hello all, I know there are a few donkey people here on TAS

Wondering what everybody does regarding restricting grazing in summer, or rather, when do you lift grazing restrictions as autumn rolls in?
My two have been on a track system since April and it's kept them really trim all summer, and they seemed happy enough. But now not sure when I should give them the run of the whole field again. It has only rained here once in 5 weeks, so while there is still a lot of grass about (3 acres, quite rough in parts, sharing with flock of Soay sheep) the grass is looking rather 'old' now.
I'd love to let them run around as they please again, but worried it might still be too early? (this was my first summer with them.
As ever, thank you for any advice!