If its not fenced off from the sheep then they will eat the new shoots of whatever grows, so planting a forage crop (kale, turnips etc) would be a waste of time. If you can fence it off then I would plant a mixture of forage crops - either for the sheep to eat later this year, or if you are getting more pigs later in the year, for them.
If you just leave it then chances are you will get a lot of weeds coming up (depending on how weedy it has been in the past).