If you breed two sheep, the baby will eat very rapidly (as expected per the wiki 1/50 chance for babies vs 1/1000 for adults). It appears that when baby sheep eat grass, it acts the same as feeding them wheat. I did a test where i put sheep into 2 5x5 pens, one with grass and one with sand floors. The baby in the grass pen grew to adult in a little less than 3 minutes instead of the expected 20.  The baby in the sand pen took the full time.

To reproduce

1. Place two baby sheep at the same time inside two separate pens, one with grass blocks in it, the other without
2. Observe the sheep
   → Note that the sheep in the pen with grass grew up significantly quicker than the other sheep

Code analysis

Code analysis by pr0cess can be found in this comment.