Hay Bales are crafted with 9 wheat.
According to http://www.minecraftwiki.net/wiki/Horses#Foods 1 wheat heals 1 heart, a hay bale heals 10 hearts which is nearly no mismatch.
But looking at how fast one can speed up the growth of a foal, there is a huge mismatch:
1 wheat = 1 minute
1 bread = 3 minutes
1 hay bale = 3 minutes, too
So you could speed up the growth of a foal with 3 bread by 9 minutes, but with 1 hay bale only 3 minutes, like with 1 bread, that is crafted with only 3 wheat ?
Either the speed growth of a foal must be larger for a hay bale, or a hay bale should be crafted with 4 and not 9 wheat!
If you craft a hay bale with 4 wheat, then the speed growth rate of a hay bale could be set to 4 minutes, and for a golden apple then something more (up to you; at the moment a golden apple seems to speed up 4 minutes)
If the hay bale would be crafted with 4 wheat, then the amount of heart it heals must be reduced of course in the same step.
As 1 wheat heals 1 heart, it should heal 4 or 4.5 hearts then I suppose.
(1 wheat = heals 1 heart, 1 bread = heals 3.5 hearts)