equine poisonous plants

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Poisonous Plants and Equine

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8/6/2019 Equine Poisonous Plants

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Poisonous Plants

and

Clark E. Israelsen

Scott S. McKendrick

Clell V. Bagley

Equine

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IntroductionPoisonous plants can be found in almost allplant communities. Like most other speciesof livestock, equine/horses will not usuallychoose to consume weeds or poisonousplants when quality forages are available.They do have discriminating tastes andexhibit a preference for quality legumes

and/or grasses over less desirable poisonousplants. However, horses deprived of greenvegetation when confined to stalls and feddry hay, or horses restricted to marginalpastures, may sample a large number of green plants and accidentally or intentionallyingest poisonous plants into their systemwhen allowed to graze. If large enoughamounts of toxic plants are ingested, serioushealth problems and even death can result.

In most cases, horses consume poisonousplants only when their pasture is overgrazedand quality forage is not available. Thisis because most poisonous plants are lesspalatable than quality forages. It is alsopossible for poisonous plants to be harvestedand introduced to the horse simply as amatter of consuming its regular feedingof cured hay. Another scenario may arisewhen lawn clippings are fed to horses

(never a good idea). A surprising number of ornamental trees and shrubs that are usedfor landscaping are toxic to horses. Greenlawn clippings, often contaminated withcuttings from ornamental plants, are verytempting to horses, especially those thathave been on dry or limited feed. Horsescan easily ingest toxic poisonous plantsas they anxiously gulp down green grass

clippings. Horse owners should talk to kindand helpful neighbors and ask them not tofeed any clippings to horses because of therisks involved.

It is also important to recognize thatpoisonous plants are usually moreconcentrated in their toxins during certaintimes of the growing season. Sometimespoisonous plants ingested in small quantitiesare not a problem, while at other times, even

small quantities become lethal. Occasionallya horse will develop an appetite for certainpoisonous plants and must be watchedclosely when grazing at home or on thetrail. Mineral deficiencies in the diet mayalso cause the horse to seek out poisonousplants. A well balanced and adequatelyavailable ration will usually alleviate thisproblem.

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Prevention Suggestions:It is important for horse owners to carefullysurvey the local plant population withintheir area and learn to recognize as manypoisonous plants native to their geographicregion as possible. This fact sheet willidentify common poisonous plants found inUtah horse pastures, but the list is by no

means complete.

Persistent efforts to eliminate or controlpoisonous plants in the home pasture areessential. If there are only a few plants,they can be dug out by hand and disposedof before they spread to a larger area. If the infested area is too large, troublesomeplants can be sprayed with an appropriateherbicide.

Perhaps the best defense against weed

species is cultivating a healthy stand of grasses or legumes that can compete withthem. This can be accomplished with properfertilization, irrigation, controlled grazing andtimely mowing. Too often, small pasturesbecome overstocked, becoming holding pensinstead of productive pastures. Mowingpastures before weeds and toxic plants headout and go to seed will significantly minimizespreading. Care must also be taken when

feeding harvested hays or when riding onthe trail. Each county has a local ExtensionAgent who can help identify plants andcontrol methods unique to their area. TheUSDA Poisonous Plant Research Lab is also

on the campus of Utah State Universityin Logan. Researchers at that lab are avaluable resource to horse and livestockowners and veterinarians.

Symptoms: 

Symptoms of equine poisoning are variedand diverse. Some cases may consist of slight illness and the horse’s inability toperform to its fullest potential for a few

days. More serious symptoms may includeslobbering, tremors, uncoordination, erraticbehavior, convulsions or even suddendeath. A common symptom, especially forlight-skinned animals, is photosensitization- which consists of blisters, swelling andlesions (like severe sunburn) on the white orlight colored areas of the skin. Unfortunatelythere are other illness and conditions thatmay cause similar symptoms. As such, if poisoning is suspected call a veterinarian

immediately. Because horses cannot vomit,other methods must be utilized to help ridthe horse’s digestive system of troublingplants.

