Written By: Mike Murphy, DVM U of M
Clover is a desirable feed source for most horses whether used in pasture or in hay because it provides useful energy and acceptable protein and fiber. Problems may rarely arise with clover, just as they can with most desirable feed sources. Clover may be "too rich" at times for horses. The early rapid growth phase of some clovers, like other forages, may contain high amounts of soluble sugars. The soluble sugar content of the plant will decrease as it matures. These soluble sugars and other carbohydrates are sometimes associated with colics and founder in horses fed only pasture in the early spring.
Growth of mold on clover is occasionally encountered. Two mold problems are generally associated with the common pasture clovers (red, white and alsike). They are associated with weather above 80°F and humidity above 60%. The most well characterized problem is "slobbers." Horses can literally fill several 5 gallon buckets full of saliva in one day. This condition is caused by slaframine, which is produced when red clover is infested with a mold. The mold is generally a rust color seen on the upperside of the leaf. This mold normally "runs its course" in 2 to 4 weeks, depending on weather conditions.
The second problem in these clovers, black blotch disease, is not quite as well characterized, but has been reported in Minnesota, Washington, and areas of Canada. The mold literally causes black blotches to occur on the underside of the clover leaves, usually closer to the ground where the humidity is higher. Horses ingesting clover with black blotch have been known to develop excessive "sunburn," which is really a thickening and reddening of the white areas of skin due to liver damage. Black haired horses also get the liver damage but the "sunburn" is not visible.
A third mold condition affects a different clover both white and yellow sweet clover. These clovers are not common in pasture mixes, but are more frequently seen along roadways. The problem arises not from clover in pastures but if sweet clover is harvested for hay AND gets moldy as the hay is baled. Crimping the sweet clover at cutting reduces, but may not entirely eliminate, this problem. An unknown mold converts the naturally occurring cumarol in the sweet clover to dicumerol a blood thinning drug. Horses may bleed if moldy sweet clover hay is a substantial amount of a horse's diet over a number of days. Dicumerol clears quickly, so taking the horse off the hay is the best choice. Injections of vitamin K or blood transfusions may be necessary in extreme bleeding situations.
However, even with these potential problems, clover is still considered a desirable forage for horses.
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