What does everyone feed their yearlings to make sure they get enough nutrients without getting too fat? Some of these "growth" feeds have such high feeding rates (4+lbs/day) I can't imagine any horse being able to eat that and not be a blimp!

I do not grain my yearlings at all, other than as a small treat occasionally. I now have 2 turned out in a large enclosure on a round bale feeder with a good bale of alfalfa/grass and free choice mineral.  I find that in my situation its better to not hurry the growth and let them eat free choice.  Over the years this plan has worked out great for me.  We have winter here (we have had 10 days below zero in February alone) they have a good fat cover (but not excessive) and manes and tails are really growing right now.

I do feed mine (Ultium Growth) but my repro vet has told me they'd be fine on a ration balancer. 

Nothing but pasture grass and they have access to mineral and salt. If cattle aren't in the pasture we have started using equilix tubs. If they are in the corral for any reason I am adament about free choice hay, we use grass that might have a dab of alfalfa and Bluebonnet feed but I never feed the recommended amount. In 20 yrs of raising horses we have never had joint or OCD issues and our horses are ridden as hard as anyones. They aren't normally started until they are 3 in the spring though too.

I feed mine 3 lbs am and pm.  Alfalfa in the am. Coastal in the pm. GRO STRONG minerals. Regular worming. Mine have all mature at 15-3 to 16-2. 

I feed mine just a little grain,Hubbard Summit, mixed with a cup, measured AFTER they are soaked, alfalfa pellets along with Dac Colt Grower plus free access to grass round bale. My 2 yr old that was fed that same way is around 15.2 or 3 already and is a April baby. 

BlueBonnet Intensify Growth and Development

Feed for a target of 15% protein for the entire diet. How you get there may vary. If you have very high quality pasture or hay with some alfalfa in it, you may be there already. With very poor quality grass hay only, you can get there by adding alfalfa if you have access to it, or with a ration balancer (this is the proper place for ration balancers). The challenge of a ration balancer is that just about all of them are soy based, and some horses are sensitive to soy. Calcium / Phosphorus balance is very important for proper soft tissue to bone balanced growth. Avoid high protein alfalfa and a ration balancer in combination. Also keep grain based feed to a minimum or eliminate it completely as that will unbalance your calcium to phosphorus ratio.  Poor hay, resulting in low total protein in the diet, will result in less than optimal growth that will never be recovered. On the other end, too much protein can result in the wrong kind of growth leading to developmental issues and future lameness. It is a balancing act, and hitting the sweet spot of the proper nutrition allows a colt to grow to its genetic potential without developmental issues.