The toxins associated with red clover and alsike clover poisoning are not presently known. There is some evidence that the toxic effects may be caused by a mycotoxin-producing fungus that is commonly associated with these clovers.
The horse seems to be the only animal species susceptible to poisoning by red clover and alsike clover. The toxins in these clovers cause liver damage with secondary photosensitation.
Poisoning of horses by alsike clover and red clover has been recorded in Canada, the United States and several other countries but there are no confirmed Australian reports in the literature.
Clovers are usually present in pasture as part of a mix of legumes and grasses. Most reports of red clover and alsike clover poisoning in the literature have noted that the pasture grazed by affected horses was dominated by clover, or that ingested hay contained large amounts of clover.
Researchers estimate that a diet containing greater than 50% red clover or alsike clover can cause liver damage in horses. The condition can occur at any time of year but it seems to occur most often after an unusually long and wet spring.
The effects of the toxins are cumulative and signs of poisoning can appear within 2 – 4 weeks of ingesting red clover or alsike clover but in some cases it may be months before signs appear. The time of onset and the severity of symptoms depend on the amount of toxic clovers in the diet of the horse.
Signs of alsike clover and red clover poisoning:
The initial sign of poisoning is usually photosensitisation that is a secondary effect of liver damage. Horses with areas of unpigmented skin are most commonly diagnosed with red clover or alsike clover poisoning and this is probably because they are more susceptible to photosensitisation.
Horses with dark skin may show no sign of photosensitisation before the appearance of other signs indicating liver damage. These horses are at most risk because the disease is often at a more advanced stage when diagnosed.
The signs of poisoning are variable and may include:
- Photosensitisation - redness and swelling of the skin in lips and around the eyes are areas commonly affected. The skin may crack and weep fluid.
- Depression
- Aimless wandering
- Head pressing - the horse may push its head against a wall or other surface
- Incoordination
- Loss of appetite
- Yawning
- Muscle tremors
- Mild colic
- Blindness
- Inability to swallow
- In severe cases the horse may progress to episodes of violent excitation, then coma and death unpigmented areas. The nose, lips and around the eyes are areas commonly affected. The skin may crack and weep fluid.
There is no specific treatment for red clover and alsike clover poisoning. Veterinary attention should be sought if red clover or alsike clover poisoning is suspected in a horse. A comprehensive liver function test can assist in the diagnosis of horses showing signs of red clover or alsike clover poisoning.
Mild cases will usually recover if the toxic clover is removed from the diet. Horses displaying signs of photosensitisation should be kept out of the sun to help the skin heal. Veterinary treatment and supportive care may help with recovery. Horses showing signs of advanced liver disease have little chance of survival.
Identification note:
Clovers can be very difficult to tell apart. Before implementing any management strategies in regards to red clover or alsike clover it would be prudent to have the clover species positively identified. See page 5 for more information on obtaining a precise identification of a plant species.
Red clover:
Trifolium pratense [Fabaceae]
Growth and identification:
Red clover can be upright or the stems may droop downwards. The stems are up to 50 cm long and may be hairless or covered in hairs. The plant has leaves made up of 3 leaflets. The leaflets are up to 5 cm long and 1.5 cm wide and each leaf has a light green or white inverted crescent-shaped marking in the middle.
The pink or purple flower heads are spherical in shape and are around 2.7 cm across. The flower heads are borne on stalks. Each flower head is made up of around 100 small flowers.
Alsike clover:
Trifolium hybridum [Fabaceae]
Growth and identification:
Alsike clover can be upright or the stems may droop downwards. The stems are up to 60 cm long and may be hairless or covered in hairs. The plant has leaves made up of 3 leaflets. The leaflets are up to 3 cm long and 1.8 cm wide and they are borne on the end of long stalks. The leaflets do not have the leaf markings that appear on the leaflets of red clover.
The white or pink flower heads are spherical in shape and are around 2.5 cm across. The flower heads are on a stalk that is usually longer than the stalk of the leaves. Each flower head is made up of 20 – 40 small flowers.
Management notes:
- Avoid grazing horses on pasture that is dominated by red clover or alsike clover.
- Check hay for red clover or alsike clover.
Selenium poisoning:
Selenium is an essential dietary nutrient for horses but excessive amounts of selenium in the diet can cause toxicity. The amount of selenium found in a plant depends on the species and the soil selenium level where the plant grows. Soils in some parts of Australia have particularly high levels of selenium and some plant species that grow in these areas can accumulate selenium in their leaves at potentially toxic levels.
