Toxic plant species and symptoms

Cyanide in sorghums and other grasses

Some fodder and pasture grasses - particularly sorghums and, to a lesser extent, couch grasses (Cynodon and Brachyachne spp.) and occasionally other grass species - can accumulate cyanide (prussic acid). This is more likely during overcast periods or very hot weather when plants wilt during the heat of the day.

Death is usually very rapid; however, sick animals may show rapid deep breathing, salivation, rapid weak pulse, muscle twitching or trembling, spasms, staggering and sometimes a bluish discolouration of the gums. Urgent veterinary treatment is required to save the animal.

Read more about cyanide poisoning.

Nitrate in sorghums and other grasses

In addition to cyanide, sorghum and other grasses - including oats (Avena sativa), ryegrass (Lolium spp.), maize (Zea mays), button grass (Dactyloctenium radulans) and liverseed grass (Urochloa panicoides) - can accumulate toxic amounts of nitrate. This is more likely when growing in nitrogen-rich soils (e.g. cattle camps, fertilised pastures) and when the plants wilt or the weather is overcast.

Other non-grass plant species which tend to be associated with cases of nitrate poisoning in livestock include pigweed (Portulaca spp.), mintweed (Salvia spp.), capeweed (Arctotheca calendula) and variegated thistle (Silybum marianum).

Death is often rapid, but animals may be seen with rapid, gasping breathing, bluish gums and convulsions. Blood is usually chocolate-coloured, which can make the membranes of mouth and eyes even look muddy or brown. Urgent veterinary treatment is required to save the animal.

Read more about nitrate poisoning.

Pyrrolizidine alkaloid in plants

Some plants normally accumulate toxins and are generally always toxic. One such plant group is Crotalaria spp. (rattlepods). This group accumulates pyrrolizidine alkaloids, which cause liver disease and, if severe enough, will kill the animal.

In cattle, symptoms include poor growth or wasting, weakness and collapse. There may be aimless walking, staggering and apparent blindness. Occasionally photosensitisation, jaundice straining, scouring, prolapse of the rectum and drooling may be seen.

Horses usually show similar signs of weight loss, slight jaundice and abnormal behaviour. Other signs include sleepiness, lethargy, yawning, muscle twitching, uncoordination, irritability, apparent blindness and compulsive walking. Some may show paralysis of tongue and larynx with associated breathing difficulty.

Generally, there are no specific treatments for pyrrolizidine alkaloid poisoning. Stock may recover with supportive therapy, but damage is usually permanent and cumulative. Affected animals should probably be culled to slaughter after recovery, as further exposure or other stresses may result in the animals failing to thrive or succumbing to other diseases.

Other toxic plant species

Other plants that might be a problem include:

  • Noogoora burr (Xanthium strumarium) - Mass germination can occur readily in post-flood conditions, exposing animals to a palatable, concentrated source of toxin. The toxin found in noogoora burr (and green cestrum) is acutely and severely toxic to the liver, with animals mostly found dead. Those still alive may show depression, abdominal pain and/or nervous signs.
  • Bracken (Pteridium esculentum) and mulga fern (Cheilanthes sieberi) - Fern species are often drought-tolerant with rapid revival after rain. The toxin ptaquiloside may cause different syndromes depending on the dose and duration of exposure. Acute poisoning often results in fatal infections and/or haemorrhages due to bone marrow suppression.
  • Nardoo (Marsilea spp.) - Common on floodplains and around transient water bodies. Thiaminase can be found in nardoo (and bracken and mulga ferns), with the highest concentration in lush young fronds (and rhizomes). Horses tend to develop incoordination, while sheep show signs of polioencephalomalacia, including star-gazing, head pressing and apparent blindness.
  • Fungal-infected grass seed heads - A number of native and introduced grass species may become fungal-infected following warm, moist conditions, with resultant mycotoxin production and poisoning of stock. Signs can vary but may include nervous signs or ‘staggers’.
  • Darling peas (Swainsona spp.) - Shoots from perennial rootstock following floods. Generally, more than than 2-4 weeks of consumption is required, with signs of weight loss, incoordination or erratic behaviour resulting.
  • Lantana (Lantana camara) - May be attractive to stock on lush pasture, as it is often drier and provides more bulk. Signs include jaundice, dehydration and photosensitisation. Not all colour varieties are toxic.
  • Castor oil plant (Ricinus communis) - Grows especially along streams. Poisoned animals will develop severe diarrhoea.

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