Growing up in Arkansas, there were oaks everywhere. Never had a problem with the leaves at all, but the acorns were like crack for some horses. THe acorns aren't so bad after a good rain because it leaches the tannin out of them (the poison). They can founder from it, either the big dietary change or the excessive tannin, but we just penned our "crack heads" off the oak trees until a good soaking rain. The other horses never seemed to care. Usually, the horses that are stalled a good deal of the time, or dry lotted (and obsese which is probably related to a metabolic problem and they don't know WHAT they need or don't) that were just desperate to chew on most anything. Of course, we had a forest of them, not just one or two, so a couple of trees don't pose as much of a threat overall, They probably consume the nuts as they fall, rather than waiting for all the nuts to fall, then gorging on them. Or, if all else fails, try hosing the nuts down pretty good, but I wouldn't cut down good shade trees over it.