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Poisonous Pasture Weeds and Livestock

Poisonous Pasture Weeds and Livestock

By Dwight Lingenfelter

During drought and other poor environmental conditions that reduce forage growth, there are concerns for poisonous weeds in pastures and hay. Livestock may be forced to graze on weeds that normally they would not, or they may eat weeds out of curiosity. Scout your pastures and remove these weeds before they cause livestock health problems. Keep in mind there are numerous poisonous plants that could invade an area or pasture. Many plants contain potentially poisonous substances that may be toxic to livestock if consumed. In addition, certain plants may be problematic because of mechanical irritation when eaten, photosensitization, and disagreeable tastes or odors in meat, milk or milk products. If you suspect livestock poisoning, call your local extension educator or veterinarian immediately. If death occurs, the stomach contents should be examined for consumed herbage. Identify the suspected plants and remove livestock from the grazing area until all poisonous plants have been removed or destroyed. The table below lists only some common weeds and their poisonous properties; many other plants can be toxic to livestock.

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.
  • Recently, there has been some research that suggests that for every pound of weeds present in pastures, available desirable forage is reduced by one to one and a half pounds! So, if a pasture is really weedy, there is a lot of forage that is not being consumed by the livestock or is unable to compete with the weeds.

For additional information and resources on plants that are poisonous to livestock see these sites: Cornell UniversityColorado State University; and the Weed Science Society of America. In addition, there are numerous other websites that contain information on this subject. Simply conduct a web search for poisonous plants and livestock

Selected poisonous plants of the Northeast

(Information adapted from Fishel 2000; Hardin 1973; and Hill and Folland 1986 and D. Wolfgang, (retired, Penn State))

Common nameProblem/symptomsToxic ingredient – toxicity dosage
Bouncing betLeaves and stem – delayed for several days; depression, vomiting, abdominal pain, diarrheaSaponin – amount equivalent to 3% (dry wt.) of sheep wt. killed within 4 hr.
ButtercupsLeaves and stem especially in flower. Dried hay loses toxicity – anorexia, salivation, weakness, convulsions, breathing difficulty, deathProtoanemonin – toxicity reported to vary with species, age, and habitat. Generally 1-3% of body weight necessary.
Cherry, blackLeaves (wilted leaves are worse), stems, bark and fruit – anxiety, staggering, breathing difficulty, dilated pupils, bloat, deathCyanogenic 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.
Clover speciesVegetation – Hairballs; Sweet clover: nose bleeding, anemia, abdominal swellingCoumarin with sweet clover - varies
Fern, brackenEntire plant – Dullness, fever, bleeding, loss of appetite, and salivationGlycoside thiaminase – Cattle fed 50% bracken for 30 to 80 days was toxic. Others report that only 20% of diet for 30-60 days was toxic.
Garlic, wildAll plant parts – tainted milk and meatOnly toxic in large quantities
Hemlock, poisonAll plant parts – nervousness, salivation, vomiting, diarrhea, weakness, paralysis, trembling, dilation of pupils convulsions, and coma, deathConiine and others (pyridine alkaloids) – 0.5 to 4% (fresh wt.) equivalent of cattle wt. is toxic. In horses, 0.25% of body weight.
JimsonweedEntire plant (seeds are most toxic – Thirst, mood swings, convulsions, coma, deathSolanaceous alkaloids – 0.06 to 0.09% (dry wt.) equivalent of animal body wt. is toxic.
Locust, blackLeaves (especially wilted), seeds, and inner bark - Causes weakness, depression, anorexia, vomiting and diarrheaPhytotoxin 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.
MilkweedsEntire plant – depression, muscle tremors, spasms, bloat, difficult breathing.Glycosides and galitoxin – 0.3 to 0.6% of body weight.
MustardsAll parts (especially seeds) – oral and gastrointestinal irritation, shaking, salivation, abdominal pain, vomiting, and diarrheaThiocyanates, irritant oils, and nitrates (large quantities generally necessary for toxicity)
Nightshade speciesVegetation, unripe fruit – loss of appetite, salivation, weakness, trembling, paralysisSolanine – toxic at 42 mg/kg (LD50). 0.1 to 0.3% of body weight.
Pigweed speciesFoliage (worse in drought) – kidney disease, weakness, edema, rapid respirationNitrates nitrate oxalates, unknown – 0.5 to 1% of diet. Sheep, hogs, and young calves most susceptible.
Pokeweed, commonEntire plant, especially roots - gastrointestinal cramps, weakened pulse, respiration, salivationPhytolacctinm – 10 or more berries can result in toxicity to humans. Unknown for livestock, but perhaps 100-200 berries/1000 lb.
Snakeroot, whiteLeaves 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. JohnswortFlowers and leaves – photosensitivity which leads to redness of muzzle, around eyes, and around white hair.Hypercin - uncertain


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