Yes and no Acorn poisoning does sometimes occur and some horses seem to get a taste for them but there are plenty of ways of preventing excess consumption. The tannins in the leaves and bark are at lower levels and can be a useful addition to the diet. I sometimes pick the odd twig for ours and they usually want them when offered. If the trees are young they may not even have started producing acorns yet, but even mature trees have years where they produce few or none. Mast years (lots of acorns) are usually obvious before they fall, so the tree(s) can be fenced off or raking up the acorns can be factored into the daily routine. They're more of a problem if grazing is short or the horse(s) are restricted for weight loss but equally it's easier to pick acorns up from short grass.
I have 2/4 sides with Oak, but on the other side of the fence, so not actually directly accessible to the horses. Not a problem, ( I fence off when the acorns drop, wildlife removes acorns very swiftly) horses don't seem to eat the fallen leaves, but occasionally nibble at any green ones they can reach. I did have to get a leaf blower out this year as the wind came from a different direction and failed to blow the leaves clear of the field , never happened before, but I probably wouldn't have bothered had I not reseeded that field in the autumn.
Tends to depend on the horses. 1 of mine doesnt bother nibbling on oak leaves, the other will grab a few mouthfuls if low enough to reach…a mouthy curious gelding.
As a ‘just in case’ i took off the lower limbs that were hanging low enough for easy reaching. Lots of leaf fall, and they like eating fallen leaves but tend to leave the fallen oak leaves.
I have one that seeks them out, yet the others don’t bother. I’ve seen a horse almost die of acorn poisoning so it’s not something I’d risk. Mine are fenced off when there are acorns falling and until they’ve all rotted away.
Yes, but watch them in the autumn. If there is plenty of grass they should be fine, but if they are on restricted grazing you may find that some horse will crunch through acorns like they're sweeties. My elderly Arab made herself quite poorly doing this a few years ago. Some years there don't seem to be many acorns, which makes life easier, but some years there are lots. Just something to keep an eye on, and be prepared to fence off oak trees if needed.