Toxins and Poisons

 Knowledge Nuggets | Fact Sheets | Research Papers

Knowledge Nuggets

  • The level of toxins produced in harvested forages will depend on moisture level, amount of heating, subsequent molding and forage maturity. As well, toxins may be introduced through weed contamination and insect infestation.
  • Molds are produced when excessive moisture is present in harvested forage. The toxins that are produced depend on moisture, temperature, forage type and nutrients. The physical characteristics of mold growth are of little value in determining the toxic effective. Mycotic abortions can be caused by moldy forages and occur sporadically.
  • Improper fermentation can produce silage that causes botulism or listerosis. Botulism is a potential problem when the silage pH is higher than 5.0. The toxin produced by the bacterium Clostridium botulinum is one of the most potent toxins known. Cattle with botulism are weak, can't get up and may die 1 to 3 days later. Listerosis can also be present in poor quality silage, allowing rapid growth and accumulation of the bacteria. Some animals may eat the silage and develop a brain infection resulting in a dysfunctional central nervous system that causes the animal to walk around in circles.
  • Nitrate accumulation can occur when annual crops are under stress during the growing season. Heavy fertilization, drought, diseased plants, soil mineral deficiencies, chemical injury, frost or hail are factors are causes. In general, perennial forages do not accumulate toxic levels of nitrates. Nitrates convert to nitrite in the blood, the nitrite complexes with hemoglobin to produce methemoglobin that reduces the oxygen carrying capacity of the blood and can cause death due to suffocation. Sick, hungry, lactating or pregnant animals are more susceptible to nitrate toxicity.
  • Spoiled sweet clover hay or silage can cause hemorrhagic anemia or Sweet clover poisoning. Some sweet clover varieties contain large levels of coumarin that is converted to dicoumarol by fungal growth after harvest (molding). The fibrous stems of sweet clover make it hard to harvest without having some mold growth. However, spoilage does not guarantee that the toxin is present, nor does the lack of visible signs of spoilage rule out its presence. It is best to use varieties that have low coumarin levels, make sure that the hay is dry before baling or ensile at the proper moisture content.
  • Prussic acid or hydrogen cyanide poisoning may occur when animals are fed some varieties of bird's foot trefoil, sorghum grains, sorghum grass, sudan grass, hybrid pearl millet, foxtail millet, Indian grass, flax, chokecherry and elderberry. These plants may accumulate hydrogen cyanide (HCN), especially when young plants are stressed due to drought or freezing. Poison levels are higher in fresh forage than in cured forage. If the hay is not properly cured, toxic levels of HCN can be present.
  • Some tall fescue and perennial ryegrass turf varieties are infected with endophytes, a fungus that lives inside of a plant. The principal toxin that is produced is ergovaline. These toxins constrict blood vessels and reduce the circulation to the outer parts of the animal's body. After 10 to 20 days of feeding endophyte infected forage clinical signs may appear. Affected animals have difficulty regulating their body temperature causing 'fescue foot'. Fescue foot is characterized by tissue death in the legs, ears and tails characterized by lameness and swelling of the legs. Two weeks later, the loss of tips of tails and ears or the sloughing of hooves can result.
  • Several species of blister beetles contain an irritant chemical, canthardin, which affects the gastro-intestinal tract. The irritant is very stable and will remain in cured hay indefinitely. This chemical is especially toxic to horses and ingestion of only 1 or 2 beetles can cause colic. If hay is cut with a swather, most of the beetles will move out of the swath before it is baled. If the hay is cut with a mower conditioner, the crimped dead beetles stay in the hay and may cause some problems.
  • Harvested forages contaminated with weeds may contain toxins. Some of the most toxic weeds include: spotted water hemlock, poison hemlock, seaside arrow-grass, death camas, tall and low larkspur and timber milk-vetch.
Fact Sheets

Blue-Green Algae Poisoning

Endophyte Toxins in Grass Seed Fields and Straw Effects on Livestock - available in PDF format only

Grazing Cover Crops: Toxicity considerations

Infertility due to Phyto-oestrogens

Molybdenum Poisoning

Nitrate and Prussic Acid Toxicity in Forage - available in PDF format only


Polioencephalomalacia in cattle: a consequence of prolonged feeding barley malt sprouts

Problems Caused by Specific Forages

The Merck Veterinary Manual

Toxic Contaminants in Harvested Forages - available in PDF format only

Research Papers
Send us your research papers relating to toxins and poisons and we will be happy to post them here!
  For more information about the content of this document, contact Grant Lastiwka.
This document is maintained by Linda Hunt.
This information published to the web on October 27, 2003.
Last Reviewed/Revised on February 16, 2018.

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