Know your Plants, Protect your Horses

Looking at my backyard last night and all of the different species of plants that have cropped up over the last week or so made me thankful I only have a small space to manage, one where everything is free to grow! This would not be the case if I were managing a horse property.

Effective pasture management is essential to maintaining safe and healthy grazing areas for horses. This can be partially achieved through effective pasture management that includes rotational grazing. Bare patches of overgrazed grass present an ideal opportunity for more opportunistic plant species to grow, in many cases, ones that are toxic to horses. In Canada, the following 10 plants have been identified as being the most toxic to horses. Take some time to learn which are native to your area and how to identify them. Prompt removal of any toxic plants from paddocks or pastures the horses have access to is recommended.

Yew – ornamental, often used in hedges. Yew can be lethal if enough is ingested (0.5 pounds for a 1000 pound horse). Symptoms: muscular tremors, staggering, convulsions, difficulty breathing, collapse, heart failure. 

Water Hemlock – found in ditches and wet areas throughout Canada. Ingestion of just 1 root of Western Hemlock can lead to fatality in just a few hours. Symptoms: salivation, muscle spasms, violent convulsions, coma, asphyxiation. 

Poison Hemlock – found in ditches, streams, wet meadows and along roadsides throughout North America. Poison Hemlock can result in death from respiratory failure within 2 – 3 hours. Symptoms: frothing at the mouth, uneasiness, dilated pupils, weak, rapid pulse, convulsions, clamping of jaws, muscle tremors.

Oak – found throughout Canada. While not fatal, if regularly ingested, Oak can cause gastroenteritis and kidney damage. Symptoms: inappetence, constipation followed by bloody diarrhea, abdominal pain, depression, blood in the urine.

Rhododendron – ornamental plant. 2 pounds of Rhododendron per 1000 pound horse can be lethal within a few hours of ingestion. Symptoms: diarrhea, weakness, repeated swallowing, impaired vision, bradycardia, tachycardia, coma.

Cow Cockle – found in pastures, along roadsides, in cultivated fields and waste areas. Cow cockle seeds are toxic with a lethal dose of 2.45 pounds per 1000 pound horse. Symptoms: restlessness, grinding of teeth, salivation, colic, diarrhea, coma.

Cocklebur – found in farmyards, cultivated fields, streambanks and beaches. Cocklebur is not considered lethal but can result in extreme discomfort if ingested. Symptoms: weakness, unsteady gait, twisting of neck muscles, depression, nausea, laboured breathing, rapid/weak pulse.

Jimsonweed – found in cultivated fields and farmyards across most of southern Canada. Lethal at 1 pound per 1000 pound horse, Jimsomweed also has a narcotic effect that may be fatal to livestock. Symptoms: dilation of pupils, impaired vision, rapid/weak pulse, nausea, loss of muscular coordination, violent/aggressive behaviour, trembling. 

Nightshade – found in fencerows, shrubs and wood edges in southern Canada. Nighshade can be fatal at 1 pound per 1000 pound horse. Symptoms: abdominal pain, dilation of pupils, loss of appetite, diarrhea, loss of muscular coordination. 

Oleander – may be grown as an ornamental shrub in Canada

Other plants toxic to horses include:

  • Alsike Clover
  • Arrow Grass
  • Barnyard Grass
  • Bracken Fern
  • Burdock
  • Field Horsetail
  • Johnsongrass
  • Lupine
  • Mikweed
  • Puncture Vine
  • Spear-Leaved Goosefoot
  • Sneezeweed
  • Spurge
  • St. John’s Wort
  • Stinging Nettle
  • Stork’s Bill
  • Tall Buttercups
  • Tansy Ragwort
  • White Snakeroot
  • Witch Grass

Sources

Lawseth, A. undated. “Pasture Perils – Plants Toxic to Horses.” Horse Journals Retrieved from https://www.horsejournals.com/horse-care/feed-nutrition/pasture-perils-plants-toxic-horses

Ontario Ministry of Agriculture, Food and Rural Affairs. “Common Weeds Poisonous to Grazing Livestock.” Retrieved from http://www.omafra.gov.on.ca/english/livestock/dairy/facts/poisonousweeds.htm

 

Selenium Deficiency in Horses

Happy New Year! This is part of Owl Equestrian's new series, Health Spotlight, featuring a number of topics related to equine health. 

