My horsey newsgroup suggested this cross-post.

There are a few gaps / weak points in one of our hedges. It's a mixture of
allsorts; ash, alder, sycmore, thorny things. What can we plant that might grow
reasonably quickly and not get eaten by horses? On limestone, fairly exposed,
hilltop in S Gloucestershire. (I already have a list of things that are toxic
to horses, to filter against)

Thanks...SteveR