The ingestion of the crotalarias known as trefoil rattlepod and Chillagoe horse poison can induce severe ulceration on the lining of the oesophagus and the upper part of the stomach in horses.
Horses will readily eat these species of crotalaria and may selectively graze the plants even when other forage is available. Horses are at an increased risk of developing Chillagoe horse disease when rain produces a flush of growth in these plants.
Despite the wide distribution of these plants, cases have been recorded only from central coastal and northern Queensland. It is likely that large intakes of the plants are required before poisoning occurs. The toxins that cause these effects in horses are presently unknown.
Other species of Crotalaria can cause pyrrolizidine alkaloid poisoning if ingested.
Signs of Chillagoe horse disease:
- Ulceration of the oesophagus
- Frequent licking of lips
- Teeth grinding
- Drooling
- In severe cases the oesophagus may become partially or completely blocked and the horse may be unable to swallow food or water. Severely affected horses can die if the oesophagus becomes completely blocked.
Veterinary attention should be sought and the affected horse should be removed from pastures containing crotalaria if Chillagoe horse disease is suspected. Veterinary treatment and supportive care may help with recovery. In severe cases a veterinarian can try to pass a stomach tube past any blockage in the oesophagus to supply nutrition to the horse while the oesophageal ulcers heal.
Trefoil rattlepod:
Crotalaria medicaginea var. neglecta (formerly known as C. trifoliastrum) [Fabaceae]
Growth and identification
Trefoil rattlepod is an upright annual or perennial that grows to 1 m in height. The leaves have 3 narrow egg-shaped leaflets that can be up to 3 cm long by 1.5 cm wide. The lower surfaces of the leaves are densely hairy. The plant flowers throughout the year. Flower spikes appear at the tips of the branches and they can be up to 10 cm long. The numerous pea-shaped flowers are yellow and are sometimes streaked with red. The plant produces small hairy or hairless pods.
Chillagoe horse poison:
Crotalaria aridicola ssp. aridicola [Fabaceae]
Growth and identification
Chillagoe horse poison is a semi-upright perennial that grows 50 – 60 cm in height. The plant has slender branched stems and a woody taproot. The leaves have 3 wedge-shaped leaflets. The plant flowers in autumn to winter. Flower racemes appear at the tips of the branches and the flowers grow in crowded clusters. The peashaped flowers are yellow. The plant produces triangular-shaped pods.
Management notes
- Avoid grazing horses on pasture infested with crotalaria.
- Isolated small plants can be pulled out by hand or spot sprayed with herbicide.
- The use of herbicides can be an effective control measure for larger infestations. Seek advice from a local agronomist or weed management agency to select the most appropriate herbicide and spray program for your property.
Avocado:
All parts of avocado trees are poisonous to horses but the leaves contain the highest levels of toxins. The leaves of avocado trees are toxic even when fallen and dried. The toxin in avocado trees is called persin but the mechanism by which it causes toxicity in animals is not known.
Cases of horse poisoning after ingestion of material from avocado trees are uncommon but have been reported in Australia and overseas.
Signs of avocado poisoning:
The signs of avocado poisoning in horses are variable and may include:
- Swelling of the lips, mouth, head, neck and chest
- Colic
- Diarrhoea
- Lethargy
- Loss of appetite
- Shortness of breath and heart problems in severe cases Severely affected horses may die suddenly from heart failure or respiratory failure
Veterinary attention should be sought immediately if avocado poisoning is suspected and treatment will depend on the symptoms the horse displays. Most horses fully recover from avocado poisoning, although milk production may not return to normal levels in lactating mares. Horses displaying signs of heart and lung damage may have ongoing complications.
Avocado
Persea americana [Lauraceae]
Growth and identification
Avocado trees can be tall and upright or low and spreading, depending on the variety. Tall growing varieties can grow to 15 m in favourable conditions. The trees are evergreen but the leaves are shed and regrow throughout the year, in cooler regions the tree can be semi-deciduous.
The leathery leaves are dark green and glossy and are variable in size depending on the variety. The tree has small greenish-yellow flowers that grow where the leaves join the branch. The plant produces pear-shaped avocado fruit. Flowering and fruit development can occur at different times of the year depending on the variety.
Management
- Avocado trees are not suitable for planting paddocks, along paddock fences or in areas where horses are kept.
- Do not put clippings from avocado trees in areas where horses may access them.
