Rather than look to supplements to bring him up to par, you should take a look at what you are supplementing with. Look at the energy and protein provided in the feed. Is this feed designed for an older or younger horse? A horse of this age should receive feed that has at least 14 to 16% protein and will need to have a feed with enough digestible energy to meet its needs. You could contact an equine nutritionist or feed manufacturer and ask them for help in finding the right feed to meet the colt's needs. Also, consider how you are providing the feed. Is it accessible to the mare? Perhaps she is eating the feed, preventing the colt from getting it. If you are not already doing so, you should consider using a creep feeder, which is a feeder with bars in it that prevent a mare from accessing the feed, only allowing room for the colt to access it. Are the mare and colt turned out with other horses? Perhaps more dominant horses are getting the colt's feed. If the foal is getting the feed and consuming all of it and still not gaining weight, you will want to look at what type of feed you are providing and consider feeding one higher in energy, perhaps one that includes fat as an energy source.
Also consider your de-worming program. Foals should be de-wormed on a regular basis beginning by 30 days of age. If your foal has not been de-wormed, begin with a panacur or strongid paste that covers the foal's weight. It is always a good practice to use a weight tape if a scale is not available. If the weight tape indicates the foal weighs 300 pounds, add 100 pounds to that weight and give it 400 pounds of de-wormer. Weight tapes are not 100% accurate, so this way you cover the possible error.