A number of significant health impacts are associated with selenium deficiencies in the equine diet, including severe muscle disorders like white muscle disease, myodegeneration, and acute exertional rhabdomyolysis (tying-up), the latter of which may not actually be a sign of deficiency at all. Hypothyroidism has also been associated with selenium deficiency, and a link has recently been found between low blood selenium levels and the stereotypic behaviour of crib-biting.

White Muscle Disease in Foals

A degenerative disease typically classified as a developmental disease and most commonly observed in foals of selenium deficient mares, acute or subacute white muscle disease can also affect adult horses, often in the form of masseter (cheek muscle) myopathy. Horses affected by this have difficulty opening their mouths and therefore, difficulty eating, which can lead to complications from anorexia, or death from starvation.

In foals,  the age group most commonly affected by white muscle disease, observable signs of severe deficiency include recumbency, increased heart rate, arrhythmias, rapid or slow and laboured breathing, stiffness and muscular pain in the limbs, back and neck, discoloured urine, and in extreme cases, sudden death. At the subacute stage, foals may exhibit severe weakness, a stilted gait or inability to stand, muscular twitches, lethargy, and difficulty suckling and swallowing. Common complications include aspiration pneumonia and starvation.

A retrospective study of approximately 270 horses tested for selenium levels at the Cornell University Equine and Farm Animal Hospital between 1996-2011 indicates that of the horses tested, 65% in the less than 30 day old age group had low selenium concentrations, with decreasing incidences of selenium deficiency in older horses, 43% in the 30 day to 2 year-old age group, and 37% in those older than 2. The higher incidence of selenium deficiency in foals less than 30 days old was directly associated with nutritional myopathy and low blood serum selenium in dams.

In some instances, when the mare is selenium deficient during pregnancy, the foal will be born with white muscle disease, additionally, said mares are at an increased risk for “early embryonic death, retained placentas, cystic ovaries, and weak or silent heat periods”. Minimum daily requirements of selenium for foals, pregnant mares, and lactating mares, all with an assumed body weight of 500kg, are presented below.

Minimum Daily Requirements of Selenium

FoalsPregnant MaresLactating Mares
1 month .16 mg
2 months .23 mg
3 months .28 mg
6 months .43 mg
12 months .64 mg
1 mg1.25 mg





(National Research Council, 2007)


Myodegeneration in Adult Horses


At the 2016 American College of Veterinary Internal Medicine Forum, Dr. Andrew Allen presented a 2014 case in which 13 horses on a farm had died. All of the horses had exhibited similar clinical signs. Two of these horses had been examined post-mortem and were found to have muscle cell death of the head and skeletal muscles (myonecrosis), fluid accumulation beneath the skin (subcutaneous edema), fluid accumulation in the chest cavity (pleural effusion), and ulcerated and inflamed tongues. The remaining herd of 12 horses was subsequently examined and all were found to have low body condition scores. Of the 12, 3 horses had elevated heart rates and one had edema in the abdomen. Blood was drawn from 4 of the horses, those that exhibited the most severe clinical signs, and all of them had low blood selenium levels and elevated creatine kinase activity and aspartate aminotransferase levels, indicative of muscle damage/degeneration. The entire herd was prescribed selenium injections and supplementation. When tested after several months, even the most severely affected horses had attained normal blood selenium level

In 2015, Dr. Allen investigated a second farm where there was a similar incidence of “herd-wide myodegeneration secondary to selenium deficiency”.  During this second investigation, the owners indicated they had learned about the toxicity of selenium and chose not to supplement their horses’ diet. Both of these cases underscore the role of selenium in maintaining muscle integrity in adult horses.

Acute Exertional Rhabdomyolysis

Commonly known as ‘tying-up’, exertional rhabdomyolysis can cause muscle necrosis and in severe cases, kidney failure. Veterinary attention is necessary within 24 to 48 hours of mild cases, and immediately in acute cases. In suspected acute cases, amongst other tests, blood serum levels will be tested for creatine kinase 4 to 6 hours after the episode, and aspartate aminotransferase 18 to 24 hours after elevated levels of which are indicative of muscle damage. It has been suggested that there is a relationship between selenium and vitamin E deficiencies and these elevated levels, however, horses with acute exertional rhabdomyolysis typically have adequate selenium and vitamin E serum levels, and increased supplementation has not proven to be beneficial.

