feeding for yearlings info needed - Susan G?

[Deanna German <finishis2win@columbus.rr.com>]

I'm worried that a well-intentioned fellow boarder is misguided in her
notions of proper feeding. She just bought about 30 bales of the sweetest,
greenest, leafiest alfalfa I have ever seen and intends to feed it as the
only source of forage for her QH yearling. I haven't seen the yearling, but
her 6YO is quite the butterball.

I'm one of two or three close candidates to advise this owner since I
purchased my horse as a yearling.  But at that time, I left the feeding up
to the barn manager who told me it's best to keep horses that young on the
lean side (not skinny, but lean). My understanding is that you don't want
fat young horses because it's just that much more strain on developing
joints. Is that true? What about other nutritional concerns? We're located
in Ohio where we don't have the probs with alfalfa that owners who feed
alfalfa grown in the southwest do.

There's plenty of good grass mix hays available. I feed a very nice,
consistently sweet smelling and green, consistently baled grass hay with a
little alfalfa that's easy to obtain. Wouldn't that be better to feed the
yearling?

TIA 

Deanna (Ohio)