If cattle get hold of Hypericum (St. John's wort) they become super sensitive
to the sun's rays and they get burnt, especially if they are fair skinned in
colouring - many cattle species in europe are cream or white.  When they get
burnt their skin splits, it is quite horrific to see and the animal is in
intense pain.  They have to be placed in a darkened building for a week or
more whilsts the skin heals, given pain killers and other medication and the
plant's effects dissipate.

helen in UK