University of Maryland Extension

Toxic Plants and Accidental Poisoning

Sara BhaduriHauck

Many plants that are commonly found in and around pastures and hay fields can cause poisoning in livestock animals. In many cases it may be impossible to completely remove all toxic plants, but having the proper knowledge about how to manage against poisoning by toxic plants can be enough to prevent it.

In most cases, livestock will avoid eating toxic plants as long as there is adequate other forage available. Livestock are more likely to consume toxic plant materials:              

  • when they have finished their forage ration and are bored;
  • during times of the year when pastures are sparse, such as during a drought, when pastures are snow covered, or when pastures are overgrazed;
  • during times of the year when plants might be stressed and may become more toxic, such as during drought;
  • if the hay you provide is very weedy and there is no other forage available to them;
  • if toxic weeds and/or their seeds collect at the bottom of a hay feeder and the feeder isn’t cleaned out;
  • or if new plant material is introduced to animal access areas, such as a neighbor or the power company dumping yard waste over a fence, fallen tree limbs due to a storm, or after repairs or construction like alteration of a fence line.

It’s also important to remember the saying “the dose makes the poison.” In the case of most toxic plants, an animal must eat a significant quantity to be poisoned. Usually, an animal will not exhibit any symptoms if they’ve only consumed one or two plants or a few mouthfuls. (Two exceptions to this rule are hemlock and yew, which are extremely poisonous even at low doses.) Different types of animals also respond differently to different toxins. Some plants are poisonous only to ruminants and not to non-ruminants, and vice versa.

There are hundreds of plants that can cause toxicity, but not all of them are present in all areas. Scout your pastures and hay fields to see what is growing on your farm; be familiar with the clinical signs of poisoning from those plants and of a few of the most common toxic plants you think your animals might encounter from purchased hay. A working knowledge of the principles of plant toxicity can help you prevent poisoning and recognize it quickly if it does happen.


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