Avoiding sheep poisoning

Some plants are toxic to sheep. Any period of heavy rainfall or flooding, or a spell of very high temperatures, can increase the abundance of poisonous plants.

To prevent contact with poisonous plants, you should avoid situations where your sheep are forced to graze plants they would not normally favour. For example, avoid introducing hungry stock to lush pastures or pasture recovering from a flood, or allowing them to overgraze.

You should also prevent an excess of fluorine in your flock's diet (through water or pasture contamination), as this is toxic to sheep and can cause fluorosis.

How plant poisoning affects sheep

Some pasture plants such as silk sorghum, and common native couch grass, can accumulate prussic acid. The annual urochloa and button grasses can accumulate nitrites, especially if growing in nitrogen-rich soils (e.g. in fertilised pastures). Both prussic acid and nitrites can poison and kill an animal in under an hour if left untreated.

The crotalaria group of plants, including rattlepods, accumulates pyrrolizidine alkaloids, which cause liver disease and, if severe enough, will kill the animal.

Other plants that can affect sheep health include:

  • noogoora burr - will readily germinate to produce highly toxic seedlings in post-flood conditions
  • lantana - may be attractive to stock on lush pasture, as it is often drier and provides more bulk.  Lantana can cause photosensitisation and neurological symptoms
  • castor oil plants - contain poison within the seeds and are often found along streams
  • fireweed - germinates in abundance during autumn, including downstream of previous infestations, and causes damage to the liver.

How to detect plant poisoning

The symptoms of poisoning can come on very quickly in sheep. Sick animals may show rapid deep breathing and weak pulse, salivation, muscle twitching or trembling, spasms, staggering, and sometimes a bluish discolouration of the gums or jaundice.

Identifying and eradicating poisonous plants

It's important to become familiar with the plants your stock may come into contact with when grazing. This will help you to spot any unfamiliar species.

Identify new plants as soon as possible so action can be taken if needed. Move your sheep away from the affected area until the plants can be identified as safe or, if poisonous, eradicated.

Fluorosis in sheep

Fluorine in small quantities in the diet of sheep has a beneficial effect on the teeth and bone, but in excess it is toxic.

The main sources of excess fluorine in the diet are:

  • water containing dissolved fluorides
  • foodstuffs containing high levels of fluorides (e.g. rock phosphate).

Sheep can exhibit 2 types of fluorine toxicity:

  • acute toxicity - when sheep ingest large quantities of fluorine rapidly and death occurs
  • chronic toxicity - caused by the continuous consumption of toxic concentrations of fluorine. This causes defects in tooth enamel of young animals, softening of the bones, osteoporosis, and damage to other organs, including the kidney and heart.

Acute toxicity is rare in Queensland, as sheep are seldom exposed to high-fluorine phosphorus supplements. In chronic cases, the teeth become chalky white, mottled, pitted, and wear excessively. The bone of the lower jaw thickens and bony outgrowths may develop.

Preventing fluorine poisoning

There is no treatment for fluorosis, so prevention is essential.

You should restrict the opportunity for sheep to consume high levels of fluorine. When using rock phosphate in a diet, use low-fluorine phosphate or defluorinated rock phosphate.

You should also restrict access to high-fluorine water. Where possible, allow sheep to drink from bores or surface storage areas (e.g. dams, tanks or creeks). Low-fluorine water should be given to young sheep first, as animals under 3 years of age are more severely affected than older ones. Older sheep should be rotated so they may have 3 months on high-fluorine water and 3 months on low-fluorine water.

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