Veterinarians and horse owners often wonder if there is some insidious weed growing in the pastures/paddocks when a vague disease syndrome, such as colic, alopecia, chronic itching and laminitis, occurs during the pasture period. An investigation into plant poisonings requires knowledge of horse-feeding behaviour, common agricultural practices and a common sense approach.

The investigation of potential plant poisonings is similar to finding a needle in a hay stack. One can gain considerable knowledge by walking a pasture and identifying the predominant plants that are growing, by collecting samples of plants the horses are eating and looking at stocking rates of the pastures. Samples can later be referred to experts or identified by viewing references. Many pastures have fence rows full of potentially poisonous plants. These will not cause a problem unless the horses have nothing else to eat. Therefore, being knowledgeable about horse-eating behaviour and being able to identify the common pasture plants will help in the preliminary investigation of problems.

Horse Behaviour

Horses are selective grazers and normally graze some areas like a golf green while leaving other areas (roughs) long and defecating in them. However, they still retain the tendency to browse when something succulent is available. They also seem to change their eating behaviour from summer to fall to winter and will browse on some trees in the fall. In general, there is no use worrying about the toxicity of ungrazed plants in the roughs. In most cases, poisonous plants can be present in horse pastures and the horses will not touch them unless there is nothing else to eat. This is a key to the investigation. The best method of investigating a problem is to walk the pasture alone, without the distraction of others talking or directing your investigation. Look for places where the horses are leaning over the fence. Look for areas where the horses have browsed trees and plants and where they have not. All plants are suspect, including trees both inside and overhanging the pasture, pasture plants, plants growing in the water and plant material that has been thrown over or piled near the fence by neighbours.

Plant Dynamics

Excellent resource material on how plants grow, how to identify the common pasture plants and weeds, as well as an understanding of their growing habits can be found in the OMAFRA Publication 505, Ontario Weeds and Publication 30, Forage Production. Weeds in pasture can be best described by dividing them into whether they are annual, biennial, or perennial.

Annual weeds are usually only a problem in the first year the pasture is established (seeded). When the ground is disturbed, there is an abundance of annual weed seeds which have laid dormant in the soil and now germinate. Of the annual weeds, bristly foxtail (Setaria verticillata) and wild mustard (Sinapis arvensis) have the most potential to be bothersome. Bristly foxtail have sharp-pointed, bristly segments with forward pointing barbs. Since these barbs will only slide in one direction, they can easily become embedded in the tongue and gums where they act as a foreign body, stimulating excessive salivation and/or deep ulceration. Wild mustard and canola have the potential of being poisonous to horses. Cattle in Canada were reportedly poisoned after they ingested hay containing large quantities of wild mustard seed. Wild mustard and canola contain glucosinolates similar to other Brassica species (kale, cabbage). Glucosinolates are chemicals that can inhibit the function of the thyroid gland. Reports indicate that cattle initially salivate excessively, after which they stagger, collapse and die. No horse toxicities have been reported.

Annual weed problems in new pasture/hay-field seedings can be handled in a number of ways. For spring seedings;

  • use a selective herbicide during the seeding year or
  • cut the fields at a height to remove the flowers before the seed heads are set. This should only be required in the first year of pasture/hay-field establishment.

Changing to a late summer - early fall direct seeding will allow the annual plants to germinate but they will die over the winter without developing seed heads.

Weeds in old horse pastures are usually perennial weeds and a result of poor pasture management or the presence of old fence lines. For example, field horsetail (Equisetum arvense), which resembles a small pine tree, will grow in damp areas of the pasture that have been overgrazed. It is commonly found in horse pastures. Horsetail contains thiaminase, an enzyme which causes thiamin (B1) deficiency in horses. Thiamin is normally produced by microbial synthesis in the intestines. Unless horses are consuming horsetail every day (as in hay), sufficient thiamin will be synthesised to prevent a problem from the occasional ingestion of the plant. Bracken fern (Pteridium aquilinium), similarly, will cause a thiamin deficiency. It is a native fern that grows across most of Canada and is mainly a problem when incorporated in hay.

The presence of many weeds in old pastures can be prevented and/or reduced by preventing overgrazing (with pasture rotation), regular applications of fertilizer, the occasional clipping of pastures, spot applications of herbicide at key periods of weed development and frost seeding.

Of the perennial weeds, milkweed, thistles, buttercup and burdock have the most potential to be bothersome. Milkweed (Asclepias spp.) are native perennials found in eastern Canada in fields, ditches, and waste places and are common nuisance weeds. They reproduce by seed and by horizontal spreading underground roots, which produce new leafy stems. The plant contains cardiac glycosides that are toxic to animals. Normally, livestock find the plant distasteful but, during a drought, sheep have been poisoned. Milkweed are difficult to eliminate due to their ability to shut off the root system if herbicides are detected.