Cache Valley pasture full of goatsrue

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Common Utah PlantsPoisonous to Equine:

Poison Hemlock A relatively common range orpasture plant often found alongroadsides, fence lines, creek

beds and in pastures. This plantkills a number of cattle, sheep

and horses each year and is also toxic tohumans. This plant is not very palatableand is unlikely to be ingested when otherforage is available. However, since poisonhemlock is one of the first plants to grow inthe spring, horsesmay eat this plantin the absenceof other green

feeds. Early inthe spring, beforethe plant flowers,the leavesare especiallypoisonous.Commonsymptomsof poisoninginclude nervoustrembling,uncoordination and dilated pupils. Thehorse may be disoriented and appear to beunaware of his surroundings. Humans arepoisoned when they confuse poison hemlockroots with wild parsnips. The toxic potentialof this plant diminishes significantly as theplant dries. However, the seeds (both greenand mature) remain toxic. As such, smallquantities of poison hemlock accidentallybaled into properly cured hay prior to seedproduction, are not considered a high risk.The best way to prevent exposure to poisonhemlock is to make every effort to eliminateit from the field. It is also wise to provideadequate quality forage to horses.

 Yellow Starthistle andRussian Knapweed

These invasive weeds arebecoming more prevalent in Utahpastures and rangelands. Horseswill not eat these plants unlessgood quality, palatable feed is

gone. These plants are alsotoxic when dried in hay. Usually,horses must eat the plant over anextended period of time before

symptoms appear. The most commonsymptom is the horse’s inability to chew orswallow, although it may attempt to do so.An infected horse may chew feed only to spitit out when it cannot swallow. Owners maythink the horse has something caught in itsthroat. The horse may also chew without

having anythingin its mouthand move itslips and tonguein a peculiarfashion.Drinking isdifficult and thehorse will oftenlap the waterlike a dog, and

dehydration frequently occurs. Since theingested toxins cause damage to the brain,an infected horse may yawn frequently, rollits tongue, toss its head, hang the head low

and/or pushagainst solidobjects. Theprognosisfor recoveryonce clinicalsigns appear

is usuallypoor, usuallyresulting ineuthanasia.

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HoundstongueThis biennial weed, often foundin pastures and along roadsides,forms a rosette the first yearand produces a flowering stalkthe second year. The leaves

are rough and resemble a dog’s tongue.This plant is toxic, containing pyrrolizidine

alkaloids that cause liver damage. This mayresult in jaundice, possible photosensitization(sunburn) and/or death. Houndstongue isnot very palatable so horses generally donot eat this plantin pastures, unlessthere is a limitedamount of qualityfeed. However,houndstongueis invading anincreasing number of alfalfa and grass hayfields and when baledinto hay, horses areless discriminatingand may ingesttoxic amounts of theplant. Clinical signsmay not be apparentfor several months after ingestion and theeffects are cumulative, even if the periods of ingestion are months apart. The syndromeof liver failure progresses rapidly over a fewdays to a week. Clinical signs include weightloss, weakness, sleepiness, uncoordination,and with a yellowish discoloration to themucous membranes. The horse may haveshown no signs, but upon being ridden mayseem to tire quickly and the clinical conditionmay deteriorate rapidly to death. Onceaffected, there is no effective treatment.

The best prevention is to properly maintainpastures and hay fields to encourage theproduction of quality forages and to be surethat hay fed contains no houndstongue.