Hypothyroidism

There has been speculation that because two selenoproteins, iodothyronine deiodinase, or tri-iodothyronine (T3), and thyroxin (T4) are integral components of thyroid hormone activation, that a horse deficient in selenium will also develop hypothyroidism. A 2010 study by Muirhead et al. found this to be untrue. In a study of 201 horses on Prince Edward Island, blood serum was measured to determine levels of selenium, vitamin E, T3, and T4. In instances where an animal is in a hypothyroid state, both T3 and T4 concentrations will fall below the range of what is considered normal. The majority of the horses in the study had low or deficient selenium concentrations, however, T3 serum concentrations fell within the normal range for all horses, while T4 was more varied, leading the researchers to conclude that there is not a significant correlation between selenium deficiency and hypothyroidism in horses.

Crib-biting

A recent study by Omidi et al. found a direct correlation between low blood serum concentrations of selenium and crib-biting in horses. Blood samples were drawn from 10 horses with an established history of crib-biting during basal conditions and during or immediately following cribbing episodes. The samples were compared to blood samples of a control group of non-crib-biting horses.

Low levels of blood serum selenium were observed in the crib-biting horses both at rest and during cribbing episodes, with the lowest levels of concentration recorded when the behaviour was performed, suggesting that low levels of blood serum selenium may have a role in crib-biting in horses, possibly as a result of oxidative stress. To give more credibility to their study, Omidi et al. reference several recent studies that have linked low Selenium levels to stereotypic behaviours in humans with psychiatric conditions, like schizophrenia.

To learn more about the role selenium plays in the equine diet, visit Health Spotlight: The Role of Selenium in the Equine Diet

References

Aleman, M. (2008). “A review of equine muscle disorders.” Neuromuscular Disorders 18, pp. 277-287

Cavanagh, K. and S. Ternan. (2014). From the Horses’ Mouth: Nutrition, Feeds & Feeding. Fonthill, ON: Matrix Multimedia

House, A. (undated). “Selenium in the Equine Diet.” American Association of Equine Practitioners. Retrieved from  https://aaep.org/horsehealth/selenium-equine-diet

Larson, E. (2016). “Is Selenium Deficiency Deadly to Horses?” The Horse Aug 29, 2016. Retrieved from https://thehorse.com/18527/is-selenium-deficiency-deadly-to-horses/

Muirhead, T.L., Wichtel, J.J., Stryhn, H. and J.T. McClure. (2010). “The selenium and vitamin E status of horses in Prince Edward Island.” The Canadian Veterinary Journal, 51(9), pp. 979-985. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2920172/  

Multiple Authors. (2001). “Tying-Up in Horses: Causes and Management.” The Horse. Retrieved from http://www.thehorse.com/articles/12674/tying-up-in-horses-causes-and-management

National Research Council. (2007). Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. https://doi.org/10.17226/11653

Omidi, A., Jafari, R., Nazifi, S. and M.O. Parker. (2018). “Potential role for selenium in the pathophysiology of crib-biting behavior in horses.” Journal of Veterinary Behavior, 23, pp. 10-1

Patton, K.M. (2008). “Hiding in Plain Sight: Selenium deficiency masks itself as different diseases.” The American Quarter Horse Journal May 2008. Retrieved from https://themustangproject.files.wordpress.com/2010/01/aqha-hiding-in-plain-sight-selenium-deficiency-masks-itself-as-different-diseases.pdf

Streeter, R.M., Divers, T.J., Mittel, L., Korn, A.E., and  J.J. Wakshlag. (2012). “Selenium deficiency associations with gender, breed, serum vitamin E and creatine kinase, clinical signs and diagnoses in horses of different age groups: A retrospective examination 1996–2011.” Equine Veterinary Journal, 44, pp. 31-35

Thunes, C. (2011). “Supplementing Selenium.” The Horse Jan 11, 2011. Retrieved from https://thehorse.com/113890/supplementing-selenium/

Valberg, S.J. (undated). “Exertional Myopathies in Horses.” Merck Manual: Veterinary Manual. Retrieved from https://www.merckvetmanual.com/musculoskeletal-system/myopathies-in-horses/exertional-myopathies-in-horses

The Role of Selenium in the Equine Diet

Happy New Year! This is part of Owl Equestrian's new series, Health Spotlight, featuring a number of topics related to equine health.

