I suspect many people are unaware that clover can cause adverse health effects on the horse.  Alsike clover grows throughout the United States, and has creamy white to pink "flowers" that generally bloom between May and October (see attached photo).  The cause of toxicity may be a mycotoxin that grows on the plant (thanks to moist conditions), or that the plant produces when stressed.  

It is true that overconsumption of Alsike Clover (Trifolium hybridum) is toxic to horses.  The main symptom is photosensitivity (sensitive to sunlight).  Photosensitivity most often occurs when the horse consumes wet Alsike clover.  If the horse's skin is moist, exposure to damp Alsike clover may cause the development of lesions or sores.  Swelling of the tongue is also a concern, and at worst may inhibit the horse's normal eating habits.  Symptoms of photosensitivity may also manifest similar to the way sunburn would, with red skin around the muzzle or any area that contains unpigmented (pink) skin.     

The liver may also be affected.  Long-term overconsumption (over the course of a year or so) of Alsike clover may result in an enlarged liver, the result of tissue destruction and replacement with connective tissue, causing progressive loss of liver function. The horse may further develop jaundice and weight loss.  Your veterinarian can run blood tests to assess liver fuction and prognosis.  

Note that other varieties of clover, such as Red Clover (Trifolium pratense), Sweet Clover (Melilotus species) and Crimson Clover (Trifolium incarnatum) are also toxic to horses, with symptoms including slobbers (drooling), colic, bloating, diarrhea, weakness, abnormal heartbeat and abortion in pregnant mares.  

Usually the main treatment for clover toxicity is to remove the horse from grazing areas that contain the plant.  Other cures or treatments are unknown.  Many times, symptoms will resolve quickly but don't hesitate to call your vet if you suspect something is serious.  If your pasture contains a lot of clover, you may need to consider a complete pasture renovation to make it suitable for horses. 

A great resource on toxic plants is a handbook entitled, "Horse Owner's Field Guide to Toxic Plants" by Sandra Burger. A good deal of information provided here is attributed to this source.  See also the "Ask The Expert" section of the Rutgers University's Equine Science Center website, at http://www.esc.rutgers.edu/ask_expert/ate_pp.htm#wac.  

Thank you very much. This was very helpful. I always knew horses could get excessive salivation due to moldy red clover, but never knew of any of the other possible reactions.