Over the last several weeks I have received many calls about managing weeds in different types of pastures and rangeland. These calls always seem to happen this time of year as warmer and drier weather settles in and people notice and identify undesirable plants in their pastures, both large and small. Examples across the state of Oregon range from bulbous bluegrass to bulrushes. Though the window of time for effective cultural and chemical control of many of these species closes quickly this time of year it is never too late to consider managing weeds in pastures. Often the added reliance on maximizing forage in all types of pastures and rangeland leads to the potential to overgraze and degrade them. Taken to the extreme, this may force livestock, such as cattle, sheep and horses, to graze plants that they normally wouldn’t eat. It is under these situations that toxic plants and livestock poisoning become an issue for landowners who are either unaware of toxic weed populations in their pastures or who simply can not afford to buy additional forage.
The following blog post is a part of a series of publications I have written on this topic over the years on the description and management of poisonous plants in pastures. A partial listing of the types of toxic compounds found in plants, plant species containing these toxins and symptoms exhibited by livestock after consuming these toxins is provided here. What problem species are you dealing with? We would like to hear from you.
Pastures often contain weeds that are potentially dangerous to livestock. However, weeds with toxic compounds are generally not palatable to livestock unless no other forage is available. The toxic compounds in plants are usually a defense mechanism against predation and have a distinct, unpleasant odor or a bitter taste and are therefore avoided. However, the consumption of unpalatable plants will increase under some circumstances.
Plant growth stages can influence the palatability and toxicity of certain plants, as can climate and time of year. Some plants, like those that accumulate nitrate, can increase in toxicity after rainfall or on cool, cloudy mornings and evenings. Some plants become more palatable after a frost. Many toxic plants have specific growth stages or plant parts that are the most toxic. Avoiding grazing pastures that contain these plants when they are most toxic will greatly reduce the chances of livestock being harmed. Minimizing overgrazing, knowing the locations of established weed populations, and managing weed populations to reduce the size and spread will ensure the health of pastures and reduce the risk to livestock.
Nitrate Poisoning
Plants absorb nitrates from the soil and metabolize them into plant proteins. If plants absorb excess nitrates and are consumed by livestock before they are converted to proteins, nitrate poisoning can occur. Forage crops that are over-fertilized before being harvested or grazed can be a common cause of nitrate poisoning. However, excess nitrate accumulation also occurs readily in some common pasture weeds. Nitrate concentration can vary widely among plants and growing conditions. Nitrates are highest in plants in mornings and evenings, and on cool, cloudy days (when plant metabolism is slower). Drought, fertilization, and nutrient deficiency can result in nitrate accumulation. Highest concentrations occur generally in stems rather than leaves, flowers, or fruit/seed.
An animal’s metabolism converts nitrate (NO3) to nitrite (NO2), which is toxic. This occurs less frequently for horses, which do not readily convert nitrate to nitrite. In small quantities, nitrates are reduced by the rumen’s bacteria to microbial proteins—it is the rapid absorption of large quantities of nitrates that can lead to poisoning, overwhelming the rumen’s ability to convert nitrates into protein. Increasing the carbohydrates (energy content) in an animal’s diet can prevent poisoning as it allows the conversion of nitrates to proteins to occur more quickly, reducing the likelihood of nitrate poisoning.
Symptoms of nitrate poisoning include drowsiness and weakness followed by muscular tremors, increased heart and respiratory rates, staggering gait, and recumbency (inability to remain standing without support). Sub-lethal doses can cause abortion and reduced milk production. Animals suspected of having nitrate poisoning should be kept stress free and the suspect food source removed. Forages assumed of being high in nitrates, especially if they have been heavily fertilized with N-fertilizer or experienced drought, should be tested.
Oxalate Poisoning
Rather than absorbing excess nitrates, some plants store high quantities of potassium and sodium oxalates (salts). If large quantities of oxalate accumulating plants are eaten, the rumen is overwhelmed and unable to metabolize the salts and they are absorbed into the bloodstream. In the bloodstream they form insoluble salts that precipitate in the kidney, causing kidney failure.
Sheep are most susceptible, then cattle. Cattle are able to detoxify large quantities of oxalates in their rumen, reducing chances of poisoning. Animals can develop a tolerance for oxalate accumulating plants by building up the concentration of oxalate-degrading bacteria in the rumen. If eaten in small amounts over time, with other feed to dilute the concentrations in the rumen, oxalate accumulating plants cease to be a problem.
Within a few hours of poisoning, animals develop muscle tremors, tetany (calcium deficiency), weakness, and recumbency. Coma and death can follow within 12 hours of consumption. Livestock should be adapted to oxalate plants over four days, incrementally increasing the time allowed to graze the plants, before being left in pastures containing high concentrations of oxalate-accumulating plants.
Common Pasture Plants Causing Nitrate and Oxalate Poisoning
- Amaranthus retroflexus (Redroot pigweed)
- Chenopodium album (Common Lambsquarters)
- Malva neglecta (Common mallow)
- Rumex spp. (Dock species)
Plants Affecting Liver Health
Photosensitization
Plants causing liver disease and photosensitization (sensitivity to sunlight) are often grouped together, as photosensitization is often, but not always, a secondary symptom of liver disease caused by poisonous plants. As chlorophyll breaks down, it becomes phylloerythrin, a phototoxic compound. In healthy animals, the liver filters phylloerythrin from the blood, preventing any damage. If the liver is compromised by toxins, it is unable to remove the compound from the blood, and photosensitization occurs. Some plants contain compounds that, once absorbed into the bloodstream, react to UV exposure, without any effect on the liver. Photosensitization resembles severe sunburn. Plants from a variety of families can impact liver health or cause-related nutrient deficiencies. Photosensitization symptoms are most significant on white-skinned animals or white-skinned portions of animals, around the face, and near hooves.
