Amsinckias
(Amsinckia, yellow burrweeds, fiddlenecks)
Amsinckia calycina, Amsinckia lycopsoides, Amsinckia intermedia and Amsinckia menziesii [Boraginaceae]
There are 4 species of Amsinckia in Australia. It is currently thought that these plants may not be separate species but different forms of one species. For the purposes of this book they will be discussed as one species.
Amsinckias are native to North America and South America. Amsinckias are important weeds of crops and pastures. They are declared noxious weeds in Victoria, South Australia and Western Australia.
Amsinckias are usually unpalatable to horses and are unlikely to be consumed in amounts that would cause poisoning. Horses may eat amsinckias if other fodder is scarce and the plants may contribute to pyrrolizidine alkaloid poisoning when consumed along with other toxic plants in horse grazing areas.
The main risk for horse poisoning by amsinckias is through their potential to contaminate grain. They are an important cause of poisonings in horses in other countries but there have been no confirmed reports of horse deaths from amsinckias in Australia.
Growth and identification:
Amsinckias are short-lived perennials that grow 20 – 70 cm tall. The plants germinate in autumn giving them a competitive advantage over other desirable species that germinate later in the year. The seeds are contained in a burr that can become attached to animals and can be dispersed to new areas. Seed can also be spread by vehicles and clothing and as contaminants of cereal crops.
The young plants form rosettes from which flowering stems are produced as the plant matures. The plants have upright or semi-upright stems that are branched or unbranched. The stems are covered in long stiff hairs and short soft hairs. The green leaves can grow to 20 cm long and 2 cm wide and they are covered in stiff hairs.
The rosette leaves are generally the largest and they have short stalks. The stem leaves have no stalks and they are alternately arranged along the stem. The leaves are lance-shaped and have a pointed tip. Flowering occurs from late winter to spring. The flowers are tube-shaped and are yellow or orange.
Each flower is up to 1.5 cm long and the flowers are arranged along one side of a 5 – 15cm long curved flower spike.
Management notes :
Avoid grazing horses on pastures infested with amsinckia. Check feed grains for amsinckia seeds. Amsinckia seeds are contained in a group of 4 ‘nutlets’ that each contain a seed and are around 2 – 3.5 mm long. The nutlets are brown or black and have a rough surface. The nutlets are surrounded by spiny bracts to form a burr. Grain can contain the burr or the released nutlets.
Individual plants can be pulled out by hand or spot sprayed with herbicide. The use of herbicides on larger infestations can be an effective control measure. Seek advice from a local agronomist or weed management agency to select the most appropriate herbicide and spray program for your property.
Graze adult non-pregnant goats or sheep on heavily infested paddocks as they are more tolerant to pyrrolizidine alkaloids and will readily eat the weed. Do not leave these animals on the infested pasture continuously as toxicity problems can occur. Mow or slash pastures before flowering to reduce seed production. If done repeatedly this practice will deplete the soil seed bank.
Cropping areas:
- repeated harrowing before crops are sown can destroy amsinckia seedlings.
- Planting pasture legumes on land used for cereal crops can suppress amsinckia establishment in between crops.
Tryptamine alkaloids:
Phalaris grasses contain tryptamine alkaloids. Most phalaris species are harmless to horses and phalaris grasses are valuable grazing species for all livestock in Australia. However two closely-related phalaris species, blue canary grass and paradoxa grass, have been implicated in the death of a small number of horses in Australia. All commercially available phalaris grass seed is the species Phalaris aquatica and this grass does not contain toxins.
Since the early 1980s there have been sporadic reports of horse deaths in Australia that have been attributed to blue canary grass, with cases occurring in New South Wales, Victoria and South Australia. In 1999, three horses died suddenly on a farm in Narrabri, New South Wales and their deaths were linked to paradoxa grass. Other horse deaths related to paradoxa grass have been occasionally reported in Australia.
Blue canary grass and paradoxa grass are thought to contain alkaloids closely related to tryptamine alkaloids. Rapid growth after rainfall is thought to increase the amount of tryptamine-type alkaloids in these grasses. The action of the toxins found in these grasses is unknown, as post-mortems of affected horses have not shown conclusive findings. It is thought that the cause of death might have been sudden heart failure.
Signs of tryptamine alkaloid poisoning:
In most reported poisoning cases the horse has been found dead in the paddock after no apparent ill health. In a few cases an owner has been present at the time that their horse died. These owners have reported that there was nothing to indicate their animal was unwell before a sudden onset of symptoms. One horse became suddenly unsteady, collapsed and died. Another horse apparently reared before collapsing and dying. A third horse became excited while being moved from a paddock, galloped for about 400 m, collapsed and died.
