What are your thoughts on the calcium supplement? I'm worried I may be causing (or could cause in the future) him an imbalance...The owner was worried about him growing too fast and had put him on it so his bones would support his growth.
Get an anylasis done on the hay. Most hay has sufficient calcium already as well as oats. I would skip the calcium supplement and just go with the source. The additional calcium will put the calcium phosporus ratio way off what the requirements are.
Agree with QueenRider, I'd cut out the calcium supplement, get my hay checked out or at least find a supplement that works with the type of hay that you are feeding (grass vs. legume). I'd recommend a Select The Best vitamin/mineral supplement, but I think you can only buy the product in Canada. They have one to pair with grass gay and one to pair with legumes so that the calcium:phosphorus ratio stays balanced.
PS. I've never used Source before, but from the little info on the site that you provided it looks like a decent supplement to be using with fescue hay.
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I don't know specifically about young horses, but some years ago there was a lot of people supplementing young large breed dogs with too much calcium on the same theory about bone growth. It created some serious problems. So you have to consult a vet or nutrition expert and look at the whole diet. In general calcium has to be in balance with all other minerals such as magnesium. Also things like their Vitamin D from sunshine and cured hay has as much to do with Calcium absorption as just supplying the mineral alone.
The calcium supplement is probably to balance the inverted calcium to phosphorus ratio in oats. If you switch the horse from oats to a formulated feed that has a balanced calcium:phosphorus ratio, you can (and should) eliminate the calcium supplement.