A trace mineral required in minuscule quantities, selenium is a powerful antioxidant, protecting tissues from the free radicals that are formed during the conversion of food to energy. Selenium is also found in a number of selenoproteins (selenium-containing proteins) in the body. While often toxic in doses exceeding 5 mg per day, selenium’s role in maintaining cellular and muscular integrity is imperative in functions related to respiratory, cardiac, thyroid and muscular health. It may also have a role in crib-biting behaviour in horses.

As an antioxidant, selenium contributes to the formation of an enzyme (glutathione peroxidase) that destroys free radicals (lipo- and hydrogen peroxides) by converting them into harmless alcohols, an antioxidant activity that protects the integrity of cell membranes. It is important to mention that selenium does not act alone in this process, rather it works in coordination with vitamin E. The relationship between this vitamin and mineral duo is essentially a balancing act, where they act in concert to protect cells from the damaging effects of oxidation. For example, vitamin E decreases the formation of lipid peroxides in cell membranes, while selenium in the intracellular fluid removes any lipid peroxides that do form. If there are insufficient amounts of either selenium or vitamin E, oxidation-induced cellular damage may occur, with effects similar to deficiency.

The simplest way to think of this balance is:

  • inadequate vitamin E = increased peroxide formation, requiring more selenium to remove peroxides, or
  • inadequate selenium = decreased removal of peroxides, requiring more vitamin E to decrease peroxide formation

Clinical Signs & Effects of Selenium Deficiency

Clinical signs of selenium deficiency vary with the degree of deficiency in a horse’s diet. In mild cases, a horse’s immune system may be depressed, resulting in increased susceptibility to disease. Fertility may also be reduced. Signs of severe selenium deficiency include impaired cardiac function, respiratory distress, cardiomyopathy, and white muscle disease. Horses may also engage in crib-biting when selenium is deficient.  More detail on these conditions is available here.

Maintaining a Proper Selenium Balance

Dietary Sources of Selenium

In an ideal situation, a horse will be able to attain appropriate levels of selenium from forage. However, as the map below illustrates, extensive areas of soil across North America are selenium deficient. Supplementation is recommended for horses that are kept solely on hay and/or pasture in selenium deficient areas. While a complete vitamin and mineral supplement may seem like a logical approach to ensuring horses are receiving adequate amounts of selenium, unlike other trace minerals, balancing selenium requires a great deal of caution. Too little can result in a deficiency and too much can be toxic.

Source: Pard, K. (2016)

 

Supplementation

While horses fed concentrates may receive adequate amounts of selenium through feed, organic selenium yeast is the best source of supplementation for horses consuming a diet consisting solely or primarily of selenium-deficient forage. It has a high level of bioavailability and is the most similar to the selenium found in plants. Selenate, a byproduct of the mining industry, is also an acceptable selenium supplement, however, in areas with high concentrations of iron or sulphur, it will not be effective because both iron and sulphur will oxidize the selenate before it can be absorbed by the body. In cases of dangerously low selenium levels, particularly if clinical signs are observed, horses should receive therapeutic doses of selenium and be retested after a few months to monitor improvements or adjust supplementation levels accordingly.

Prior to adding selenium to the diet, it is recommended to determine the amount of selenium the horse is getting from forage and feed to determine whether supplementation is necessary, and in what quantity. Other factors to consider include, age, activity level, and stage of life. For example, pregnant and nursing mares will require higher levels of selenium than a performance horse fed a high concentrate diet, which will also differ from a horse with low-to-moderate activity levels kept at pasture.  

