This link describes how to identify poison hemlock. http://ohioline.osu.edu/b866/b866_6.html
Poison hemlock is very toxic to humans, sheep, cattle, swine, horses, and other domestic animals. They can be poisoned by eating either green or dried plant material.
Mechanical destruction of the plants by mowing or cutting with a weed-trimmer before poison hemlock flowers can be effective.
Plants should be treated in the spring before they begin to flower with triclopyr, 2,4-D plus dicamba, or metsulfuron. These products will work well for lawn or pasture areas as it won’t harm grasses. Glyphosate products (e.g. Roundup) work also but they kill grass as well as broadleaf plants. Repeat applications may be needed. Follow all precautions for handling herbicides.