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Awareness of Potential Plant Toxicity to Grazing Animals


How important is the subject of toxic or poisonous plants to livestock or equine producers?

Effects on animal health from consuming or contacting potentially toxic plants can range from none to death. Potential deleterious effects include tainted milk; liver or kidney damage; cardiovascular, nervous system, musculoskeletal, or gastrointestinal problems; photosensitization or allergic reactions; birth defects or reproductive failure; nitrate poisoning; or sudden death. To protect the financial and often emotional investments in animals, it is important to have an awareness of how toxic plant poisoning most often occurs. In addition, all weeds negatively affect the longevity and productivity of pasture and hay crops. Therefore, effectively scouting for toxic plants and other weeds is important. It is difficult to make generalizations about toxic plant species. Plant species with toxic potential (the ability to cause detrimental effects) are found among a wide variety of plants. The toxic potential of a plant species depends upon several factors, including the species of animal exposed and the dose or amount of toxin consumed or contacted. The age, size, health, sex, or reproductive status of the animal may also be important. Toxins within a plant can be confined to one or more plant parts (seeds or nuts, rooting structures, cotyledons, stems, leaves, or bark), or all parts can be toxic.

For example, only the seeds and cotyledon stage plants of common cocklebur contain toxins, the bulbs of Star-of-Bethlehem contain higher toxin levels than other parts, and almost all parts of yew and boxwood are highly toxic. Toxin levels can fluctuate with certain environmental conditions (alsike clover during high rainfall; high humidity), during certain seasons of the year (spring fructan toxicity with ryegrass species, oats, and others), or when a combination of factors occurs. In some species (poison hemlock), the toxic principle or compound remains in preserved forage (hay or silage). In other species, these practices reduce (hairy vetch) or eliminate (buttercup species) toxin levels. Some tree leaves are more toxic after they have wilted (red maple, wild cherry). Toxic compounds associated with forages are generally termed antiquality compounds. Several of the forage related disorders result from toxins released by fungi (fungal endophytes) growing in association with the living host forage plant (tall fescue toxicosis, ryegrass staggers, red clover slobbers, etc); fungi in moldy hay, silage, or clippings (botulism, sweet clover poisoning); or undesirable bacteria in improperly ensiled forage (listeriosis). Blister beetle poisoning in horses results from consuming a toxin found in dead beetles bailed into alfalfa hay.

A complete list of plants that are potentially toxic to all grazing animals would be daunting, and will not be attempted in this information piece. There are many sources of detailed information, both printed and internet-based, on poisonous or toxic plants. Some references are included at the end of this document.


A sampling of plant families and species with toxic potential (either naturally occurring or fungal related) are listed below.

  • Beech family – oak species
  • Box family (highly toxic) – boxwood, pachysandra
  • Buckwheat family – buckwheat, dock species, rhubarb
  • Buttercup family – buttercup species, baneberry, larkspur species
  • Heath Family (highly toxic) – azalea, rhododendron, mountain laurel
  • Legume family – black locust, crown or hairy vetch, golden chain tree, lupine species, pea species, alsike clover, sweet clover, red and white clover
  • Lily family – autumn crocus, lily-of-the-valley, star-of-Bethlehem, hyacinth species, skunk cabbage, onion and garlic (both wild and cultivated), tulip, asparagus
  • Maple family – red maple
  • Nightshade family – nightshade species, jimsonweed, ground cherry, horsenettle, tomato, potato
  • Rose family – wild black cherry, chokecherry
  • Yew family (highly toxic) – English or American Yew
  • Other species – black walnut (shavings used as bedding), bleeding heart, burning bush, cocklebur, dogbane species, foxglove, ground-ivy, horse chestnut species, horsetail, johnsongrass, milkweed species, oleander, pigweed (amaranthus species), poison hemlock, pokeweed, privet, ryegrass species, shattercane, St. John’s wort, tall fescue, water hemlock species, white snakeroot

This list does not include several species that are highly toxic but are rarely consumed by animals, or all species that have the potential to accumulate nitrates under certain conditions. Toxic plant or antiquality issues most often occur under one or more of the following circumstances.


