Personally I think slow, steady growth is better than sprouting up like a weed in the space of 6 weeks. The cutting horse trainer I worked for several years ago fed all of his babies under 4 years free choice grass hay and a little bit of alfalfa (I think it was 2 flakes?) in the evenings. They spent their 2 year old year in light work so only received beet pulp and some whole grain when they were underweight. 3 year olds were worked a bit harder so were on Purina Strategy full time with added beet pulp if they lost weight (which didn’t happen more than once the entire 4 years I worked for him). All of his horses looked great, nice shiny coats, lovely muscle, and the babies grew up steadily with good weight on them.
I bought an underweight, wormy 2 year old last fall and in the time since he’s gained just over 100 pounds and filled out nicely on about 3/4 lb of Omolene 200 and 1/2 lb of Amplify, plus free choice grass hay. : )
Hope this helps,
I call you Far Rider, not for your races and your fine pony, but because you are one who rides far from himself, and wishes not to look home.