is this safe for a 3y/o filly?

My farrier suggested that I should feed a skinny 3 y/o filly that I take care of senior feed, or tizwhiz, or both mixed together. Now I may go for the tizwhiz, but I thought senior feed could be only fed to horses like 6-7 years and up. Is really safe to feed to her?

Senior feed is safe to feed horses 2 years of age or older. However, it's not the most cost effective choice for putting weight on a horse. Free choice forage, a pelleted ration balancer and supplemental fat would work as well unless there are dental issues.

The reason SR feed is often used on severly underweight horses is because it is a complete feed. Depending on the brand etc for the most part you do not have to limit the amount given. But, as said before, it is not very cost effective.