Alsike clover is a short plant, usually growing no taller than 1-2 feet, if that. It has flowers that are white to purple.
This plant is found throughout the United States and Canada. It prefers soils that are moist, and therefore tends to grow better in areas that receive adequate rainfall. It is often found in roadside ditches, pastures, yards, and practically any other growing area.
The entire plant is toxic, both in its fresh and dried state.
The exact reason for the toxicity is still unknown, though it has been known to be poisonous for almost 100 years. When horses are exposed to the clover for short periods, they exhibit signs of photosensitivity. This is usually in the form of reddened skin followed by the formation of bumps that ooze and then eventually dry up and fall off.
It essentially looks like a really bad sunburn. Usually the white-haired areas of the face and legs are the most affected body parts.
If horses are exposed to the clover for long periods of time, they can develop "big liver disease", which leads to an enlargement of the liver with excess connective tissue. If the condition progresses far enough, it can lead to neurological signs (walking in circles, disoriented, and head-pressing, among others) as well as jaundice and weight-loss.
The best and easiest way to prevent poisoning, and take care of the photosensitization is to remove access to the clover. If the horse shows signs of photosensitivity, keep the horse indoors during daylight hours.
When you are seeding pastures, ensure that the mix does not contain alsike clover, as it is present in many seed mixes.