I love Triple Crowns 30% supplement over any actual feed I have ever used. Even for youngsters. For my area, It seems to be the perfect balance to our grass hay. Its very low starch and most horses seem to thrive on it.
If I need more calories than what they can eat in a days worth of good grass hay and the RB, I add Alfalfa hay pellets or cubes in 5 lb increments typically , rice bran, cool calories and/or Triple Crown Sr feed with the RB (percentaged out to keep it balanced lb for lb) depending on the situation.
I try to only feed forage and a RB as my base diet and some extra fat in the form of rice bran pellets or cool calories if possible for needed calories. If that fails, I up the forage quantity/quality, and if that still isn't enough, Ill change the feed to something like TC Sr. That protocol has never failed in all ages and breeds of horses. If it still doesn't work, im looking for things like ulcers and hind gut acidosis or problems with deworming/encysted strongyles, tapes, resistant bugs etc.