There are many small plant species that are toxic to livestock when grazed. Below is a collection of common poisonous forbs, grasses, algae and crops.
Blue-green algae (Cyanobacteria spp.)
Poisonous parts
It is very difficult to distinguish toxic algae from non-toxic species without professional assistance.
Toxic blooms will often produce an odour.
Location
Throughout Australia in fresh and brackish waterholes. Water samples containing bloom material should be sent to laboratories that specifically test for algae toxicity. It is recommended to use a NATA (National Association of Testing Authorities) accredited laboratory.
Symptoms of poisoning
Sudden death near a water source is the most commonly found symptom of blue-green algae poisoning. If observed quickly, cattle may tremble, stagger and become comatose.
Treatment / management
Treatment of poisoned livestock is often unrewarding but if provided early, activated charcoal can assist mildly affected livestock.
Stock must be denied access to the water source until the toxic cyanobacteria can be removed. The New South Wales Government conducted a webinar regarding blue-green algae in May 2024; the notes from this webinar answers questions such as how to manage a toxic algal bloom.
Notes
The risk of toxic algae blooms is much higher when there is increased concentration of nutrients in still or stagnant bodies of water. These nutrients can come from both animal waste and fertilisers. Blue-green algae is also harmful to humans, so care should be taken around any blooms that are suspicious.
More information from the Government of Western Australia can be found here: Blue-green algae poisoning in livestock, or from Queensland Government, here.
Cyanide or prussic acid poisoning — Sorghum
Poisonous part
Young sorghum plants growing rapidly, particularly after a period of stunted growth. Crops that are wilted, crushed, frosted, trampled, or have hail or insect damage, are also a risk. Leaves contain the highest concentration of toxin, followed by stems, then seeds.
Toxin
Cyanide glycoside.
Symptoms of poisoning
Cyanide or prussic acid poisoning generally happens very quickly (within an hour), and animals are usually found dead.
When cyanogenic glycosides in plants (commonly sorghum) is metabolised in the rumen, hydrocyanic acid is released preventing the essential oxygen exchange from the blood into tissues and organs. Death can be rapid as the major organs start to fail from lack of oxygen.
If you were to see clinical signs, they could include:
- accelerated and deepened breathing
- irregular weak pulse
- excessive salivation or frothing
- twitching muscles
- spasms
- staggering
- unconsciousness.
Bluish discolouration of the gums is sometimes witnessed before death. Blood is bright red in colour and doesn’t clot readily.
Treatment / management
Livestock should not be allowed access to young plants that are stressed or less than 75 cm high. Cattle should have a full belly before being provided access to sorghum.
Cyanide content can be measured in the plant by sending samples off to an approved facility, details can be found here.
Notes
In a sorghum crop, the risk of cyanide toxicity is greatest during a period of drought. Early regrowth and young plants contain the most toxin. The problem will persist in hay made from affected crops.
As with all cases of toxicity – the severity of poisoning will vary by the size of the animal and concentration of toxin consumed. Failed sorghum crops can be made into silage and fed in conjunction with other roughage as an alternative to livestock being allowed to graze toxic plants.
Theoretically, a sodium thiosulphate solution can be administered by intravenous or oral methods for treatment of toxicity by a veterinarian. However, in most cases, toxicity and death occurs too rapidly to treat animals successfully. Surviving stock should be removed from the toxin source and drenched with sodium thiosulphate even if they are not showing clinical signs of poisoning.
More information is available from the Queensland Government here: Cyanide poisoning from sorghum.
A fact sheet on prussic acid from Feed Central can be found here.
Darling pea (Swainsona spp.)
Poisonous parts
All parts are poisonous. Once grazed consistently for a period of weeks, stock appear to become ‘addicted’.
Location
Australia wide.
Toxin
Swainsonine.
Symptoms of poisoning
Swainsonine inhibits the normal processing of the sugar, mannose, leading to a build-up of this sugar, interfering with cells particularly in the brain, followed by liver and then other organs.
Clinical symptoms of consumption for 10 weeks or less include:
- a loss of condition
- nervous malfunction where animals are easily excited
- awkward in gait
- head shaking and head pressing against objects
- star gazing (head tilted upward and staring).
Grazing of Swainsona on a long-term basis will lead to infertility and abortion.
Treatment / management
It may take 3-4 months for animals to die if grazed constantly. They may recover if they are removed from infested pastures in the early stages of poisoning. Non-pregnant cattle and sheep can graze pastures for up to four weeks without serious effects. Horses are more susceptible and should be removed after two weeks of grazing infested pastures and not returned for another four weeks.
Notes
Swainsona species are known to grow rapidly after rain and may develop in large stands in areas that frequently flood.
Australia’s poisonous plants, fungi and cyanobacteria book states the following Swainsona species are poisonous to stock:
- Swainsona canescens
- Swainsona luteola
- Swainsona brachycarpa
- Swainsona galegifolia
- Swainsona greyana
- Swainsona luteola
- Swainsona procumbens
- Swainsona swainsonioides.
The Northern Territory Government has more information on Swainsona poisoning.
