The differences between Hemlock (Conium maculatum) and Cow's Parsley (Anthriscus sylvestris; also known as Wild Chervil)

As you see they are very similar in flower but the leaves are different. Leaf of Cows parsley is like that of a fern. Leaf of Hemlock is flat.

If you are unsure do not give your horses anything from the hedgerows.