How does poison hemlock affect horses?

Poison hemlock is a coarse biennial herb with a smooth, purple-spotted, hollow stem and leaves like parsley. It grows 3 to 6 feet tall and in late summer has many small white flowers in showy umbels. Its leaves are extremely nauseating when tasted.
Although sometimes confused with water hemlock, poison hemlock can be distinguished by its leaves and its roots. The leaf veins of the poison hemlock run to the tips of the teeth; those of the water hemlock run to the notches between the teeth. The poison hemlock root is long, white, and fleshy. It is usually unbranched and can be easily distinguished from the root of water hemlock, which is made up of several tubers.
Piperidine (nicotinic) alkaloids in Conium include coniceine, coniine, N-methyl coniine, conhydrine, and pseudoconhydrine. The alkaloid content is variable with the stage of development and the stage of reproduction of the plant. During the first year of growth, the plant alkaloid content tends to be low. Plants in the second year, however, have alkaloid contents of approximately 1% in all plant parts. The alkaloid content is somewhat higher after sunny weather, as compared to rainy weather. The highest concentration of alkaloids occur in the seeds which can contaminate cereal grains. Coniine (2-propylpiperidine) and N-methyl coniine progressively increase in flowers and fruits, while coniceine decreases during plant maturation. In the vegetative stage – i.e. early growth – coniceine (W-2 propylpiperidine) is the predominant alkaloid. Coniceine and coniine are the primary teratogenic alkaloids of Conium.
Clinical signs: Susceptible species include cattle, pigs, elk, and poultry. Toxicosis has been experimentally reproduced in sheep and horses as well.
Systemic effects: After having eaten poison hemlock, animals may lose their appetites, salivate excessively, bloat, and have a rapid but feeble pulse. They also show evidence of muscular incoordination and appear to have great abdominal pain. Other signs include muscle tremors, frequent urination and defecation, recumbency, mydriasis, and “nervousness” followed by severe depression. In animals that die, breathing ceases due to respiratory paralysis before cardiac arrest. Convulsions, which occur in water-hemlock poisoning, do not follow the eating of poison hemlock.
Teratogenic effects: Birth defects due to ingesting poison hemlock occur in (at least) calves and piglets and may include crooked legs (crooked calf disease, arthrogryposis), cleft palate, and kinked tails. Arthrogrypotic skeletal malformations occur in calves when poison hemlock is ingested by pregnant cows between days 40 through 70 of gestation. Similar skeletal lesions occur in pigs between days 40 through 61 of gestation. Cleft palates can occur in piglets if pregnant swine ingest poison hemlock between days 30 through 45 of gestation.