I posted before about how we’re fencing off some of my lawn for day-rotation pasture. My lawn has a small maple tree in one spot. I’d prefer not to remove it because we don’t have a lot of trees. How far away from the tree do my pastures need to be, and are horses likely to graze the leaves if they have a steady supply of food?
we had maples in Kentucky, I never remember any of the horses eating those…but if fencing off… whatever distance you think is enough double it since at least our horses can really reach for things that they are not supposed to have …they have concealed extend -a-necks things I believe
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Fortunately my horses tend to not eat things they shouldn’t–currently their pasture has a lot of buttercups because it’s been so wet. It makes me cringe but they’re really good at avoiding them.
I have no idea how tempting or palatable maple leaves are, or if they’re toxic while still on the tree. We have a cherry tree too that they will DEFINITELY not have access to, but it’s in a less critical location.
In Kentucky we had horses in pastures that had wild cherry, walnut and maples… all are very common trees in Kentucky, the horses never ate any if they did at least none died
but once here in Texas, the horses ate all the peach and pear trees to the ground
As for fencing off, I would keep them ten feet or more away from the tree trunk , might even put a hot wire around the thing also
But the only interest would be if the grass is not desirable then they just need to be moved elsewhere
There is plenty of desirable grass. They’re going out because lawns are a waste 
This will give us 8 rotation spots on 4+ acres for 4 horses. I don’t intend for them to be on any one spot for more than a couple days, and when my fourth guy comes home I may split them into pairs.
There are red maples everywhere down here and I have come to accept that there will be red maple leaves somewhere in my pastures (surrounded by forest) in the fall/winter and buttercups in the late spring. That said, they have good grass and hay, they are not messing with anything but that. (when I took over the worst pasture it had been overgrazed horribly by a pony who needed some groceries and yet she managed to never touch a red maple leaf, so I truly believe it is bored horses in dry lots that are at the biggest risk, and I always keep ALL leaves out of the dry lot).
My only precaution is when we have high winds/tropical storm blowing through, something that will blow off living leaves, I pull them off the most tree-filled pasture until those leaves are well past the wilt stage and firmly in the dead/brown stage. (when I took over that pasture it had been overgrazed horribly by a pony who really needed some groceries and yet she managed to never touch a red maple leaf, so I truly believe it is bored horses in dry lots that are at the biggest risk, and I always keep ALL leaves out of the dry lot).
If it is a “planted” tree in your yard, it may be a sugar maple not a red maple. As I understand it, sugar maples are okay.
Otherwise, I agree with the other posters. Have lots of red maples, not yet a problem for my horses.
Red Maples are toxic. Other types of Maples are OK. But, they have to eat a considerable amount of leaves. If they aren’t usually tree browsers, I’d not worry about it. If its a seedling or young tree they might accidentally bend or break, then I would just put a cylinder of stiff 4’ welded wire around it to protect it (maybe a foot or two in diameter).
First I would determine if the tree is definitely acer rubrum. A lot of maples are red in the fall but not all of them are red maple /acer rubrum which has one of the highest levels of toxicity to horses. The leaves of the true red maple look like little dog paws and are silvery on the leaf underside but there are a lot of hybrid varieties as well. If they are actually red maples as opposed to another maple species I would be far more cautious. I might even cut them down if they are within reach of a horse and I hate cutting down mature trees. Wilted green leaves have the highest concentrations of toxic compounds (gallic acid or hypoglycin a) a from my understanding.
For other maple species I would still take precautions but I would keep the trees and manage them; I have Norway maples lining my driveway outside the pasture fences and although some sources report they contain no toxins I keep them well pruned and blow the leaves out regularly in the fall just to be on the safe side.
This is is not the definitive source on the topic but might be helpful to you. The first link has a decent table of red maple varieties and the second discusses other maple species that may or may not be risky.
http://www.horsedvm.com/poisonous/red-maple/
https://www.horsetalk.co.nz/2016/04/17/not-all-maple-species-risk-horses-research/
If you are after protecting the tree and it hasn’t got a well developed root system then any tree fence should go out the the edges of the tree canopy so the horses feet don’t damage them. The roots that is. Agree with others that my horses never ate the red maple though I did remove downed branches with green leaves so they wouldn’t have a chance to eat wilted leaves. They left most of the trees alone but the old guy used to spend time pruning back the osage oranges. I couldn’t figure out how he managed that around the thorns those things had, but my DH sure missed him after he passed over and DH had to try to mow under those thorny trees.