LocoweedLocoweed is a range plant. Someyears there is very little growthand other years there can besignificant growth. Horses usuallyavoid Locoweed, but after they

have sampled it a few times, they canbecome accustomed to it. Symptoms do not

appear until horses have grazed this plantfor a period of time and the most obvioussigns may not appear until after the horsehas stopped eating it. Common symptomsinclude aimless wandering, altered gait,tremors, weakness and erratic behavior.Horses poisoned with locoweed may alsohave impaired vision and actually bump intothings or try to jump imaginary objects.This cumulative effect of alkaloid poisoningmay be irreversible, though mares fed goodquality hay may regain their reproductivefunction. Some neurological damagemay be permanent and horse ownersshould be warned that recurrent, aberrant

behavior may cause the horse to becomeunpredictable and dangerous, especiallywhen stressed, loaded onto trailers orforced into working conditions. Horsesare very sensitive to the toxin. Sheepcan be managed and can graze locoweedintermittently, but horses should be kept

away from ranges which contain it, especiallyduring years of abundant growth. Locoweedseeds will survive in the soil for years,waiting for the right growing conditions.

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Field HorsetailField horsetail is a smallherbaceous perennial that prefersmoist, cool areas such as ditches,meadows or road sides. Like mostpoisonous plants, horsetail is mostlikely to be ingested when horses

have very little quality vegetation to eat.

Young horses are more readily affected thanmature horses. Horsetail contains severalcompounds, butthiaminase is theonly one of clinicalimportance.Thiaminase isan enzyme thatbreaks downthiamine, anecessary vitamin

for metabolismand centralnervous systemfunction. Horsesneed to ingestfield horsetailfor severaldays, or evenweeks, beforeadverse clinical signs of muscle weakness,uncoordination, paresis, convulsions and

potential death appear. Therapeutic dosesof thiamine, administered under veterinarysupervision, followed by subsequent vitaminadministration may prove successfulin overcoming the effects of poisoning.Obviously, poisoned horses should beremoved from horsetail infested areas andprovided access to clean, good quality feedand water.

 YewsYews are common smallornamental evergreen shrubsor trees that are toxic during all

seasons of the year. Yews contain a cardiacdepressant called taxine. Symptoms includetrembling, uncoordination, collapse, aslowed heart rate and cardiac failure. Thesesymptoms may not appear for several hoursof they may occurwithin minutes afteringesting the plant.Some dead horses

have even been foundwith yew leaves ortwigs still in theirmouths. Because thistoxin acts so quickly,there is seldom timeto initiate treatmentand thus far, noknown antidote isavailable. Most yewpoisonings are the result of homeownerscarelessly dumping lawn and shrub clippingsinto pens or pastures with curious horses.As stated earlier, horse owners should talk tokind and helpful neighbors and ask them notto feed any clippings to horses because of the risks involved.

MilkweedThis herbaceous perennial iscommon along roadsides, ditch

banks, in pastures and even incultivated fields. Colonies spreadby underground rootstalks that

are not disturbed during tillage practices.The milky latex sap that exudes when theseplants are cut or grazed isnot palatable to livestock.The western whorledmilkweed and labriformmilkweed are the mosttoxic of the milkweedspecies. This plant iseasily controlled withproper management.

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DeathcamasThis native perennial is one of the earliest plants to emerge inthe spring and grows on foothillsand in meadows. As such, mostlivestock poisonings come from

early spring grazing when other plants arestill dormant. The

underground scaly bulbsare often mistaken forwild onions and cancause severe illnessin humans as well.All parts of this plantcontain a poisonousalkaloid at all stagesof growth. Pasturescontaining deathcamasshould not be grazed

in early spring andshould be sprayed withherbicides early in theseason when otherplants are still dormant.After other forages become available, horseswill usually not consume this toxic weed.Symptoms of poisoning include excessivesalivation, rapid breathing, weakness,staggering and convulsions. Severelypoisoned animals usually die, while those

less affected may recover.

 ArrowgrassArrowgrassis a nativeperennial thatstarts growthin earlyspring and

reproduces from seed

and rhizomes. This plantcontains hydrocyanic(prussic) acid, especiallywhen drought or frost

stressed, making it highly toxic to horses.Symptoms include nervousness, convulsions,respiratory failure and sudden death. Curedhay containing arrowgrass has also beenreported toxic to young stock. Supportivemedical treatment may prove valuable, butdeath is common.

