It tends to grow in moist pastures and meadows where it has the potential to out-compete more desirable native species. Perhaps the largest impact of a poison hemlock invasion is its toxicity to livestock. Serious livestock losses can occur when animals graze on fresh forage, harvested silage, or hay contaminated with poison hemlock. All parts of the plant are extremely poisonous. The lower portions of the stem and root are particularly deadly.
Environmental factors can alter the alkaloid composition and concentration.
Of the eight known alkaloids in poison hemlock, the principle toxins are the piperidine alkaloids coniine and coniceine. Coniceine is eight times as toxic as coniine. These two principle alkaloids affect the reproductive system and the central nervous system of animals. All classes of livestock and wildlife are affected by the toxins of poison hemlock. Cattle, goats, and horses are the most susceptible domesticated animals because they are least able to metabolize the toxic agents.