Poisoning of cattle often depends on the palatability of the toxic plant and how attractive it is to livestock, the stage of development, the portion eaten or the amount eaten. Some of these plants are never eaten unless the animals are short on feed or confined and bored. Some toxic plants are found in certain locations and climates, but not in others.

Different toxins affect animals in various ways. Some have neurologic effects, while others affect the liver, kidneys, heart or other body system. Some of the most common toxic plants are neurotoxicants. The following information is from an overview of plant poisoning in livestock, adapted from a presentation by Alice Harty, veterinarian, and Adele Harty, South Dakota State University (SDSU) Extension cow-calf field specialist.

Milkweed (Asclepias) is a neurotoxicant, and the narrow-leafed variety is the most toxic. Galitoxin is found in all vegetative parts of the plant and can affect cattle and horses, but sheep are most susceptible. Clinical signs include profuse salivation, incoordination, violent seizures, bloating in ruminants, increased or decreased heart rate, irregular heartbeat, low blood pressure or low body temperature. Death may occur one to three days after eating a toxic dose.

Water hemlock (Cicuta maculata, C. douglasii) is found throughout North America near streams or swampy areas. The toxin is a resinoid (cicutoxin), concentrated in the roots and base of the stem. Roots are easily pulled out by grazing animals when the soil is damp. Clinical signs include acute-onset tetanic seizures (similar to strychnine poisoning, with all muscles tightly clenched) and salivation, followed by muscle twitching that progresses to seizures, chomping of jaws, coma and death.

Locoweed (Astragalus) has more than 200 species; not all are toxic. The flowers are usually white or purple. Locoweed grows in dry, alkaline soils on the Great Plains and Rocky Mountains. These plants have various toxic principles, including miserotoxin (a glycoside), which ruminants hydrolyze to a toxic form that releases nitrite, and swainsonine (an alkaloid). Other locoweeds are selenium accumulators. Swainsonine poisoning occurs two or more weeks after ingestion; once they start eating it, cattle and horses may seek it out. Swainsonine can be passed through milk.

Signs of miserotoxin poisoning are incoordination, exaggerated leg movements, clicking dewclaws, difficulty breathing, cyanosis (bluish gums due to lack of oxygen), sudden collapse, and death within four to 24 hours with large doses. Signs of swainsonine poisoning are incoordination, weight loss, staggering, inability to properly place the legs, vision impairment, abortions, birth defects (contracted tendons, etc.) and occasional male infertility. Signs of selenium poisoning (from species that are selenium accumulators) may include loss of hooves or tail hair.

Russian knapweed (Centaurea repens) is an annual plant in the sunflower family — an invasive plant mainly found west of the Missouri River. Horses are most susceptible, and signs develop after many days of eating these plants. Clinical signs include contraction of facial muscles, tongue paralysis, chewing motions without swallowing, and inability to eat/drink, leading to weight loss.

The nightshade (Solanaceae) family includes buffalo bur (S. rostratum) and horsenettle (S. carolinense). The toxic principle is solanine (a steroidal alkaloid — the same thing that creates bitter taste in sun-exposed, green-skinned potatoes). The toxin is present in leaves and green berries (ripe berries are nontoxic). Death or recovery usually occurs within 24-38 hours after eating the plant. Clinical signs include depression, incoordination, tremors, weakness in the hindquarters, inability to get up, salivation, vomiting, diarrhea or constipation.

Poison hemlock (Conium maculatum) is a perennial member of the parsley family and grows up to 6 feet (ft.) tall with smooth, hollow stems covered with purple spots; leaves are finely divided like parsley/carrots. It grows all across the United States in fertile, moist soils. Toxins include coniine and pyridine-type alkaloids in roots and young plants. As plants mature, foliage loses alkaloid content, but seeds accumulate it. Animals that eat it are often found dead or die within a few hours. Clinical signs include salivation, vomiting and diarrhea; then muscle tremors, weakness, dim vision, convulsion, coma and respiratory failure. Pregnant animals (exposed in midgestation) that don’t die may give birth to offspring with crooked limbs, cleft palate or crooked tails.