Senna:
The toxic plants in this group that occur in Australia and may be a risk to horses are Senna occidentalis and Senna obtusifolia. Both of these plants are declared noxious weeds in several states. Many Australian plants in the Senna genus were previously in the Cassia genus and this can cause some confusion when identifying plants from these genera. See page 5 for more information on obtaining a precise identification of a plant species..
The toxins in plants of the Senna genus have not been identified. Poisoning mainly occurs in cattle but poisoning cases in horses have been reported in Australia. All parts of the plants are toxic, especially the seeds. The plants are unpalatable to horses and are usually avoided. Poisoning could occur through contamination of grain with senna seed or plant material.
Signs of senna poisoning The toxins contained in senna cause muscle damage and severe liver damage in horses.
The signs that may be seen include:
- Depression
- Muscle tremors
- Incoordination and swaying gait
- Shortness of breath
- Heart irregularities
- Death
Senna poisoning in horses is uncommon but is usually fatal. Veterinary attention should be sought immediately if senna poisoning is suspected. There is no specific treatment for senna poisoning but veterinary treatment and supportive care may increase the chance of survival and help with subsequent recovery.
Coffee senna:
(Nigerian senna, ant bush, arsenic bush, sickle pod, stinkweed, stinking pea, septicweed)
Senna occidentalis (formerly known as Cassia occidentalis) [Fabaceae]
Senna Coffee senna is native to the tropical and subtropical regions of the Americas. It is a declared noxious weed in the Northern Territory and Western Australia.
Growth and identification:
Coffee senna is an upright annual or biennial shrub that grows to 2.5 m in height. The plant produces seeds that are spread by water run-off, as contaminants of transported hay and grain or through animal and vehicle movements. The branched stems are reddish-purple and are somewhat square-shaped or grooved when young. Mature stems are greenish-brown and are more rounded in shape. The leaves are alternately arranged along the stem. The leaves are up to 20 cm long and are compound, having 3 – 5 pairs of oppositely arranged leaflets. Each leaflet is around 3 cm long and is egg-shaped with a pointed tip. There is a dark gland at the base of each leaf stalk.
Flowering occurs in summer and autumn. The 5-petalled flowers are bright yellow and are about 3 cm across. The flowers appear in clusters of 2 - 6 in the forks of the uppermost leaves. The plant produces cylindrical pods. The pods turn dark brown as they mature and they have paler brown stripes at the edges. The pods can be up to 13 cm in length.
Sicklepod:
(Java bean, Chinese senna, coffee weed, foetid cassia, arsenic weed)
Senna obtusifolia (also known as Senna tora and formerly known as Cassia obtusifolia or Cassia tora) [Fabaceae]
Sicklepod is native to the Caribbean region and tropical America. It is a declared noxious weed in Queensland, Western Australia and the Northern Territory.
Growth and identification:
Sicklepod is an upright annual or biennial shrub that can grow to 2.5 m but is usually less than 2 m in height. The plant produces seeds that are spread by water run-off, as contaminants of transported hay and grain or through animal and vehicle movements.
The branched sprawling stems are covered in soft hairs. The leaves are alternately arranged along the stem. The leaves are compound, having 2 – 3 pairs of oppositely arranged leaflets. Each leaflet can be up to 6.5 cm long and the leaflets furthest from the stalk are the largest. The leaflets are egg-shaped and have a rounded tip. The leaflets usually have tiny hairs along their edges. Flowering occurs from late summer through to early winter. The 5-petalled flowers are bright yellow and are about 1.5 cm across. The flowers grow on stalks up to 3 cm long and appear in the forks of the uppermost leaves. The plant produces rounded, sickle-shaped pods. The pods turn greenish-brown as they mature and they are slightly indented between the seeds. The pods can be up to 18 cm in length.
Management notes:
- Coffee senna and sicklepod should be removed from horse paddocks if they are found growing as weeds.
- Isolated young plants can be pulled out by hand or spot sprayed with herbicide. Ensure all parts of the plant are removed from the ground.
- The use of herbicides can be an effective control measure for larger infestations. Seek advice from a local agronomist or weed management agency to select the most appropriate herbicide and spray program for your property.
- Slashing infested pasture can keep the plants to a manageable size and will weaken them, allowing desirable pasture species a chance to recover. Use blunt slasher blades to shatter the stems for maximum effect.
- Establish competitive pastures. Heavily infested areas should be cultivated and sown with desirable pasture species.
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