Common Pasture Plants Affecting the Liver and Blood and/or Causing Photosensitization
- Alliums pp. (Onion species)
- Descuriania sophia (Flixweed/Tansy mustard)
- Equisetum spp. (Horsetail and Scouring rush)
- Hypericum perferatum (St. Johnswort)
- Pteridium aquilinum (Western brackenfern)
- Thermopsis rhombifolia (False lupine)
- Tribulus terrestris (Puncturevine)
- Trifolium spp. (Clover species)
- Vaccaria pyramidata (Cowcockle)
- Xantium strunarium (Cocklebur)
Pyrrolizidine Alkaloid Poisoning
Pyrrolizidine alkaloids are the most common cause of liver damage. Found in numerous species, pyrrolizidine alkaloids are most toxic for pigs, then poultry, cattle, horses, goats, and sheep, with sheep being the least susceptible. These alkaloids cause photosensitization, liver and kidney damage and can also cause cancer and heart failure. Animals will not readily eat plants containing pyrrolizidine alkaloids unless no other forage is available. However, plants become more palatable when dried and will be readily eaten in hay, with little loss of toxicity. Effects are cumulative, so symptoms may not appear until long after the toxic plant was eaten.
Common Pasture Plants Causing Pyrrolizidine Alkaloid Poisoning
- Amsinkia intermedia (Fiddleneck)
- Cynoglossum officinale (Houndstongue)
- Senecio spp. (Ragworts and Groundsels)
- Symphytum spp. (Comfrey)
Plants Affecting the Nervous System or Causing Sudden Death
Cyanogenic Glycosides
Cyanogenic glycosides are present in many plants and are converted to hydrogen cyanide or prussic acid when plant cells are damaged. The concentration of cyanogenic glycosides within a plant is variable: growth stage, moisture, and time of day can all influence plant cyanogenic glycosides levels. Fertilization and herbicide application can increase cyanogenic glycoside concentrations. Chronic cyanide poisoning from eating sublethal doses over time causes loss of nerve function. Acute cyanide poisoning causes sudden death. Care should be taken to remove or exclose plants containing cyanogenic glycosides from pastures.
Common Pasture Plants Affecting the Nervous System
- Acroptilon repens (Russian knapweed)
- Apocynum cannabinum (Hemp dogbane)
- Centaurea solstitialis (Yellow starthistle)
- Cicuta douglasii (Western waterhemlock)
- Conium maculatum (Poison hemlock)
- Daucus carota (Wild carrot)
- Delphinium spp. (Larkspur species)
- Prunus spp. (Black cherry and Chokecherry)
- Trifolium spp. (Clover species)
- Triglochin spp. (Arrowgrass)
Plants affecting cardiovascular health
Cardiac glycosides are the most common toxin affecting cardiovascular health and are found in a wide variety of plant families. Generally, all parts of the plant are highly toxic and lethal if eaten in small quantities. Plants containing cardiac glycosides are not a significant cause of livestock death, as most plants are not very palatable.
Common Pasture Plants Containing Cardiac Glycosides
- Asclepias spp. (Milkweed species)
- Digitalis purpurea (Foxglove)
- Solanum spp. (Nightshade species)
Plants Causing Irritation
Some plant compounds will irritate the digestive tract, mouth, or skin if consumed. Death seldom results from the consumption of these plants. Plants are not generally palatable and are generally avoided.
Common Pasture Plants Causing Irritation
- Conyza canedensis (Horseweed/Fleabane)
- Euphorbia esula (Leafy spurge)
- Juniperus occidentalis (Western juniper)
- Ranunculus spp. (Buttercup)
- Vicia villosa (Wooly vetch)
Plants Causing Physical Injury
Many plants have characteristics that can cause injury to grazing animals. Many grasses can be palatable when young, but injurious once they have matured with long awns that cause injury to the nose, eyes, mouth and ears of grazing animals.
Common Pasture Plants Causing Physical Injury
- Bromus spp. (Ripgut brome, Cheat, Downy brome, etc.)
- Cenchurus lonispinus (Longspine sand bur)
- Holcus spp. (Velvetgrass species)
- Hordeum spp. (Foxtail and Hare barley)
- Taeniatherum caput-medusae (Medusahead grass)
- Tribulus terrestris (Puncturevine)
- Urtica doica (Stinging nettle)
Summary
- Survey pastures-know what toxic weeds are present and understand their biology/ecology
- Manage grazing to minimize risk and maximize forage
- Manage pastures to minimize weed spread
- Control weed populations when necessary
- Make an adaptive management plan based on the resources available
- Keep the plan flexible-evaluate effectiveness and change methods/timing if necessary
- Monitor successes and failures in your plan
- Use multiple management techniques to minimize costs and to avoid unintended effects
Additional Resources About Poisonous Plants
Brown, D. Cornell University Poisonous Plants Informational Database. Cornell University.
Knight, A.P. and R.G. Walter. 2001. A Guide to Plant Poisoning of Animals in North America. Teton NewMedia Publishers, Jackson, WY.
Knight, A.P. Colorado State University Online Guide to Poisonous.
Oregon State University Weed, Poisonous Plant, and Pest Resources for Small Farms Weed Management and Control.