Whenever horses are grazing on pastures where clover is present, certain health concerns for the horse need to be kept in mind. It is of value to recognize the type of clovers present in the pasture, as well as the type of health problems that might ensue with ingestion of particular types of clover.
Clover in moderate amounts brings benefit to the health of any pasture as clover is a nitrogen-fixing plant, and when consumed in appropriate amounts, clover can be a valuable nutrient for the horse. However, consumption of clover becomes problematic with certain types of clover whenever the percentage of clover increases in the pasture or forage/hay mix beyond 20 per cent. Concern for the horse is further amplified if the clover itself is not healthy and contaminated with a fungus or other toxins.
The three most common clovers that appear in horse pastures are white, red, and alsike. Although sweet clover is also a problematic clover for horses, causing a bleeding disorder, it is rarely found in pasture situations. It is more commonly found in hay mixtures harvested from roadsides where the deep-rooted sweet clover is seeded as a soil stabilizer.
White clover is commonly found in many horse pastures because it will survive close grazing. It is a low-growing plant and has no upright stems. White clover produces stolons and stems that run along the surface of the ground. Each of the three leaflets on the plant will have an inverted white “V” marking on them. This marking is commonly referred to as a watermark. Flowers are white in colour.
Red clover has an erect growth habit, with larger reddish-purple flowers at the end of each hairy stem. Although a common pasture legume, it does not tolerate continuous grazing, and as a result it is not normally found in overgrazed pastures. The leaves of the red clover plant, like white clover, also have a watermark.
Alsike clover, like red clover is a tall upright plant. However, unlike red clover, its flowers are smaller and pink and its leaves have NO watermark or white inverted “V” on the leaves.
The phrase “clover poisoning” is generally associated with ingestion of alsike clover and refers to two related syndromes, photosensitization and liver disease. Photosensitization is usually the initial sign of poisoning and is secondary to the effects of liver damage. The liver, damaged by alkaloids in the clover, does not remove the toxic metabolites of plant ingestion via bile so they are deposited in the skin. The metabolites are photo-reactive and produce harmful oxidative products in the presence of sunlight, creating tissue damage and inflammation. The non-pigmented (pink-skinned areas) of the horse are primarily afflicted with clover poisoning, since they are susceptible to photosensitization. Although horses with dark skin are spared the signs of photosensitization, their livers will still suffer damage. The acute lesions of photosensitization resemble “sunburn” and are characterized by reddening of the skin, followed by weeping, raw and painful open wounds, covered with a crusty discharge. Effects of the toxins are cumulative and severity of symptoms depends upon the amount of toxic clovers in the horse’s diet. Liver cirrhosis and/or scarring generally follows several months of plant consumption.
Horses with symptoms suggestive of alsike clover toxicity need to be immediately offered new feed — one with NO clover. There is no specific treatment for clover poisoning. Mild cases will usually recover once the toxic clover is removed from the diet. Horses displaying signs of photosensitization will find relief by reducing their exposure to sunlight. Allowing them to graze at night while providing them with shelter during the sunlight hours will help the skin lesions to heal. Horses showing signs of advanced liver disease have little chance of survival.
In addition to the toxic alkaloid that is present in alsike clover it, like the other two clovers, is susceptible to a fungal infection under certain environmental conditions. More specifically, during periods of high humidity, a “black” fungus proliferates quickly on both clovers and legumes, accumulating harmful mycotoxins. The fungus is visible on the plant leaves as gold, brown, and/or black spots or rings and is aptly named “black patch disease.”
The hot days and cooler nights typical of late summer and early fall precipitate a heavy dew, which seems to create ideal conditions for fungal growth.
If these infected clovers and resulting mycotoxins are ingested by the horse, they can stimulate profuse salivation and induce a condition known as “slobbers.” Excessive salivation usually begins several days following initial consumption of the fungus-ridden plants and is generally self-resolving, once the offending plant is removed from the horse’s diet. Although the slobbering is a nuisance, the horse rarely suffers any health effects providing salt and water are readily available.
There also appears to be a casual relationship between the “stressed” clovers and the spike of “scratches” or caudal heel dermatitis which typically occurs during the August and September months. Either ingestion of the mycotoxin or perhaps even skin contact with the irritating mycotoxin may be a contributing factor to this skin reaction.
Excess clover consumption can also be responsible for paradoxical mammary gland development and lactation in barren mares. This is likely related to the higher levels of phytoestrogens found in many clovers.
Management strategies to “living” with clover in horse pastures include acknowledging the clover indexes in pastures and recognizing sensitivities to clover when they appear in horses. Horses vary in their preferences for clover and their individual sensitivities, therefore it is not uncommon for certain horses in any pasture situation to be affected greater than others.
Pasture management may include mowing, resting or rotating pastures and even reseeding the pasture to allow the grasses a competitive edge over the clover. Unfortunately once established, clover can be difficult to control even with broadleaf herbicides. As clover begins to dominate the pasture’s mix the likelihood of health problems to horses increases. If clover remains at 10 to 20 per cent of the pasture mix, it will likely not be a problem for an otherwise healthy horse.