This is something I actually thought about the other day, as I was bringing horses inside at feed time. And it just so happens that a local vet wrote about this very topic in his recent monthly column that I read just last night! LOL! (Dr. J is a great guy, and is my horses' chiropractic vet. If I did not already have an AWESOME veterinarian, I would use Dr. J as my regular vet!)
Every year in the winter, I see dark orange or even red urine spots in the snow. It always jars me for a moment, but it is perfectly harmless. Why is that??
I cannot remember who told me years ago about why horse's urine turns red/orange in the snow, but I clearly remember being worried about it the first time I saw it! LOL! While my mare was totally fine, I assumed the worst. Usually dark urine in a horse is a sign of tying-up. But when a horse pees in the snow, mucus or minerals will darken it and certain proteins can oxidize it. Therefore the urine spot will appear dark orange or even red! It can be alarming but is no cause for concern, especially if the horse is fine otherwise.
Ahhhhh, the joys of horse ownership! LOL! ;-D