Originally Posted by tealamutt
The leaves are more toxic in the spring but in the fall it is more common to see poisoning from the acorns. Maple leaves are the ones you really want to worry about in the fall as the wilted leaves are toxic.
There are LOTS of varieties of maples and oaks, only a few maples are toxic (red maples for one), and the toxicity of the oaks varies. It is best to actually find out what types you have around before worrying too much...and the toxicity drops the longer they are on the ground, especially if it rains as the toxins are leached out.
We have lots of maples and oaks around and the only ones we had to cut were the red maples. Our mares happily eat some of the maple leaves, generally don't touch the oaks leaves, and do eat acorns. They have free choice hay, so they're not hungry, and we actually see them head for the acorns after they've filled up on hay. They've been doing this for years without any ill effects.
If you have any questions or concerns, though, check with your agg extension agent to be sure. Better safe than sorry.