I am a fan of Rejuvenaide and use it on all of my sucklings. It has helped me avoid dealing with physitis and also works well to improve lax and tight tendons / ligaments. My foals are not on ration balancer until they are around 4 months. Once they are consuming the appropriate amount of the RB, they don’t need Rejuvenaide.
A nice side effect is that they love the taste. After having something yummy squirted in their mouths every day, all of my homebreds are super easy for deworming and oral medications. 
Posted at the same time as HHR, didn’t mean to be repetitive.
Progressive’s website:
http://www.prognutrition.com/
BTW, their foal starter and creep does wonders for a foal if the dam doesn’t produce enough milk. It is milk based, but not a milk replacer.