Just my two cents, but Water Hemlock is a native plant and is only really dangerous if you ingest it. It's not particularly unique in that regard, many common plants naturally contain toxic chemicals. Tomato leaves, for example, contain a variety of poisonous alkaloids. At least one person is known to have died from drinking tea made of tomato leaves. Water Hemlock is obviously far more toxic than tomato leaves, but the number of people who have been killed by Water Hemlock in modern times is about the same as the number killed by tomato leaves (statistically zero). A century or so ago, it wasn't super uncommon for children to be killed by Water Hemlock because they used the hollow stems as straws, but when is the last time you saw a kid make a straw out of any kind of plant?
Livestock avoid eating Water Hemlock and most cases of livestock deaths are the result of husbandry problems (i.e. overgrazing a pasture to the point where the hemlock is the only thing left to eat). I used to manage about 10,000 acres of native wet meadows, primarily with a combination of prescribed fire and cattle grazing, and most of the pastures had Water Hemlock in them. We never lost any livestock to hemlock. For that matter, whenever we did wetland restoration projects, we'd hand harvest seed from Water Hemlock and include it in the seed mix. If nothing else, it's a great butterfly plant.
You probably know this already, Dave, but if you do decide to spray it, be careful with your livestock for a few weeks afterward. Plants often become much more attractive to grazing animals after they've been sprayed with a herbicide.