Burdock is a bothersome biennial because of its burs which tangle the horses' manes and tails. Mechanical irritation of the mouth, tongue and teeth can cause excessive salivation, drooling or frothy saliva, or oral ulcers. Similar mechanical irritation can occur with bristly foxtail, long-spined sandbur and raspberry canes. Burdocks can also cause ocular lesions and, if caught in the forelock may "seed" the eye. The best time to control burdock is in June and July when the first year rosettes are easily identified by their large, heart-shaped leaves with wooly undersurface. Spot treatment (spraying each rosette) with 2,4-D at this time will prevent a mess the following year in horses' manes and tails.

Fresh Versus Dry

Some plants are toxic, both in the fresh state and the dried state, while others lose their toxin once the plant is frozen or dried. The buttercup (Ranunculus acris) is an example of this. In the fresh state, buttercup contains a bitter, acrid juice that causes severe pain and inflammation and a local dermatitis of the muzzle, much like solar dermatitis. However, this volatile chemical dissipates within a few days of the plant being cut in hay or being frozen. Horses are often seen eating the buttercups in the pasture a few days after a hard frost.

Red maple (Acer rubrum) is a native tree found in eastern Canada. Leaves of this plant in the wilted state have poisoned horses in the eastern United States. Hemolytic anemia occurs within 1-5 days of ingestion of wilted red maple leaves. The Cornell Poisonous Plants Informational Database lists "gallic acid" as the toxic principle. The signs of toxicity are similar to those oberved with the Brassica poisonings (canola, kale, cabbage). Fresh leaves do not cause any symptoms when eaten but wilted and dried leaves remain toxic for 30 days.

Black walnut trees, nuts or leaves have never been documented as causing founder (laminitis). However, the shavings of black walnut are well known to cause founder in horses. Within a few hours of being bedded on bedding containing as little as 20% black walnut shavings, horses will exhibit signs of founder.

Yew toxicity is an example where consuming both fresh leaves as well as clippings of the branches will cause acute death. The yew family includes: English yew (Taxus baccata), Japanese yew (Taxus cuspidata) and Canada yew (Taxus canadensis). The fleshy fruit pulp is considered to be the only nontoxic (or low-in-toxicity) part of the plant. Taxine (taxin) is the toxin. It is a complex mixture of alkaloids that is rapidly absorbed from the digestive tract and interferes with heart action. Taxine's toxicity is not reduced by drying. The lethal dose is estimated at 0.5-2 g/kg for horses (0.05% of body weight or 0.5 lbs. for a 1000-lb. horse).

Alsike clover is known to cause two syndromes, photosensitization and big liver syndrome. Although the toxicity has been described for 70 years, the actual toxin is unknown. It is, however, thought to be a mycotoxin from a fungus. Horses show the photosensitization syndrome with short-term exposure, while liver damage and enlargement is usually associated with long-term exposure. This problem has occurred mainly in horses and, occasionally, in cattle. Therefore, when seeding pastures/paddocks, ensure that alsike clover is not present in the seed mix.

Buckwheat (Fagopyrum esculentum) and St. John's wort (Hypericum perforatum) cause primary photosensitization. Photosensitivity is a sunburn-like reaction commonly seen as a result of ingesting photodynamic agents. These agents circulate to the skin, are exposed to UV light, fluoresce and cause oxidative injury to the cells of the skin and loss of layers of the skin. Secondary photosensitization occurs with tansy ragwort (Senecio jacobaea), groundsel (Senecio vulgaris), fiddleneck (Amsinckia intermedia), common heliotrope (Heliotropium spp.), vipers bugloss (Echium vulga), and rattlebox (Crotolaria spp.). They contain pyrrolizidine alkaloid that causes hepatogenous (liver-associated) photosensitivity.

Plant poisonings can usually be prevented through regular pasture management, e.g., weed removal, and familiarization with the more common poisonous plants in your geographic area.

Plant Hitchhikers - Fungi and Mycotoxins

Pasture grasses, hay, grain and straw can all support the growth of various fungi. The fungi can exist as saprophytes, obtaining nutrients from the plant with no benefit to the plant, or exist within the plant in a symbiotic relationship, providing benefits to the plant while obtaining nutrients from the plant. This topic is too complicated to explain in the confines of this information sheet. However, a brief overview is provided with references to the more detailed resources.

