Hay fed to some California horses was not tainted, according to a University of California, Davis (UC Davis), professor, but instead contained a naturally occurring, yet-to-be-identified chemical that made those animals photosensitive.

Photosensitivity is a condition that causes horses to be particularly sensitive to light. Affected horses can develop a sunburn-like skin condition, skin cracking, blistering, or peeling. Some highly sensitive horses can also experience liver damage if they ingest highly photosensitive plants.

Chuck Burt, co-owner of the Diamond C Hay and Feed Co., in Phalan, California, said area horse owners purchased alfalfa hay from the feed supplier in late July. After feeding it to their animals, however, some owners reported that their horses developed sunburn-like signs while other owners said their horses' signs of disease as chemical-like burns, Burt said. Some owners attributed their horses' conditions to tainted hay.

“It was nice hay,” Burt said. “It was raised well. It was just beautiful; it's a mystery