  • Fact: Most grazing animals do not “prefer” toxic plants while being provided an adequate quantity of safe, nutrient balanced forage.
    • Circumstance: Grazing animals consume toxic plants when they are undernourished in an attempt to provide caloric intake (energy) or balance a nutrient deficiency. Undernourished animals may consume large quantities of toxic plants, possibly over long time periods.
    • Solution: Maintain forages to produce a nutritious feed source; or provide grazing animals with supplemental feed such as high quality hay or prepared feed.
    • Circumstance: Grazing animals consume toxic or antiquality compounds when the available forage is not suitable for the given class of animal. Quantities consumed will be large unless grazing time is limited.
    • Solution: Provide forage types and varieties that are suitable for the class of livestock being grazed. If unsuited forage must be used, limit grazing time and provide supplemental forage or feed.
    • Circumstance: Grazing animals consume toxic compounds when certain forage species are grazed during the wrong growth stage (prussic acid poisoning), while experiencing environmental stress (nitrate poisoning), or during rapid growth and imbalanced fertility (grass tetany). Quantities consumed will be large unless grazing time is limited.
    • Solution: Understand when specific forages species should not be grazed or are subject to accumulated toxins; maintain appropriate fertility, do not overapply nitrogen (particularly during drought), and have forage tested when conditions for potential toxicity exist.


  • Fact: Most grazing animals are curious eaters, and will sample newly introduced or discovered food sources.
    • Circumstance: Grazing animals consume toxic plants that were previously unavailable when they are introduced to a new pasture, woodland grazing area, or other food source containing toxic plants. During the adjustment phase, the quantity consumed may be small unless the new food source is dominated by the toxic species or the animals are undernourished.
    • Solution: Scout new or renovated pastures or grazed woodland areas for potential problem species before turning grazing animals onto them. Also scout fence lines and other areas where animals can reach plants that might attract their attention. Apply herbicides for weed control when needed.
    • Circumstance: Grazing animals consume toxic plants that are accidentally introduced into the animal’s current environment, or the animal accidentally enters a different environment. Quantities consumed may be small unless the introduced toxic species are the only “green” food source or the animals are undernourished.
    • Solution: Place signage along fence lines where the public or other people have access to the pasture requesting that they do not feed the animals or throw other plant material into the pasture. Ask close neighbors to honor the same request. Keep fences and gates repaired and posted.


  • Fact: Grazing animals require a minimum period of time (up to three weeks) to safely transition between food types (i.e. grain and/or hay to pasture, different forage species, etc.)
    • Circumstance: Otherwise suitable feed, hay, or forage has toxic effects when animals are transitioned too quickly (colic, laminitis, bloat). Quantities consumed will be large if transitioning is ignored.
    • Solution: Introduce animals to new feed sources gradually. Initially limit grazing time to a few hours a day after having fed a ration of hay or grain to help limit animals’ appetites. Incrementally increase grazing times on the new introduction to the desired maximum over a two to three week period.


To summarize the solutions, grazing animals should be provided adequate pasture that meets their nutrient and energy requirements, or their diet should be supplemented with a balanced ration to reduce the chance that they will consume enough toxic plants to cause severe problems. Carefully control what food sources become available, make sure new foods are safe for the animal species, and make gradual transitions to new foods. Be aware of environmental, seasonal, and fertility conditions that may cause forages to accumulate toxic compounds and test if necessary. Scout for toxic plants, implement good forage and weed management practices, apply herbicides for weed control when needed, and remove dangerous plants from your property. Minimize the potential for accidental introductions of toxic plants via downed tree limbs, yard cuttings (especially ornamentals), or well-meaning neighbors. Provide animals with safe bedding materials. Scouting for toxic plants and other weeds should take place on a regular basis. Keep dated records of which weed species are found in each pasture or hayfield and where large concentrations occur, particularly for problem weeds (biennial, perennial, woody, and poisonous species). Scouting in late fall, mid-summer, and early spring, when forages are not as lush, allows for easier location and identification of problem weeds. Scouting should also occur after each grazing cycle in a rotationally grazed system, monthly in a continuously grazed system, or one to two weeks after each hay cutting. Visits to pastures or hayfields to conduct other management practices (fertilizer applications, soil sampling, manure removal or dragging, etc.) provide excellent opportunities to scout. Proper weed identification is vital. Small plants in the cotyledon or seedling stages are more difficult to see and identify than mature weeds in the later vegetative or flowering stages, but small weeds are easier to control with herbicides. Use previous year’s scouting records that noted concentrations of mature weeds to direct scouting efforts when weeds are in the seedling stage. Try to cover a good crosssection of the pasture. Understand that plant populations may change based on landscape positions such as upland areas, wet areas, forest edges, or floodplains. Pay particular attention to high traffic areas, hay feeding stations, bare spots, fence-lines, and field edges where new weed infestations most often begin. In addition to scouting pastures and hayfields, it is important to know potential hazards outside the pasture fence. Sacrifice lots usually become nearly devoid of healthy forage due to severe grazing pressure and compaction, while many weeds are fit to survive under these conditions. Grazing animals sometimes get free and wander into barnyards, trails, woods, or ornamental plantings where they can’t resist feeding. Well-meaning neighbors might think the animals look undernourished, or just enjoy feeding them, while others may find a pasture a convenient location to dispose of grass, ornamental, or tree clippings. Trees on your or adjacent properties often straddle fence-lines. Weather conditions can cause leaves, limbs, or entire trees to snap and fall into a pasture. Certain species when they begin to wilt may attract grazing animals, posing a significant risk of toxicity. Hay can also contain toxic plants (or blister beetles - alfalfa hay), particularly if purchased blind from far away or from an unknown source through a hay broker. Toxic plant material in bedding can be eaten, or cause a reaction or illness through physical or respiratory contact. For example, black walnut shavings used as bedding can cause laminitis in horses without being ingested.