Water Hemlock This plant frequently grows inmoist areas such as wet meadowsand pastures, or on the banks of ponds and streams. It resemblesthe taller poison hemlock plant,

though the leaves are different, and alsohas hollow stems with occasional purplestreaking near the lower portion of the plant.Water hemlock can be easily identified by thecross-sectional chambers in tuberous roots.

The knife used for cutting must be cleanedthoroughly or it can carry the toxicity. Allparts of the waterhemlock plantcontain a toxincalled cicutoxin.Young leaves,tubers and greenseed are especiallytoxic. Someconsider this to be

one of the mostpoisonous plants inthe U.S. Poisoningmost often occursin the spring whenyoung, vegetativegrowth is eatenbefore mostother foragesare available. Cicutoxin acts quickly as adirect stimulant to the nervous system.Clinical signs such as muscle twitching,teeth grinding, convulsive seizures andexcessive salivation may appear as early as15 minutes after ingestion. Death, resulting

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from respiratory distress, may occur as earlyas 45 minutes after ingestion. The bestway to prevent water hemlock poisoningis to consistently destroy the plants and toprovide adequate quality forage early in thespring.

Sorghum and Sudangrass

These productive grassesare excellent livestock feedswhen grown, harvested andcured correctly. Under certainconditions however, especially

when stressed with drought or frost, theseplants can accumulate cyanogenic glycosidesand nitrates. Nitrates can create problemsfor ruminants, but the cyanogenic glycosides

can affect horses. Typical signs includeloss of nerve function to the hind legs andbladder. It is most apparent as horsesdemonstrate an inability or reluctance toback up. If the horse loses the ability tourinate normally, there will be constant urinedribbling, urine scalding of the hind legs andpotential bladder infection. There are noadequate treatments available after clinicalsigns appear. It is safest to avoid pasturinghorses on Sorghum or Sudangrass.

GoatsrueThis tap rooted, perennial legumewas intentionally introducedfrom the Middle East as potentiallivestock forage, but was found tobe unpalatable and highly toxic.

Unfortunately, it was allowed to escape andhas spread primarily by irrigation water to

pastures, fence lines, roadways and marshyareas. Most goatsrue plants can be foundin Cache County, Utah, where an intenseeradication program is underway. Unlesshorses are near starving, they will not eatthis plant.

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Additional Poisonous Plant Resources:

 “Small Pasture Management Guide for Utah,” Utah State University Extension AG 508.

 “Weeds of the West,” Western Society of Weed Science, P.O. Box 963, Newark, CA.

The USDA ARS Poisonous Plant ResearchLaboratory, UMC 6300, Logan, UT 84322.

http://www.ars.usda.gov/main/site_main.htm?modecode=54-28-20-00

 “Plants Poisonous to Livestock and Horses,” Oregon State University, http://extension.oregonstate.edu/linn/content1/poisonplants.php.

http://www.horsequest.info/

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of illegal discrimination based on race, color, religion, sex, national origin, age (40 and older), disability,

and veteran’s status. USU’s policy also prohibits discrimination on the basis of sexual orientation in

employment and academic related practices and decisions.

Utah State University employees and students cannot, because of race, color, religion, sex, national origin,

age, disability, or veteran’s status, refuse to hire; discharge; promote; demote; terminate; discriminatein compensation; or discriminate regarding terms, privileges, or conditions of employment, against any

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This publication is issued in furtherance of Cooperative Extension work. Acts of May 8 and June 30, 1914,

in cooperation with the U.S. Department of Agriculture, Noelle Cockett, Vice President for Extension and

Agriculture, Utah State University.

Design by: Peak Media, Logan, Utah

Photos courtesy of: Peak Media, Cache County WeedDepartment, Richard Old, William & Wilma Follette @ USDA-NRCS PLANTS Database / USDA NRCS, Dr. Charles Hart andBruce Barnes.