Clover toxicity is caused by consumption of alsike clover (Trifolium hybridum) or red clover (Trifolium pratense). There are two forms of toxicity associated with ingestion of these two species of clover---photosensitivity and liver failure. Both species of clover are found in pastures throughout North America and grow best in cool climates with heavy, poorly drained clay soils.

Alsike clover thrives in adverse weather conditions such as cold, drought and excess rain, which often causes it to become invasive, taking over the pasture and replacing the grass.

The majority of toxic cases occur between April and November. Horses need only to ingest as little as 5% of fresh pasture or contaminated hay to begin to develop signs of toxicity. Signs usually develop 2-4 weeks following ingestion. The potency of the toxin is increased if consumed with dew or under hot and humid conditions.