My daughter's Paso Fino does not do well with sweet feeds - not only do they make her "hot" and hard to handle, but she tends to turn her nose up and the feeds that are really good for her.
Your foal is really just a baby - and the sweet feed is like feeding him candy all the time. Try slowly cutting out the sweet feed and adding pelleted alfalfa. If necessary, you can add a little water and foal lac. Some weanlings get upset tummies during the process of weaning off of mother's milk and onto regular horse feed. Most likely, your baby hasn't had a lot of experience with hay, and may not quite know what to do with it yet.
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