Invasive Plants that are Toxic to Livestock



  • Learn to identify toxic invasive plants and the most effective control options
  • Examine pastures, hay fields, roadsides and fence lines regularly for toxic plants
  • Check your hay and forage
  • Prepare a grazing strategy to avoid livestock concentrating and overgrazing
  • Fence off areas in pastures where poisonous plants occur or range an alternative livestock that is not impacted by that specific plant
  • Share your knowledge with your livestock association or club members
  • Support provincial biological control programs and work with your regional weed coordinator
  • Consult your veterinarian to correctly confirm a suspected poisoning from plants


Invasive plants may directly impact livestock health. Some plants have sharp spines and burs that puncture and scratch animals, increasing stress and veterinarian costs. Others may lead to animal fatalities, either through direct poisoning, or through an accumulation of nitrates and soluble oxalates. However, the majority of toxic weed poisonings result from animals feeding on contaminated hay; most livestock are unable to selectively avoid toxic plant material when dried.


The following information is intended to assist landowners in protecting their livestock from toxic invasive plants in the Okanagan. Although some native plants are poisonous, only non-native, invasive species are included.



Toxins may occur in some or all plant parts including leaves, roots, stems, flowers and seeds, at varying degrees of concentration. Often these toxins are only poisonous during certain stages of growth or seasons of the year and therefore may provide good forage if feed at the right time.

In most circumstances livestock rarely directly consume toxic plants. Poisonings are most likely to occur through consuming large quantities of contaminated hay or silage in the early spring and during a drought when forage quality is low. In these circumstances, livestock are either unable to detect dried plants or are so hungry that even toxic plants become a potential source of food.


Several common plants can also cause livestock poisoning through accumulation of large amounts of ingested nitrates, which then convert to nitrite – a compound that reduces the oxygen-carrying capacity of blood, stressing organ tissues, a condition known as anoxia. The most common cause of nitrate poisoning is through agronomic grasses. However several weedy species, such as lamb’s-quarters, lady’s-thumb and kochia can also contain large amounts of nitrate.



Livestock poisonings are often undiagnosed as symptoms can be as general as a decreased appetite, weight loss or unhealthy appearance. Symptoms can also be as severe as liver or nervous system damage and death. If livestock are experiencing unusual symptoms it is important to contact your veterinarian for proper diagnosis and treatment. If you suspect a poisonous plant, carefully take a specimen to your local Invasive Species Society or District Agrologist. In the case of fatal poisoning, it is sometimes possible to determine the cause of death from stomach content samples.


When consulting with your veterinarian, inform them of any changes to your pasture or range, including:

  • Sparse forage due to heavy grazing, drought or poor early season growth
  • Recently moving livestock to a new pasture
  • Recently fertilizing pasture with nitrogen followed by cool weather and slow growth
  • Feeding livestock hay from a new source.




St.John’s-wort contains a photosensitizing chemical hypericin. This chemical causes skin problems when ingested by cattle, horses, rabbits, sheep, and swine with light-coloured skin; dark skinned animals are not affected. The effects are most severe when animals ingest green plants; effects diminish slightly as plants dry. St. John’s-wort poisonings have also resulted in convulsions, staggering, and coma in some animals.

For more information on this species, click here.


Hound’s-tongue contains toxic pyrrolizidine alkaloids, which interferes with cell division causing disorders of the central nervous system and hepatic failure in horses and cattle. The alkaloids are in the highest concentrations when the plant is in the rosette stage. Cattle that have ingested hound’s-tongue often experience increased thirst, nervousness, and dairy cows can experience a drop in milk yield. Unfortunately, there is no effective treatment for animals with terminal liver damage; however you may relieve pain and stress from the secondary effect of photosensitization by providing animals with shade.

For more information on this species, click here.


Leafy Spurge has milky latex that contains 5-deoxyingenol, a compound that is toxic and may lead to death in cattle and horses. The toxin may also produce inflammation and loss of hair on the feet of horses when freshly mowed or during haying. Other symptoms include irritation to the gastrointestinal tract, dermal and eye irritation and lack of performance and weakness. Although sheep and goats may be used as a form of biological control of this weed when consuming as a portion of their diet, deaths have resulted where animals are restricted to leafy spurge only.

For more information on this species, click here.


Hoary alyssum can be toxic to horses, causing swollen legs and severe lameness. Consumption of large quantities can also cause diarrhea, leading to dehydration and miscarriage. The plant remains toxic after it is dried, and most poisonings are due to contaminated hay.

For more information on this species, click here.


Tansy ragwort contains pyrrolizidine alkaloids, which may lead to irreversible liver damage in cattle, horses and possibly goats. All parts of the plant are toxic; however, the flowers and leaves closest to the flowers contain the highest concentrations of alkaloids. Other symptoms include a peculiar bleached colour of the hair, nervousness, staggering, coma and death. Lactating cattle may also experience a rapid loss of milk production. Milk produced may have a bitter taste and an unpleasant odour.

For more information on this species, click here.