Two native Australian plants are known to accumulate selenium and have been associated with selenium poisoning of horses in Australia. Horses that graze plants containing high levels of selenium may develop chronic selenium poisoning and this is sometimes known as ‘alkali disease’.
The plants are not usually palatable to horses but may be eaten if other forage is scarce.
Signs of selenium poisoning:
- Hair loss from the mane and tail
- Horizontal cracks in the hoof wall
- In severe cases the hoof wall may slough off
Affected horses should be moved to a paddock that does not contain selenium-accumulating plants. Veterinary advice should be sought to devise a suitable low-selenium diet for the horse to help with recovery, yet still provide adequate nutrition. A gradual recovery should occur as the affected hooves grow out.
Selenium weed:
Neptunia amplexicaulis [Fabaceae]
Selenium weed is a native Australian plant.
Growth and identification:
There are 2 forms of selenium weed, one being hairless and the other being covered in fine, soft hairs. Both forms of selenium weed are found only in the Hughenden- Richmond area of north Queensland. Selenium weed is an erect or semi-erect perennial that grows to 80 cm in height.
The leaves are made up of paired leaflets along a central leaf stem. The leaflets are around 2 cm long and are heart-shaped with a tapering point. The flower heads are borne on a spike that grows from the point where the leaf meets the stem.
There are 20 – 40 small yellow pea-shaped flowers on each flower head. The plant produces pods that are dark brown when mature. Each pod contains 1 – 4 seeds.
Mapoon bush:
Morinda reticulata [Rubiaceae]
Mapoon bush is an Australian native plant.
Growth and identification:
Mapoon bush is found only on Cape York Peninsula in northern Queensland. Mapoon bush is a shrub that can grow to 1 m in height. The shrub has a straggly appearance and can grow as a climber in shaded areas. The pale green leaves are oval-shaped and can grow to 12 cm in length. The leaves are thick and have a leathery feel.
The plant has white star-shaped flowers that grow to 1.5 cm across and are surrounded by white leafy bracts.
Management notes:
- Avoid grazing horses on pastures infested with mapoon bush or selenium weed.
- Mapoon bush and selenium weed should be controlled in horse paddocks, especially if they are the dominating species. Seek advice from a local agronomist or weed management agency for advice on the management of these weeds on your property.
- Many Australia soils are marginally deficient in selenium and dietary supplementation of the element may be required. Contact a local agronomist or your state Department of Agriculture for advice on the soil types in your area. Feeding Horses in Australia (Kohnke et al, 1999) contains some very good information and recommendations concerning selenium in the diet of horses.
Chewing disease:
Prolonged ingestion of the plants St Barnaby’s thistle or hardheads can result in a disease called equine nigropallidal encephalomalacia (ENE), or ‘chewing disease’. The disease is uncommon but cases have been reported in Australia and overseas.
Several potentially toxic substances are present in these plants but the primary toxins responsible for poisoning have not been definitively identified. The exact mechanism of poisoning is not completely understood but it may involve the interference of cell function by the inhibition of important enzymes and other functional proteins. This may lead to the degeneration of nerve cells in specific parts of the brain, causing the characteristic symptoms of ENE.
Symptoms of poisoning in horses can occur after long periods (1 – 2 months) consuming significant amounts of the plants. Horses will eat these plants if there is little alternative fodder and poisoning could occur if large amounts of the plants are consumed as contaminants of hay. It has been reported that some horses will preferentially graze the plants, even when other feed is available.
Signs of chewing disease:
A horse suffering chewing disease has damage to a specific part of the brain and the effect of this brain damage is that the horse is unable to move food or water to the back of its mouth. Although horses will only develop the disease after a relatively long period of exposure to the toxins, the onset of symptoms is abrupt. A horse may seem normal one day and the next day it may seem unable to eat or drink.
The signs that may be seen include:
- Apparent inability to eat or drink. The horse may have trouble getting food into the mouth but if successful, the horse may rhythmically chew and move its tongue around for prolonged periods in attempts to move the food around the mouth. The food often dribbles out of the mouth. The horse is able to swallow but is unable to get food into a position for swallowing.
- Dehydration
- Depression
- The horse may stand with its head so low that its nose is almost touching the ground. A horse standing in this position for long periods can develop swelling of the head.
