I was at a local horse expo in March and was talking to a lady with a gorgeous Paint and I remarked on some marks on his coat. She told me she had used MTG on him and he developed burn like patches. She ended up having to shave him all the way down to the skin, then bathe him in cool water. He'd had a reaction to the MTG.
The vet told her that some horses cannot tolerate the sulfur in it? I'm not sure if that's true, but that's what she was told.
It's made me a bit leery of using it on my own horse for that reason.
"When you're young and you fall off a horse, you may break something. When you're my age, you splatter." -- Roy Rogers