Tryptamine alkaloids:
Phalaris coerulescens [Poaceae]
Blue canary grass is native to the Mediterranean countries Madeira and the Canary Islands. It is a major weed in winter crops and pastures.
Growth and identification:
Blue canary grass is a short-lived perennial, surviving for 3 – 4 years. It grows vigorously in autumn and spring, especially after rain. The grass continues its growth through winter and it has a summer dormancy period. In cool, high rainfall areas the grass maintains growth through summer.
Blue canary grass can grow to 1.5 m tall. The green stems grow from a crown that is mostly above ground. Many buds develop at the crown during spring growth and these buds are the source of new growth in the following autumn.
The numerous leaves of blue canary grass are greenish blue in colour and they are mostly upright. Flowering occurs in late spring. The flowering seed heads of blue canary grass are upright, cylindrical in shape and are up to 14 cm long and 1 – 2 cm wide. The seed heads are green, with blue or purple coloured tips.
Paradoxa grass:
Phalaris paradoxa [Poaceae]
Paradoxa grass is native to the Mediterranean region. It is a major weed in winter crops and pastures.
Growth and identification:
Paradoxa grass is an annual. It grows mainly in winter and spring, and can grow vigorously after rain. The seeds germinate in autumn to early winter. Paradoxa grass can grow up to 1 m tall. The green stems grow from a crown that is mostly below the ground. The numerous leaves of paradoxa grass are greenish blue in colour and are mostly upright.
Flowering occurs in late spring. The flowering seed heads of paradoxa grass are cylindrical in shape and are 18 cm long and 1.5 - 2.5 cm wide. The seed heads are green, with blue or purple coloured tips.
Identification note:
Distinguishing between different grasses can be extremely difficult and only a very basic description is given here. See page 5 for more information on obtaining a precise identification of a plant species.
Management notes:
- Avoid grazing horses on pastures infested with blue canary grass or paradoxa grass, especially when these grasses are rapidly growing.
- Improve pastures to increase desirable grasses.
Indolizidine alkaloids:
Many plants worldwide contain indolizidine alkaloids. The most significant indolizidine alkaloid plants that are potentially toxic to horses in Australia are in the Swainsona genus of plants. These plants contain several indolizidine alkaloids but the most important in regard to toxic effects is called swainsonine.
Swainsona species are related to other swainsonine-containing genera such as Astragalus and Oxytropis. These plants cause livestock poisonings in many countries and they are known as ‘loco weeds’ in North America. The species of Astragalus found in Australia do not produce toxic amounts of swainsonine and there are no species of Oxytropis that grow in Australia.
Swainsonine is readily absorbed from the digestive tract and is rapidly distributed throughout the body. Swainsonine induces a condition known as lysosomal storage disease. The toxin inhibits the action of two important enzymes and this causes the accumulation of a particular proteincarbohydrate molecule in cells of the brain and other organs. This leads to nerve cell damage that is largely irreversible.
Signs of indolizidine alkaloid poisoning:
Signs of poisoning do not usually occur until after the horse has consumed plants containing swainsonine for at least 3 – 4 weeks.
- Depression
- Circling
- Incoordination
- Unpredictable behaviour – some horses may fall down or rear while being ridden or otherwise handled
- As the disease progresses horses become increasingly depressed and some horses may be so depressed that they appear to be asleep.
- The horse may become too weak to stand.
- In severe cases the horse may suffer convulsions followed by coma and death.
- There are no current treatment options that have proven to be effective for swainsonine poisoning.
If access to swainsonine-containing plants is restricted when symptoms first appear there is a good chance the horse could make a full recovery. However, if the symptoms are not recognised and the horse continues consuming swainsonine plants the damage to the brain may be too extensive for the horse to fully recover. Such horses are dangerous to ride or handle because of their ongoing unpredictable behaviour.
Swainsonas:
There are around eighty-five native species of Swainsona in Australia. Some of these are considered valuable as fodder for stock in arid areas and some have been cultivated as garden plants. The group contains the iconic Australian flowering plant, the Sturt’s desert pea, Swainsona formosa.
Most swainsonas are considered safe as they contain low levels of swainsonine. The swainsonas that have been implicated in the poisoning of horses and other livestock in Australia are Swainsona galegifolia, S. canescens, S. luteola, S. greyana and S. procumbens.
Swainsonas are palatable and horses may eat the plants when growth is abundant. Some horses may selectively graze swainsona, even when other pasture is available. Swainsonas can be found in pastures, crops, grasslands, open woodlands and roadsides.
Growth and identification:
Identification note: distinguishing between the different species of Swainsona can be extremely difficult and only a very basic description is given here. See page 5 for more information on obtaining a precise identification of a plant species.
