Black Locust does spread, and can send roots 'across the street', but it will take a long time.
If you coppice it a lot, that may send the roots out, but I would imagine that coppicing would have more of an effect on growing new shoots near where it has been cut. (Just guessing on that.)
My horses ran loose in a pasture with lots of black locust around but never got sick. My sister's horse died after eating wild cherry leaves, which contain cyanide when dry. I think her horse was just bored, on low pasture., and nibbling on anything it could find. It ran to the creek and keeled over.
I believe that the key lies in having enough variety in the pasture.
Most animals are smart enough to not eat poisons unless they need to self medicate.
(Humans are about the only animal dumb enough to 'OD')
As the Swiss-born alchemist Paraclesus observed,
All substances are poisons; there is none which is not a poison.
The right dose differentiates a poison and a remedy.
I second the idea that given good forage they will be fine. Around us dairy farmers warn about having to many apples in the pasture. If the cows eat to many green apples they colic(bloat) and can die. But given enough grass our cows and sheep do just fine. It seems like smart rotational grazing would keep it safe.
If any animal has the ability to eat as stupidly as a human, it would be a horse--especially a fancy high-brow horse. MUCH more likely to get bloat from too good of pasture than poison themselves on browse.
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