Add me to the "I am not a Purina fan" but if that's your only choice use their ration balancer not a mixed feed in a bag
Age of the horse, living environment, and what it's job is can also make a big difference as to whether or not it needs specialized vitamins.
My next best thought to is contact the closest university with an equine facility and ask to speak with an equine nutritionist.
Purina has plenty of those but don't talk to them. They will tell you the world is flat unless you feed their feeds and supplements - then it will be round again