The National Research Council recommends approximately 1 mg/day of selenium for a 500 kg horse (0.1 mg/kg dry matter), however, higher intake, in the range of 2-4 mg per day for an average-sized horse is thought to be optimal to not only prevent deficiencies, but to promote optimal immune function. Up to 5 mg per day may be safe for pregnant and nursing mares, but will poses a risk of toxicity for other horses. Horses should have their blood selenium levels tested once a year to monitor for deficiency and, where appropriate, toxicity.

Take Home Points

  1. Soils across extensive areas of North America are selenium deficient, with the exception of the central regions of the continent.
  2. When balancing the amount of selenium in the equine diet, there is a fine line between deficiency and toxicity.
  3. Even in areas with selenium deficient soils, it is advisable to learn exactly how much selenium is provided by forage (and concentrates if fed) prior to adding supplemental selenium to the diet.
  4. While pregnant mares may benefit from up to 5 mg of selenium per day, this level may be toxic for the average adult horse, for which daily intake should not exceed 2-4 mg.
  5. The most bioavailable form of selenium next to that contained in plant matter is organic selenium yeast.
  6. Selenate, a byproduct of the mining industry, should be avoided in areas with high concentrations of iron or sulphur because the iron and sulphur will block the bioavailability of the selenate.
  7. Early signs of selenium deficiency may include a depressed immune system resulting in increased susceptibility to disease, and reduced fertility.
  8. Severe selenium deficiency may lead to impaired cardiac function, respiratory distress, cardiomyopathy, white muscle disease, myodegeneration, and crib-biting.  

References

Aleman, M. (2008). “A review of equine muscle disorders.” Neuromuscular Disorders 18, pp. 277-287

Cavanagh, K. and S. Ternan. (2014). From the Horses’ Mouth: Nutrition, Feeds & Feeding. Fonthill, ON: Matrix Multimedia

Crandell, K. (1999). “Selenium for Horses: How Important Is It?” Kentucky Equine Research Nov 9, 1999. Retrieved from https://ker.com/equinews/selenium-horses-how-important-it-0/

Larson, E. (2016). “Is Selenium Deficiency Deadly to Horses?” The Horse Aug 29, 2016. Retrieved from https://thehorse.com/18527/is-selenium-deficiency-deadly-to-horses/

Muirhead, T.L., Wichtel, J.J., Stryhn, H. and J.T. McClure. (2010). “The selenium and vitamin E status of horses in Prince Edward Island.” The Canadian Veterinary Journal, 51(9), pp. 979-985. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2920172/  

Multiple Authors. (2001). “Tying-Up in Horses: Causes and Management.” The Horse. Retrieved from http://www.thehorse.com/articles/12674/tying-up-in-horses-causes-and-management

National Research Council. (2007). Nutrient Requirements of Horses: Sixth Revised Edition. Washington, DC: The National Academies Press. https://doi.org/10.17226/11653

Omidi, A., Jafari, R., Nazifi, S. and M.O. Parker. (2018). “Potential role for selenium in the pathophysiology of crib-biting behavior in horses.” Journal of Veterinary Behavior, 23, pp. 10-14

Pard, K. (2016). “Understanding Selenium.” Horse Journals Retrieved from https://www.horsejournals.com/horse-care/feed-nutrition/understanding-selenium

Streeter, R.M., Divers, T.J., Mittel, L., Korn, A.E., and  J.J. Wakshlag. (2012). “Selenium deficiency associations with gender, breed, serum vitamin E and creatine kinase, clinical signs and diagnoses in horses of different age groups: A retrospective examination 1996–2011.” Equine Veterinary Journal, 44, pp. 31-35

Thunes, C. (2011). “Supplementing Selenium.” The Horse Jan 11, 2011. Retrieved from https://thehorse.com/113890/supplementing-selenium/

Wyganowska, A., Gorski, B., Jania, Danielewicz, A., and K. Andraszek. (2017). “The Effect of Selenium on Proper Body Functions in Horses”. Journal of Animal and Plant Sciences, 27(5), pp. 1448-1455

Valberg, S.J. (undated). “Exertional Myopathies in Horses.” Merck Manual: Veterinary Manual. Retrieved from https://www.merckvetmanual.com/musculoskeletal-system/myopathies-in-horses/exertional-myopathies-in-horses