Did the lady you bought your horse from say what supplement she used?? That would be what I would go with, at least temporarily.
If the horse came from a completely different environment (such as desert vs. rainy and lush), that may have to be re-evaluated.
The Horse Guard looks good, except my concern would be the Selenium. I am not a nutritionist but that 54 ppm on the G/A web page looks to be really high. Someone from this company would have to explain why that's ok. http://www.horseguard.com/pdfs/HG_Gu...20Analysis.jpg
The article in the link below states:
In this part of the country, between 1-3 mg of selenium per day is generally adequate and safe for most horses.
Notice the comment opens with "In this part of the country", meaning selenium deficiency levels (or overages) vary all over the U.S. And that is a total amount, which includes hay and pasture.
Again, not being a nutritionist and lucky to be able to balance my checkbook, I don't know how "54 ppm" in HorseGuard translates to "between 1-3mg" on the veterinary website below.
Selenium is one of those minerals that has a low toxicity level. http://www.bendequine.com/documents/Selenium.pdf
That all being said, respectfully disagree with feeding this horse grain or even a ration balancer to make sure it gets all its vits/mins. I would call the Seller and ask her what she used and go from there, since it sounds as if the Seller was trying to kindly say not to feed this easy keeper grain