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Poisonous Pasture Weeds

Posted: June 19, 2012

As hot and dryer weather arrives in Pennsylvania, poisonous plants in pastures become a greater concern. Slow growth of pasture grasses can easily result in overgrazing, which means that grazing animals might not have sufficient forage intake and start eating some less desirable plant species.

Grazing animals will very rarely eat poisonous weeds if there are other options. Keeping pastures growing rapidly and knowing which species to be most concerned about will help in minimizing the risk of poisonous pasture plants.

Table 1. Selected poisonous plants of the Northeast (adapted from Fishel, 2000; Hardin, 1973; and Hill and Folland, 1986; and Wolfgang, Penn State)
Common Problems/symptoms Toxic ingredient — toxicity dosage
Bouncing bet Leaves and stem – delayed for several days; depression, vomiting, abdominal pain, diarrhea Saponin – amount equivalent to 3% animal dry weight) will kill within 4 hrs.
Buttercups Leaves and stem especially in flower. Dried hay loses toxicity - anorexia, salivation, weakness, convulsions, breathing difficulty, death Protoanemonin - toxicity reported to vary with species, age, and habitat, Generally 1-3% of body weight necessary.
Cherry, black Leaves (wilted leaves are worse), stems, bark and fruit - anxiety, staggering, breathing difficulty, dilated pupils, bloat, death Cyanogenic glycosides - Less than 0.25 lb leaves (fresh wt.) can be toxic to 100 lb animal. Leaves from several small to mid-sized branches are sufficient to kill an adult animal.
Hemlock, poison All plant parts - nervousness, salivation, vomiting, diarrhea, weakness, paralysis, trembling, dilation of pupils convulsions, and coma, death Coniine and others (pyridine alkaloids) - 0.5 to 4% (fresh wt,) equivalent of cattle wt, is toxic. In horses, 0.25% of body weight.
Jimsonweed Entire plant (seeds are most toxic) - Thirst, mood swings, convulsions, coma, death Solanaceous alkaloids – 0.06 to 0.09% (dry wt.) equivalent of animal body wt, is toxic.
Locust, black Leaves (especially wilted), seeds, and inner bark - causes weakness, depression, anorexia, vomiting and diarrhea Phytotoxin robin, glycoside robitinm - bark extract and powder in amount equivalent to 0.04 - 0.1% of animal wt. toxic to horses, Cattle 10-times more tolerant.
Milkweeds Entire plant - depression, muscle tremors, spasms, bloat, difficult breathing Glycosides and galitoxin - 0.3 to 0.6% of body weight.
Nightshade species Vegetation, unripe fruit - loss of appetite, salivation, weakness, trembling, paralysis Solanine - toxic at 42 mg/kg (LD50). 0.1 to 0.3% of body weight.
Pigweed species Foliage (worse in drought) - kidney disease, weakness, edema, rapid respiration Nitrates nitrate oxalates, unknown - 0.5 to 1% of diet. Sheep, hogs, and young calves most susceptible.
Pokeweed Entire plant, especially roots – gastrointestinal cramps, weakened pulse, respiration, salivation Phytolacctinm - 10 or more berries can result in toxicity to humans, Unknown for livestock, but perhaps 100-200 berries/1000 lb.
Snakeroot, white Leaves and stem - constipation, loss of appetite, salivation, rapid respiration. Toxin passes through milk (milksickness) Trophine alkaloid - varies from 1 to 2% of animal body wt, after 2 weeks. Toxin cumulative.
St. Johnswort Flowers and leaves – photosensitivity which leads to redness of muzzle, around eyes, and around white hair Hypercin - uncertain

Key points about weed forage quality and poisonous plants:

  • Some weeds have excellent nutritive quality.
  • Weeds in the vegetative stage of development usually are more nutritious than more mature weeds.
  • Regardless of weed quality, livestock may avoid grazing certain plants because of taste, smell, or toxicity.
  • Some plants contain potentially poisonous substances that may be toxic to livestock if consumed – properly identify potential problem weeds and consult with a veterinarian if necessary.
  • A productive pasture is important to reduce the potential incidence of toxic weed exposure to livestock.
  • Remember to soil test and maintain the proper lime and fertility levels. If possible, routinely mow or spray to manage weed problems within and around pasture area

Contact Information

Marvin Hall, Dwight Lingenfelter and Bill Curran
  • Penn State Forage and Weed Specialists