"Most animals do not eat it other than nibbling on the leaves. To create a toxic condition requires cattle to eat 1- to 2-pounds of the material," said Cole. "Fortunately I don't know of very many deaths attributed to it around here. I suspect at this time of year the plant will be coarse enough it will be sorted out from any grass-legume hay and not be eaten. The risk to animals should be minimal unless they have nothing else to eat."
Symptoms include nervousness, trembling, incoordination and death in some cases. Pregnant cows that graze poison hemlock may experience birth defects in their calves such as cleft palate and spinal abnormalities. The critical time in the pregnancy for cattle is 40 through 70 days.
MU Extension Guide sheet G-4970, "Plants Poisonous to Livestock," is available online. There are a number of poisonous plants that lurk in Missouri pastures, ditches and fencerows that can harm and even kill livestock that eat them along with baled forage.
The most common poisonous plants of Missouri include black cherry, black locust, black nightshade, bouncing bet—also known as soapwort—bracken fern, buttercup, common cocklebur, field horsetail, jimsonweed, johnsongrass, milkweed and mustard species, Ohio buckeye, poison hemlock, common pokeweed, snow-on-the-mountain, water hemlock, white snakeroot, wild indigo and wooly croton. Some of the toxic species, like wooly croton and mustard, are relatively harmless unless the animal ingests large amounts.
If livestock poisoning is suspected, call a veterinarian immediately and remove livestock from the grazing area until all poisonous plants have been destroyed.
"Hemlock is becoming a real problem and is on the order, if not greater than knapweed in its infestation," said Tim Schnakenberg, an agronomy specialist with University of Missouri Extension. "It can be eradicated by applying a recommended herbicide early in the year and later in the year every effort should be made to minimize seed production. This may be done by mowing in the bloom stage."
Recommended control is to use Remedy Ultra, Tordon 22K or Grazon P+D before poison hemlock bolts in the early spring. It may also control it in the fall in the rosette stage.
Information about herbicide treatments is available online under the "agriculture" link at the MU Extension website.
Source: University of Missouri Extension