Thin 2 yr. old-normal?

I had a young Arab gelding once that seemed to be a hard keeper.  An Arab breeder friend suggested feeding him whole oats and no other grain.  He gained weight and it did not make him "full of his oats" as I had feared it would.  My fox trotter and Quarter horse only get oats for grain and are rather rollie pollie.

It might help.  Growth spurts do tend to make them look improperly fed.

In cold weather it sometimes helps to blanket them, then all their food can go to growing instead of trying to keep warm. 

16.1 and 2 1/2 years old? She's been growing like a weed! The name of the game is not to keep up with the growth spurts, but to temper them as she hits them. Think of it as an ongoing slow burning fire. A horse that grows evenly keeps a good even slow burn, but a horse that goes through growth spurts has the fire bursting into flame. That flame is inflammation in the joints that can lead to physitis, among others.
A balanced diet is crucial and she has a lot of growing yet to do. Calcium/phosphorus needs to be balanced for the paragraph above. Calcium/magnesium, iron/copper/zinc, sugar/starch and protein all need to be taken into consideration as well. Getting the hay tested to know where the holes are and then filling those holes will help her to temper the fire. So, don't worry about the ribs, think of the joints. If you got tall that fast, you'd be showing ribs too....patience. An equine nutritionist can put a balanced diet together for you and it would be the best move you ever made. That diet will need to be tweaked as work load, growth and changes continue.

Oats are so high in sugar and starch (especially starch) that it is used as a benchmark that all other grains are pitted against.  Keep it simple....more hay...don't feed the fire, but for true metabolic health and all needs met,  a balanced diet is a must.

I've never heard of trying to temper growth spurts.  I'm raising my 4th young horse and not once has my vet ever said anything like that and in fact has had me up feed during times of leanness due to a spurt because you want to try to keep them at a consistent weight - not to pudgy between spurts and not too lean during a growth.    That can be hard to do, but what you should strive for.

I have heard not to over feed or you will promote faster growth than the horse's body can handle and chance hurting joints due to the extra weight, But you should still feed to meet the body's current requirements.  Most people get into trouble because they aren't watching their babies close enough and up the feed to cover the growth spurt but don't cut back feed fast enough when they come out of that spurt or don't cut back at all and all of a sudden their young horse is now obese.  Or worse - feed the baby all the time like its in a constant growth spurt in hopes of getting a bigger horse faster.  A little lean is much better than over weight. 

Growth spurts tend to be part of growing - no matter what species of animal you are discussing and I would seriously talk to a vet before attempting something like tempering a growth spurt.  You could deny her fuel her body is craving and cause worse issues than riding out the spurt. 

I have a 2 yr old gelding, who when taken out of his normal enviorment, taking him to different barns where there is alot of activity, boarding barns, he rears, I have a small back yard place, with 3 other horses,He's not a "hot" horse, but tends to worry a bit, we can take him to a friends place where its about the same level of activity as home and he is just fine.
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Earl Nunes

I'm paranoid about hip bones sticking out.  I guess I didn't realize my horses were that lean but once it was pointed out to me I can't stand to see hip bones. 
(they all get SrGlo, alfalfa pellets and hay)