Swainsonas are perennial or sometimes annual plants that develop quickly after spring or autumn rains and can dominate pastures during these times. They have colourful pea flowers and they produce seedpods.
Smooth darling pea:
Smooth darling pea is a perennial that grows to 1 m in height. The plant has arching stems. The leaves are around 10 cm long and are made up of 20 – 25 leaflets that are arranged along the leaf stem in pairs. The stems are usually hairless. The plant flowers in spring. The pea-shaped flowers appear in clusters of about 20 and flower colour varies from white to deep red. Pink and mauve flowers are the most common colours.
The plant produces leathery pods that can be up to 4 cm long.
Hairy darling pea:
Swainsona greyana
Hairy darling pea is a perennial that grows to 1.5 m in height. The leaves are approximately 15 cm long and are made up of 17 – 21 leaflets that are arranged along the leaf stem in pairs. The stems are covered in hairs. The plant flowers in spring. The pea-shaped flowers appear in clusters of about 20 and they can be white, pink or purple. Some parts of the flowers are covered in hairs. The plant produces hairless seedpods that can be up to 5 cm long.
Grey swainsona:
Swainsona canescens
Grey swainsona is a perennial that grows to 60 cm or taller in favourable conditions. Grey swainsona has upright or prostrate (creeping) stems. The leaves are up to 12 cm long and are made up of around 10 oblong-shaped leaflets that are arranged along the stem in pairs. The leaves and leaf stems are covered in white or cream coloured hairs.
The plant flowers from May through to December. The pea-shaped flowers appear in clusters of 20 – 35 and are mostly purple, pink, yellow or green. The plant produces hairy seedpods that can be up to 1.5 cm long.
Broughton pea:
Swainsona procumbens
Broughton pea is a low, spreading perennial that grows 30 – 50 cm in height. The stems can be hairless or covered in tiny hairs. The leaves are up to 15 cm long and have up to 25 leaflets arranged on opposite sides of the leaf stems. The leaflets are up to 2.5 cm long. The plant flowers in spring. The pea-shaped flowers appear in clusters of 2 – 12 and are mostly purple, mauve or pink. The plant produces seedpods that can be hairless or sparsely covered in hairs. The seedpods can be up to 4 cm long.
Dwarf darling pea:
Swainsona luteola
This perennial plant grows to 50 cm high. The stems are prostrate (creeping) or semi-erect and they are sparsely covered in hairs. The leaves are up to 10 cm long and have up to 17 leaflets arranged on opposite sides of the leaf stem. The leaflets are around 2 cm long and 1 cm wide. The plant flowers in spring. The pea-shaped flowers appear in clusters of 5 – 15 and are mostly purple or occasionally yellow. The plant produces a seedpod that is oblong in shape and is up to 3.5 cm long. The seedpod is hairless or has a sparse covering of hairs.
Management notes
- Avoid grazing horses on pasture infested with swainsona.
- Improve pastures to increase desirable grasses.
- 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.
- Closely monitor horses for behavioural changes if consumption of swainsona plants is suspected.
Taxine alkaloids:
Taxine alkaloids are found in the evergreen trees or shrubs of the Taxus genus. These are known as ‘yews’ and the species grown most commonly in Australia is the English yew. All Taxus species should be considered poisonous to horses.
Horses will not intentionally eat the unpalatable leaves or seeds from yew trees. Most horse poisoning cases have occurred when clippings from yew trees have been placed in horse areas. Yew leaves mixed with the clippings of other plants or grass are a particular risk to horses.
The taxine alkaloids found in yews are cardiotoxins, which means that they act upon the heart. All parts of the plant are toxic and mature leaves in winter are reported to be the most toxic. Horses are highly susceptible to poisoning by taxine alkaloids and only small amounts need be ingested for death to occur. One report in the literature describes the death of a pony within 1¼ hours of ingesting only 160 g of Japanese yew leaves.
Signs of taxine alkaloid poisoning:
In many reported poisoning cases the horse has been found dead in the paddock. The first signs of poisoning may appear within an hour of ingesting material from a yew tree and the progression of symptoms is rapid.
- Decreased tone in the lips and tail
- Decreased blood flow - difficult to find an arterial pulse
- Incoordination, staggering and trembling
- The horse may appear paralysed.
- Breathing becomes difficult and noisy
- In the final stages the horse will collapse and die, either from heart failure or respiratory failure
There is no specific treatment for yew poisoning in horses and in most cases death is likely to occur within 12 hours. Veterinary attention should be immediately sought if it is suspected that a horse has suffered yew tree poisoning and supportive care may help with survival and subsequent recovery.
