheep, horse, pigs, poultry, domestic pets and humans.

All parts are of the green cestrum plant are poisonous. Livestock poisoning usually occurs when other feed is scarce.

Eating green cestrum can also taint animal products, such as milk.

To mitigate the spread, landholders must not allow plants to set seed. This is best achieved by destroying the plant. Herbicides are often the most effective and economical method of control. Recently sprayed and wilting plants are more palatable than fresh healthy plants and, potentially, can cause more deaths.

Pictured above: Green cestrum.

Mother-of-millions

Poisonous to livestock, domestic pets (especially dogs) and humans. Poisoning is most common in May through October during flowering.

The toxins from mother-of-millions accumulate over time. Even small amounts add to toxic build-up. Stock eating small amounts over several days may develop diarrhea (sometimes bloody), drool saliva, dribble urine and then die of heart failure.

Large amounts eaten cause sudden death. 5 kg will kill an adult cow. Poisoned stock must be treated within 24 hours of consuming the plant. If suspected, consult a vet immediately.

Pictured above: Mother of millions.

Panicum grasses

Panicum grasses, particularly Hairy Panic, are toxic to livestock. When consumed in high amounts, Panicum grasses can cause liver damage and photosensitivity in sheep. Younger stock such as lambs are particularly susceptible.

Signs to look out for include swelling of the face, ears and legs. Affected stock may be very sensitive to sunlight, actively seeking out shade. Scabs similar to sunburn may develop on the head and ears, and significant production losses can occur from liver damage.

Producers should avoid grazing lambs on paddocks containing high amounts of Panicum grasses. Spraying is an option as long as the weed is not grazed straight afterwards when the rising plant sugars cause an increase in palatability and intake. Once the plant has browned off, sheep may prefer to graze other available pasture. However, with any ongoing summer rain, further germination of Panicum grasses may occur.

Pictured above: Panicum grasses.

Pimelea

A number of native Pimelea species, including Desert rice flower, are toxic to livestock, with cattle being particularly susceptible.

Pimelea are common winter and spring annuals, however, they have been found to germinate following decent rainfalls on the back of dry seasons.

The plants are often grazed inadvertently when hungry stock are introduced to new green pasture and they do not need to ingest much of the plant at any stage of its growth for it to be toxic.

Landholders who are unsure if their stock have ingested the plant should look signs of swelling under the jaw and brisket area, severe sudden loss of weight, foul smelling scours/diarrhoea, a rough coat or sudden death.

Feeding good quality hay and keeping stock off pasture which contain the plant is the best way to avoid the potentially disastrous effects.

Pictured above: Pimelea.

Paterson's curse

Paterson’s curse produces toxins called pyrrolizidine alkaloids. These alkaloids cause liver damage in stock, which is irreversible and cannot be treated. It is also ongoing - as the affected liver cells die, the healthy liver cells next to them take up the released alkaloids and in turn are then poisoned by them.  It is generally believed that the amount of toxin in the plants increases as the plant matures.

Different livestock species have different susceptibility to poisoning by Paterson’s curse due to the way that they break down pyrrolizidine alkaloids.

Pigs and horses are highly susceptible as they have no ability to break down the alkaloids in their stomachs.

Cattle are moderately susceptible as the rumen contains bugs that can break down the alkaloids before they reach the liver, which prevents some damage. However, if they have no choice but to eat large amounts of Paterson’s curse over a period, they can still end up with chronic liver failure.

Sheep and goats are the least susceptible to poisoning. Their rumen does a better job of breaking the alkaloids down into less toxic parts. Toxicity in sheep is usually due to continued exposure over several years, especially in combination with years of ingestion of heliotrope.

Chronic liver damage can lead to death from chronic copper poisoning or chronic liver failure. Clinical signs include weight loss and jaundice (yellowing) of the carcass, or the mucous membranes such as around the eyes. Sometimes you see photosensitisation, a sunburn-like reddening and peeling of the skin in non-pigmented, non-woolled areas. Pigs, horses and cattle can sometimes show nervous signs of blindness, wandering and head-pressing. This is due to the damaged liver not filtering toxins from the body, which can then start to affect the brain.

Pictured above: Paterson's Curse.

St John's Wort

The whole St John’s Wort plant is poisonous and affects all grazing animals including horses. With repeated grazing, St John's Wort can cause photosensitisation causing livestock to become sensitive to light and develop sunburn. Skin damage is observed on pale or non-pigmented skin or areas with less wool such as the nose, face and ears of sheep. Animals also experience weight loss, agitation, reduced productivity, depression and sometimes death.

St John’s Wort is an erect, woody perennial herb. Leaves and branches are always opposite one another on the stem. The leaves are 1.5 to 3cm in length with an ovoid shape, when held up to the light oil glands appear as perforations.

Flowering in late spring to summer, the 5 petals are golden yellow, with black dots along the margins. Seed capsules form and ripen from green to brown in late summer/early autumn. These sticky capsules easily adhere to animal fur or clothing and are dispersed far and wide.

Two varieties of St John’s Wort may be found – a narrow leaf (7-9 mm at the sixth node of a flowering stem) and a wide leaf (10-12 mm). The broad leaf variety is more compact, growing to around 60 cm compared to 90 cm, has thicker stems (6 mm compared to 4.5 mm), and larger seed pods than the narrow leaf variety. The broad leaf variety also has fewer oil glands and so lower levels of hypericin.

Bright sunlight is required to activate the toxin in the bloodstream before signs of poisoning develop, but photosensitisation may occur within five hours. If you suspect St John’s Wort is causing photosensitisation in your animals, move them to another paddock free from St John’s Wort with good shade cover. Secondary infections may require treatment from your veterinarian.

The best strategies to manage St John’s Wort are to reduce its ability to establish and spread by maintaining good perennial pasture cover and preventing invasion through good property biosecurity.

Pictured above: St John's Wort.

Common garden plants that are toxic to livestock

Not all trees and plants commonly found in or around gardens are safe for livestock. On small landholdings where stock have access to household gardens, this can be fatal.

Most cases of toxicity due to garden plants occur from livestock grazing on plants located in the garden by direct access or from over the fence. Toxicities also occur when stock graze garden clippings that contain toxic plants.

In some plants, the clippings increase in toxicity as they dry out and they quite often become more palatable. So, to keep your garden and your livestock happy and healthy be careful where you put your stock and what you feed them.

Listed below are a few of the more common plants and trees you might find in or around your garden that are toxic to livestock. This is not an extensive list of all toxic plants. If you do not know if a plant is toxic or not, it is always best to assume that it is and prevent access to the plant.

Trees

Sugar Gums and Prunus spp. contain cyanide/prussic acid that can cause sudden death within hours of consumption. White cedar trees are toxic to sheep, cattle, and pigs. The small white berries are the most toxic part of the tree. Toxicity can cause muscle tremors leading to collapse, seizures, and death within 48 hours.

Shrubs

Oleander and Yew are toxic to ruminants, pigs, and horses, and cause sudden death due to a fatal heart attack. It can take as little as five oleander leaves to kill a cow.

Pictured above: Oleander.

Plants

Foxglove and mother of millions can also cause a fatal heart attack. Some succulents from the Crassulaceae family can affect the heart, gastrointestinal system and/or cause nervous signs. Rhododendrons and azaleas can affect the gastrointestinal and/or respiratory systems.