Fusarium is a fungus which grows mainly on cereal grains, e.g., corn and wheat, and produces a number of mycotoxins that may affect various animals upon ingestion or inhalation. The fusarium species can also grow on grasses used for horse pastures and hay. The toxin produced by the fungus is the main concern but fungal spores can be inhaled and become allergens and the cause of 'heaves'. Horse owners should be aware of the potential of high fusarium levels and their related mycotoxins during wet falls, especially in corn and corn byproducts. Wheat grain normally does not contain significant levels of mycotoxins. However, in growing years, when there is high precipitation during seed-head formation, high levels of fusarium and its associated mycotoxins are common. The mycotoxins can be present in the grain as well as the straw and any byproducts, such as wheat middlings or bran. See the OMAFRA information sheet, Molds, Mycotoxins and Their Effect on Horses for more details.

Neotyphodium coenophialum was formerly called Acremonium coenophialum. It is the endophytic fungus which produces mycotoxins, resulting in the disease condition called 'fescue toxicity.' The mycotoxins of endophyte-infected fescue (fungus completes its entire life cycle inside the plant) belong to the class of chemicals called 'ergot' or more specifically 'ergopeptine alkaloids.' They are well recognized as causing dystocia (difficult birth) in mares and deaths of perinatal foals (unborn and newborn) in the United States. Therefore, when seeding pastures/paddocks, ensure that the seed does not contain endophyte-infected varieties of fescue.

Ergotism is the clinical syndrome caused by the genera of fungi, Claviceps. Ergotism is probably the oldest known mycotoxicosis. The ergot alkaloids of Claviceps purpurea are hallucinogenic in humans and have been associated with historical accounts of witches. Consumption of infected rye bread has been associated with human disease dating back some 2,000 years. Claviceps can live on a variety of hays and pasture grasses and produce fruiting bodies on bluegrass and cereal rye. On an Ontario farm, late-gestation mares that consumed their cereal-rye straw bedding were affected by a clinical syndrome similar to fescue toxicity (thickened edematous placentas, agalactia and seven perinatal deaths in foals) except that the mares foaled around their normal due date. See the OMAFRA information sheet Ergot Alkaloid (Ergopeptine) Toxicity in Horse Hay and Pasture for more details.

Research into the effects of mycotoxins on horses is in its infancy, so the general rule is to feed/bed horses with hay/grain and bedding free of mycotoxins. Late-gestation mares are known to be very sensitive to the ergot alkaloids and, therefore, should never be bedded on cereal-rye straw or fed endophyte-infected hay or stubble. Consequently, it is imperative to ensure that all pasture/hay seed mixes containing tall fescue and perennial rye grass are guaranteed endophyte-free varieties.

The Investigation

Often the most difficult problem, when investigating a toxic plant concern, is knowing what is normal. Some of the basic information that should be gathered or determined is:

  • What is the stocking density - the number of horses grazing per acre?
  • Is the pasture or stored feed supplying the majority of the horses' daily nutrients? If stored feed is being fed, what is the quality of the feed? Can you estimate the protein content and, therefore, the relative feeding value?
  • What plants and trees are present in the paddock or along the fence line? If you can't identify them yourself, then bring back samples to someone who can.
  • What species of plants are being grazed? Collect samples.
  • What species of plants are not being grazed?
  • Estimate the total nutrient intake from all sources.
  • What appears to be out of place? Is there a feed source or plant type that is not normally seen on horse farms, e.g., spurge or black walnut?
  • What type of hay was purchased? What is the source of the purchased hay?

The investigation is a process of keeping your mind open, documenting plant types, amounts fed, recording the batch numbers of feed and keeping representative samples (in the freezer in triplicate) of suspect or potentially suspect material.

Many of the background resources used in the development of this information sheet are available on the Horses - Feeding and Pastures page of the website.

The following are excellent resources:

Hall JO, Buck WB, Cote LM. Natural Poisons of Horses, 2nd ed. Urbana, Illinois: National University Poison Control Center, 1995.

Kingsbury JM. Poisonous Plants of the United States and Canada. Englewood Cliffs, New Jersey: Prentice-Hall Inc., 1964.

Knight AP, Walter RG. A Guide to Plant Poisoning of Animals in North America. Jackson, Wyoming: Teton NewMedia, 2001: 142-150.

Mulligan GA, Munro DB, Poisonous Plants of Canada. Ottawa, Ontario: Agriculture Canada, Publication 1824/E, 1990.

Alex JF. Ontario Weeds. Guelph, Ontario: Ontario Ministry of Agriculture and Food, Publication 505, 2001.

Upfold RA, Wright HC. Forage Production. Guelph, Ontario: Ontario Ministry of Agriculture and Food, Publication 30, 1994.

Canadian Poisonous Plants Information System

Cornell Poisonous Plants Informational Database

For more information:
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