Diluted bleach, I believe it’s 1 parts bleach to 7 parts water. Spray and leave. Works great.
I also do like MTG. It’s messy, which I’m not as thrilled about, but it does work quite well. Also works well on scratches and “mud fever” and my big hairy draft cross wound up with rain rot on his back and scratches/mud fever on his white stockings; and then the gnats and midges were going after the bare spots as it warmed up. MTG being oily with sulfur cleared up both issues and then kept the bugs off while the skin healed and he got hair growth back.