there is nothing in senior feed to hurt a non senior horse. They are usually higher in protein and higher in fat than basic feeds but if she is giving her just a cup or so a day, you might notice her gain a few pounds but other than that there is nothing to hurt her. Just make sure she doesn't feed her tons of it or she might get fat if she is an easy keeper! I have fed senior feed to foals and hard keepers before because it works great for that too!
You know how to make a miniature horse even smaller? Leave them in the dryer a little longer!
"Don't ever regret something that once made you smile"