Poisonous Pasture Weeds and Livestock

Horsenettle in a pasture setting. (Source: D. Lingenfelter, Penn State Weed Science)

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 University; Colorado 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))

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