The bottom line of managing toxic plants is to minimize your animals’ exposure.

The following points will help:

  • Partner with your local Cooperative Extension agent, forage or weed specialist from your university, and/or your veterinarian to determine which plants on or near your property can be toxic to your grazing animals.
  • Use printed or internet resources to help identify unknown species on your property.
  • Minimize the potential for accidental introductions of toxic plants via downed tree limbs, yard cuttings (especially ornamentals), or well-meaning neighbors. Awareness and good-neighbor communication are important for success.
  • Plant appropriate forage species; practice good cultural management to keep pastures productive; scout on a regular basis; and apply herbicides to control problem plant species when needed. Keep in mind that unless grazing animals are undernourished they usually will prefer safe forages over toxic plants.
  • When the amount of forage from pasture is insufficient to provide the animal’s nutritional needs, provide species appropriate, good quality, weed-free hay or other preserved forage to supplement or replace nutritional needs.
  • Make new forage introductions slowly. Some animal species require an adjustment period of two to three weeks to adjust successfully to a new food source without the threat of gastrointestinal or musculoskeletal disorders.
  • Use the right type of bedding purchased from a reliable source.
  • If you suspect plant poisoning has occurred:
    • Call a veterinarian immediately.
    • Remove all animals from the grazing area or sacrifice lot.
    • Identify the suspected plants; consult Cooperative Extension personnel or other professional.
    • Remove accidentally introduced plant material, or eradicate toxic plant species growing in the grazing area by hand-weeding, applying appropriate herbicides, pasture renovation, or other recommended practices.
    • Monitor the grazing area over time to assure the toxic species has been eradicated.


Quintin Johnson, Extension Associate IV, Weed Science and Crop Management Richard W. Taylor, Extension Specialist IV, Plant and Soil Science David L. Marshall, Assistant Professor, Animal and Food Science Mark VanGessel, Professor/Extension Specialist, Weed Science and Crop Management College of Agriculture and Natural Resources, University of Delaware


Resources for Poisonous Plants

A Guide To Plant Poisoning of Animals in North America
Anthony P. Knight and Richard Walter
Copyright 2001 Teton NewMedia ISBN # 1-893441-11-3
Teton NewMedia
P.O. Box 4833
125 South King Street Jackson,
WY 83001 1-888-770-3165
Poisonous Plants of Pennsylvania
Robert J. Hill, 1986
Pennsylvania Department of Agriculture
2301 N. Cameron Street
Harrisburg, PA 17110 1-717-787-4737
Poisonous Plants Home Page - University of Pennsylvania
Cornel University Poisonous Plants Informational Database
National Poison Control Center
1-800-222-1222 http://poisoncontrol.chop.edu/


Resources for General Weed Identification

Virginia Tech Weed ID Page
Weeds of the Northeast Richard H. Uva, Joseph C. Neal, and Joseph M. DiTomaso
Copyright 1997 by Cornel University ISBN # 0-8014-8334-4
Cornel University Press, Sage House
512 East State Street
Ithaca, New York 14850
Usually available at www.amazon.com

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