I feed the Progressive Nutrition Grass Balancer to all of my non-show horses, to include broodmares and foals. For those that need more calories, I add alfalfa pellets. I'll have the occasional horse that needs more calories (and I don't like pouring a large amount of feed into a horse), so I'll add Envision. I've never had one refuse the Envision. Maybe you had a bad bag? My show horses are on the alfalfa balancer, alfalfa pellets and an alfalfa/timothy mix chopped hay.
As Charlotte said, the foals are born strong and up on their legs, which didn't happen before I made the switch to Progressive. I also have some horses, like my little 28.5" broodmare, that doesn't need any extra calories beyond her hay and what grass she gets, so if I didn't feed her the balancer, she wouldn't get anything unless I dry lotted her for the rest of her breeding career, and even then, I probably still couldn't feed her the recommended amounts of any of the other kinds of feed. It wouldn't really hurt her, but it would affect her foals.