- Frequent yawning and lip curling.
- The horse may be able to drink by putting its head deep into water and then tipping its head back.
- Muscle tremors and incoordination are seen in some cases
No treatment options are available for this disease at present and the damage to the brain is thought to be irreversible. Once the symptoms occur the disease is fatal and the horse will die of starvation if not humanely euthanased.
Hardheads:
(Russian knapweed, creeping knapweed, blueweed)
Hardheads is native to the region around the Caspian Sea (Russia, Turkey, Afghanistan and Mongolia). It is a declared noxious weed in all states except Queensland, Tasmania and the Australian Capital Territory.
Growth and identification:
Hardheads is an upright perennial that usually grows to around 50 cm in height but can grow to 1 m. The plant produces seeds that often remain in the flower heads and they can be spread to new areas through water run-off, as contaminants of transported hay and grain, or through animal and vehicle movements. The plants have an extensive network of underground roots that can give rise to new shoots. The plants can spread through the dispersal of root fragments during cultivation or other practices that move soil.
The branched stems are densely covered in greyish hairs when young. Mature stems have fewer hairs and are often grooved and ridged. The young plant is a grey-coloured rosette that may be hairy. Leaves are lance-shaped and may have toothed or irregular edges. The leaves are up to 15 cm long and 5 cm wide. The leaves produced along the stem are alternately arranged and are smaller towards the top of the plant.
The plant flowers in its second year during late spring or summer and flowers appear singly at the tips of the branches. The thistle-like flowers are up to 2.5 cm across and are purple, pink or occasionally white. The flowers have numerous 5-lobed tubular florets that are arranged in several rows contained by broad bracts. The bracts narrow to a fine papery tip.
Management notes:
- Check hay for hardheads contamination.
- Avoid grazing horses in pastures contaminated with hardheads.
- Control of hardheads is difficult because of its underground root system and its tolerance to many herbicides.
- Isolated plants should be pulled out by hand or spot sprayed with herbicide. Ensure all parts of the plant are removed from the ground.
- Larger infestations can be controlled with certain herbicides. Seek advice from a local agronomist or weed management agency to select the most appropriate herbicide and spray program for your property.
- The most effective control of the weed in pastures is the establishment of competitive species.
St Barnaby’s thistle:
(Yellow star thistle, golden star thistle, yellow cockspur, knapweed)
Centaurea solstitialis [Asteraceae]
St Barnaby’s thistle is native to southern Europe and western Asia. It is a declared noxious weed in New South Wales, Victoria and Western Australia.
Growth and identification:
St Barnaby’s thistle is an upright annual or biennial that usually grows 30 – 60 cm tall but can grow to 90 cm. The plant produces seeds that can be dispersed by wind, water run-off, as contaminants of hay or grain and through animal and vehicle movements.
The branched stems have spiny wings. The stems have ‘woolly’ fine hairs that are greyish in colour. The young plant is a rosette with leaves up to 20 cm in length. The leaves are deeply divided or lobed, with the lobe at the tip of the leaf being larger than the others. The rosette leaves wither as the stem grows. The stem leaves are alternately arranged and have slightly toothed edges and pointed tips.
The plant flowers in late spring and summer and the flowers appear singly at the tips of the branches. The flower heads are up to 2.5 cm long and are egg-shaped. The flowers consist of numerous yellow tubular florets that are up to 2 cm long. The flower heads are surrounded by several rows of bracts. The middle bracts end in yellow spines that can be 3 cm long. Other bracts surrounding the flower head can end in smaller spines.
Management notes:
- Check hay for St Barnaby’s thistle contamination.
- Avoid grazing horses in pastures contaminated with St Barnaby’s thistle.
- Individual plants can be pulled out by hand or spot sprayed with herbicide.
- Slash infested pasture to reduce flowering and seed production. Timing is important and the ideal time to slash the paddock is after desirable species have set seed but before St Barnaby’s thistle has set seed. Seek advice from a local agronomist or weed management agency to devise the right slashing program for your property.
- The establishment of competitive pastures can control St Barnaby’s thistle.
- Graze sheep, goats or cattle on heavily infested paddocks as the toxins in this plant do not affect these species.
- The use of herbicides can be an effective control measure for large infestations. Seek advice from a local agronomist or weed management agency to select the most appropriate herbicide and spray program for your property.
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