Paint, I'm not talking about plants that cause irritations - like stinging nettle - but plants that have photodynamic pigments which cause photosensitization in the skin of the horse. It is a reaction caused by eating these plant pigments which are exposed to UV rays in the vulnerable un-pigmented skin areas.
This is very different to a rash caused by irritation to a plant
Clover, lucerne, rye grass, buttercup and St Johns Wort are particularly bad for this
Here's a pic of two of ours who have the 'dreaded pink nose' that most people think need to be covered - these NEVER have nose guards but they are kept away from the above plants - we've had no problems with sunburn or mud fever since doing this