The type of hay a horse eats can influence his susceptibility to photic reactions. St. John’s wort, buckwheat and other plants have been found to contain compounds that can cause photosensitive reactions. Research published in 2016 from the University of California–Davis confirms that alfalfa hay can induce primary photosensitive reactions in horses, although the exact mechanism remains unknown.
Source: EQUUS, July 20, 2022. Link. Primary photosensitive reactions occur when phototoxic compounds are ingested by a horse and accumulate under his skin. When exposed to sunlight through pink skin, the compounds cause painful blistering, crusting and inflammation.
Also see: Digestibility of haylage versus grass hay for horses, The Horse, July 25, 2023. Link.