I fed my Weanling right up until she was two on Suagrbeet, Hi Fi Light, Garlic, and Allen and Pages Calm and Condition, lots of grass and some good hay.
And in the winter months we gave her a few buckets of Redi Grass

Our yearling went from being quite 'dainty' to being in halter condition when we fed her the 'Sovereign Speciality Mix' that is specially milled by Sovereign QHs. That yearling is now somewhat older and has just been started under saddle. She still has the same feed and looks fantastic. The feed has never made her hot or fizzy and all it's ever had mixed with it is a bit of HiFi Lite. (Obviously with hay and or grass!). Most of our horses are now on this same mix and it's seemed to suit young and old horses alike and not just Quarter Horses.
It's available from David Deptford at Sovereign QH's in Cambridgeshire or from Alan Payne at Payne Performance Horses in Leicestershire.

I fed my yearling on Baileys Number 3 stud cubes from when I got him last year, and will continue to feed it until he is about 3 and a half. Its readily available from most feed merchants, it is more expensive than a standard horse and pony cube, but we are still talking under a tenner a bag. He has done brilliantly on it, grown lots and really filling out now he is 2.

Everyone has different experiences and ideas on what has worked well for them, I think really you just have to go by your own horse and what works well - you could always call your vet if you get really confused - they should be able to recommend a good feed and the right amount to give.

Hi

I have my tow year old quarter horse since he was 10 months old, he has always been on Dengie Alfa A and sugarbeet, (in winter only) summer he has just grass and he looks fantastic.

I am a great beliver in not 'overdoing' a youngster.

Everyone on our yard feeds variations of Alfa A, Alfa Oil or Hi-Fi with horses all shapes and sizes.

Great products

I use baileys conditioning cubes and no 1 , hi fi and a supplement called 365 , he looks great, our problem is there is no grass just mud paddocks for turnout