The horse should be confined to a quiet area and handling should be minimised, as stress can precipitate heart failure. The administration of activated charcoal may stop further absorption of toxins from the gut. The placement of a stomach tube would likely be too stressful for the affected horse, so the activated charcoal should be added to feed and the horse should be gently encouraged to eat.
If the animal survives beyond 12 hours and symptoms improve there is a chance of full recovery.
English yew:
Taxus baccata [Taxaceae]
English yew is native to Europe Growth and identification English yew is an evergreen tree that can grow to 20 m in height. The tree has a dark trunk that is quite thick in mature plants. The needle-like leaves are dark green and are glossy on the upper surface and paler green on the lower surface. English yew flowers in spring. Male and female flowers are borne on separate plants. The male flowers appear as clusters of small scaly cones that turn yellow on maturity.
The female flowers are very small solitary green cones. The female trees produce berries that are up to 1.2 cm in diameter. The berries are cup-shaped, fleshy and red in colour.
Management notes:
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Yew trees are not suitable for planting in paddocks, along paddock fences or in areas where horses are kept.
- Do not dispose of yew tree clippings in areas where horses could access them.
Tropane alkaloids:
Tropane alkaloids are found in several plants that commonly grow in Australia. Some of these plants grow as weeds and some are grown as garden ornamentals. Cases of horse poisonings by tropane alkaloid plants are uncommon because horses will not usually eat the unpalatable plants. Most reported cases occur through the ingestion of contaminated hay or other feedstuffs. Horses may eat the plants if no other forage is available.
Tropane alkaloids are anticholinergenic, which means that they reduce the metabolic effects of an important compound called acetylcholine. This compound stimulates muscles to contract and it is important for brain activity and normal nerve function. The properties of the tropane alkaloids in some plants have made them useful in human medicine.
Signs of tropane alkaloid poisoning The signs and severity of tropane alkaloid poisoning varies between cases because of the variability in the type and amount of tropane alkaloids in particular plants. The signs that may be seen in a case of poisoning include:
- Loss of appetite
- Depression
- Colic
- Rapid pulse and respiratory rate
- Dilated pupils
- Excessive thirst
- In severe cases death may occur within minutes, hours or days of ingesting the toxic plant material.
- Death can be due to respiratory paralysis, heart failure or rupture of the stomach.
A full recovery is the usual outcome in mild cases. Veterinary attention should be sought if tropane alkaloid poisoning is suspected in a horse. The treatment options may include the administration of activated charcoal to stop further absorption of toxins and the administration of drugs and supportive therapy.
Field bindweed:
(common bindweed, morning glory, field morning glory)
Convolvulus arvensis [Convulvulaceae] Field bindweed is native to Europe, Asia and northern Africa. Field bindweed is a declared noxious weed in Victoria, South Australia and Western Australia. No cases of field bindweed poisoning have been reported in Australia. The plant is suspected of causing tropane alkaloid poisoning in a group of thirteen horses in the United States and has been shown to be toxic to mice when fed in high doses in toxicity studies.
Growth and identification:
Field bindweed is a creeping vine that can grow along the ground or on support. The ribbed climbing stems are usually 1 – 3 m long but can be up to 7 m long. The vine produces seeds that can be spread by animal and vehicle movements and as contaminants of hay or grains.
Root segments can spread by cultivation or other practices that move soil. The leaves are alternately arranged along the stem and they can be up to 6 cm long by 3 cm wide. The leaves are triangular or shaped like an arrowhead.
The vine flowers in spring to early autumn with most flowers appearing in summer. The flowers are tubular and they grow where the leaves meet the stem. They have 5 fused petals and are on stalks that are up to ?? cm long. The flowers are white to pale pink and they can sometimes have a streaked appearance. Each flower is only open for one day.
The plants produce fruit that is a smooth, eggshaped capsule with a slightly pointed tip. The capsule is up to 8 mm across and it turns brown as it matures.
Identification note:
Vines in the genus Ipomoea are, like field bindweed, known by the common name ‘morning glory’. These plants should not be confused with Convolvulus arvensis. See page 5 for more information on obtaining a precise identification of a plant species.
Management notes:
- Avoid grazing horses on pastures infested with field bindweed.
- Check hay for field bindweed contamination.
- Individual plants can be pulled by hand or spot sprayed with herbicide. Ensure all plant material is removed as the vine can regrow from root fragments.
- The use of herbicides can be an effective control measure for larger infestations.
- For best results the plant should be treated when actively growing. Seek advice from a local agronomist or weed management agency to select the most appropriate herbicide and spray program for your property.
- Improve pastures to increase desirable grasses.
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