Oh boy. I was about to start a similar thread about this myself; "Grass won't grow back?"
I'm running DF v0.31.25, and I'm having a similar problem. The areas I made activity zones for grazing pastures for my livestock hasn't quite been sufficient and my livestock number's been going up (i really need to kill most of them off), and they've started eating away the grass in the areas I penned them up in. The most visible is a huge perfect rectangle that was eaten up by some watter buffalo/yaks I had before I slaughtered them. The grass is gone, it's red sand beneath. It's been an in-game year or two, and nothing has regrown there, same for a few other patches of red sand that have been eroded out.
On the other hand, I earlier went through a project to drain some murky pools I had near my fortress entrance. Basically, all I did was build a screw pump next to it and gush all the water out of the pool and onto the ground beside it (before paving over the murky pool tiles to remove them from collecting rainwater). However, I was then left with a huuuge blot of ground that was covered in 'piles of mud'. Nothing really grew back there...until I looked at the wiki about cleaning up mud, and then paved a dirt road over the muddy area. That seemed to work, and the furrowed/paved dirt is growing grass again. However, there's one big difference. The ground is peat in this area, not red sand.
So, I wanted to know before I go any further if this makes any difference, but most of all, how can I start the red sand growing grass over it again? I need the pasture space...