A variety of plants that occur as normal constituents of native pastures can accumulate toxic compounds and represent a risk to grazing livestock if they are consumed.  Surveys suggest that 3 to 5 percent of the cattle, sheep and horses that graze native rangelands in the western U.S. are affected by plant toxicity – representing a major economic loss.

The response of any animal to consumption of toxic compounds can be quite variable.  The quantity of toxin consumed is controlled by the quantity of the plant consumed and the concentration in the plant, which is altered by growing conditions, plant maturity and often the soil on which the plant is growing.  Ingestion of a plant containing toxins may be modified by the current palatability, the nutritional status of the animal and the animal’s previous experience with a particular plant.  In addition, an animal’s response to toxin ingestion may be modified by its own capacity to detoxify the compounds consumed.  In other words, the toxic compound may not be a problem if the animal consumes it at a slow enough rate that it can be detoxified before it causes harm.

Several factors can predispose livestock to greater risk from consumption of poisonous plants.  Many potentially toxic plants grow during the earliest part of the growing season.  Initiating grazing in the spring when forage is in short supply and/or only poisonous plants are green, may force animals to consume plants they might otherwise avoid.  Unloading, trailing or grazing naïve animals, those without previous experience with a particular poisonous plant, may lead to greater consumption than animals with previous exposure to that same plant.  Forcing livestock to graze in areas where heavy stands of poisonous plants grow may promote higher consumption levels.  Inadequate water, salt or mineral supply may lead to unusually high intake of toxic plants.  Turning hungry livestock into a new pasture containing poisonous plants may also result in consumption of toxic levels.  Grazing depleted grassland that lacks a diversity of plant species will limit diet selection and may not allow livestock to consume compounds needed to detoxify particular poisons.

Research has demonstrated that cattle in higher body condition, when exposed to toxic plants, spend less time grazing those plants and consume lower quantities than cattle in lower body condition.  Because few treatment options are available for livestock consuming toxic plants, developing procedures to avoid or reduce the risk of poisoning are essential.  Procedures may include:

  • knowing which potentially poisonous plants grow in your area
  • be able to identify poisonous plants, even at the vegetative stage before flowering
  • know the season and or growing conditions that lead to highest risk of toxicity
  • develop a grazing  plan which avoids areas containing toxic plants when they are most poisonous
  • avoid overgrazing
  • avoid leaving livestock in any pasture too long
  • avoid moving livestock to new pasture when they are stressed, thirsty or hungry
  • always provide adequate water, salt and mineral
  • be especially careful with newly acquired livestock, particularly those from furthest away

Some toxic plants may be controlled using herbicides.  Many of these plants establish a substantial soil seedbank.  Without alternative management, poisonous plants may reinfest an area that is controlled.  Conservative grazing management which encourages competing plants may delay reestablishment.  One research study demonstrated that reseeding with competitive, adapted perennial grasses was effective in reducing reintroduction of locoweed.

Many plant poisoning episodes are beyond treatment.  Immediate removal of animals from a suspected pasture may minimize losses.  If animals become sick, a veterinarian should be consulted and diagnosis of the illness insured.  Treatment of symptoms may minimize losses.   If plant toxicity is suspected, locating and identifying the plant responsible is essential in order to take corrective action.

Source: Roger Gates with contributions from Adele Harty