Yep, mine comes out in tiny pimples with big bale meadow-type haylage. Really annoying! Tried to feed it several years in a row and about a week in, she comes out in a rash.
seems to be ok with Horsehage brand though as I've fed that a few times, timothy and high fibre ryegrass were ok. Very weird!
Just after Christmas 2013 we stopped feeding our 4 haylage and switched to big bale hay. My Draft mare had pastern dermatitis when I got her 3 years ago, having been treated for feather mites during the year that she had been with them. They had also fed her on haylage. We had thrown just about everything I could think of at her to no real avail. Then I noticed that she was developing small lesions round her mouth as well. I have sprayed her clipped pasterns frequently with teatree oil in a carrier oil and although still not quite cured, she is much better. Her mouth lesions have healed completely and she no longer is wet through overnight, she was over-heating and either sweating or getting condensation almost every night.
One of the Appaloosas had developed a wheeze which has stopped since changing to hay.