I hadn't heard of this before, but it looks just like a liquid multivitamin. It really depends on what else you're feeding your horse, but my opinion on multivitamins is that they generally don't contain "everything" your horse needs in addition to just hay/grass. The fact that this one has molasses as the first ingredient would also turn me off from it.
Good nutrition will "build your horse from the inside out," and a good ration balancer would be a great place to start for filling in any nutritional gaps in your feeding regimine. Ration Balancer