I know "all stock" feed isnt good for horses. I went to a western barn and they fed it, then I went to the feed store to ask, as all stock is a lot cheaper, and my friend who works there and is also a rescuer said that she would NEVER feed it to her horses and does not recommend it.
I know all stock feed isnt the same as calf feed, but I would just stick to horse feed. If there is any chance of it being bad for your horse, dont do it.
There are a couple things that can be fed to cows but not horses. I wish I could remember the ingredient, but I'm sure a vet could. Whatever it is like a tsp (or some really small amount) can kill a horse. Sorry I can't remember the specifics.