In natural settings, horses develop selective grazing habits, seeking out healthy grasses and forages and avoiding those that are harmful. Relatively few plants are poisonous to horses and, fortunately, horses tend to avoid eating these because of their low palatability.
Horses with an abundant supply of quality pasture or hay avoid temptation to select harmful plants but those in dry lots and box stalls become desperate, sampling “anything green,” and so potentially ingesting toxic plants. Hungry horses, such as can happen after a long trailer travel, may also indiscriminately ingest all sorts of plants which might normally not interest them.
Horses that lack minerals or salt may develop strange cravings causing them to make poor plant selections. Thus balanced, proper and complete nutrition plays a highly important role in poisoning prevention.
It is important that horse owners familiarize themselves with noxious plants specific to their particular locale or environment. Recognizing these plants when they are quite small, even before the flowering stage, enables more effective control. Uprooting or mowing them in a timely manner can remove the plants before they become a problem. Identification of noxious weeds in their dried states can also prove invaluable should these troublesome plants accidentally become baled in hay.
Caution is advised when clippings or trimmings of ornamental yard plants such as yew and oleander are discarded. These two plants are highly toxic to horses. Lawn grass clippings can also be detrimental. Clippings can be easily overconsumed by horses, potentially causing colic or laminitis. In addition the composting processes that occur within piles of lawn clippings results in harmful moulds and toxins.
Noxious plants and weeds are exceptionally hardy plants thriving in areas of drought, overgrazing and/or high traffic. For example, field horsetail, which resembles a small pine tree, will grow in overgrazed pastures or along trampled fencelines. Since horsetail contains thiaminase, horses consuming it develop a vitamin B1 deficiency. Bracken fern, similarly causes a thiamine deficiency and is mainly a problem when incorporated in hay.
Both these plants become problematic with repeated ingestion. Toxicity symptoms include weight loss, jaundice, weakness, unco-ordination, anxiety, paralysis, convulsions and sudden death. These are symptoms generally seen with any plant toxicities.
The toxicity of a plant may vary with weather and growing conditions. For example, weather conditions that stress legumes, especially the clovers, may cause them to host “black patch” fungus.
This fungus flourishes in cool, wet and humid conditions. It appears late spring, throughout the summer and even in fall months as tiny black specks on clover plants. When ingested the horse may begin to salivate profusely, visibly “drooling.”
Affected horses can become dehydrated and depressed, losing fluids and electrolytes. This condition known as “Slobbers” will typically disappear within a day or two after the horse stops eating infected plants. Take the horse off the offending pasture, and feed it hay while it recovers. Ensure the availability of plenty of clean water and free-choice salt. Under certain weather conditions these infested clovers may also predispose horses to photosensitization and scratches.
Plants such as burdock, buttercups and foxtail can also cause mechanical or chemical reactions similarly resulting in increased salivation and drooling. Arrow-grass, death camas, chokecherry/pin cherry leaves, tall larkspur, tansy, timber milk vetch, water hemlock, narrow-leaved milk vetch and silky lupine are other plants that have been identified in livestock poisonings. Provincial and federal governments have comprehensive publications and websites identifying noxious plants and their geographic distribution.
Awareness and prevention is the best medicine when dealing with poisonous plants, as some poisonings quickly attack the central nervous system or liver leaving little recourse for successful treatment.