Queen Anne's Lace
Reports from Europe have indicated that Wild carrot (Queen Anne's Lace) has the potential to create mild neurotoxicity in horses. I'm not aware of any cases that have been reported in Michigan. Therefore, I'm not certain on "safe levels" of exposure.
Below is a link to a MSU Extension article on controlling wild carrot in hay fields and pastures.
On another note, Poison hemlock is a plant that looks very similar to wild carrot and poison hemlock is extremely toxic to horses.