Lysine - vitamin e – selenium: supplementation of mares and its effects on foals

The vitamin E – selenium – lysine complex is typically used for performance horses, to sustain muscular health during efforts. But its applications can be taken further into the breeding industry, especially for young growing horses, from foals to two years old, and particularly for growing horses at work. Some studies have been made considering the importance of each of those nutrients separately in the reproduction and growth domains, but the cross-link between the different nutrients of this complex remains blank…


Vitamin E, also called alpha-tocopherol, is a fat-soluble vitamin, that the horse is not able to produce himself (Duren and Crandell, 2000; Crandell, 2000). Target tissues of fat-soluble vitamins are skin, bones, muscles, and blood (Crandell, 2000).


Vitamin E is most beneficial to pregnant mares, stallions, and equine athletes, and is vital to young growing horses (Duren and Crandell, 2000; Kane, 2008), with a powerful biological antioxidant role, protecting body tissues from oxidative damage by neutralising free radicals, and therefore plays a huge role in immune response and nerve and muscle function, being part of the intracellular respiration and maintaining cell membrane integrity (alpha-tocopherol is the primary lipid soluble antioxidant) (Baalsrud and Øvernes, 1986; Duren and Crandell, 2000; Crandell, 2000; Kane, 2008; Huntington, 2012; Waldridge, 2013).

Together with selenium, vitamin E helps maintain normal muscle function, prevent muscular diseases, provide antioxidant protection from oxidative damages to body tissues, particularly cell membranes, intracellular substances, and enzymes, and improve humoral immune response, helping leucocytes and macrophages survive the damages caused by toxic products created during phagocytosis (McDowell, 1989; Kane, 2008; Waldridge, 2013).


Vitamin E deficiency can lead to a panel of disorders such as depressed immune status, or muscular pathologies as nutritional muscular dystrophy and muscle degeneration impacting skeletal and cardiac muscle and thus the tongue muscle, which would induce difficulties nursing in foals (Moore and Kohn, 1991; Crandell, 2000; Kane, 2008). A deficiency would also lead to neurological conditions such as encephalomacia, degenerative myoencephalopathy, degenerative myelopathy, or ataxia (Moore and Kohn, 1991; Crandell, 2000; Kane, 2008). Other disorders such as skin oedema, steatitis, jaundice and liver necrosis, aenemia and erythrocyte homolisis can be caused by an alpha-tocopherol deficiency (Crandell, 2000). Breeding wise, decreased growth rates (due to periods of illnesses induced by a poor immune system), reduced fertility and foetal death have been linked to vitamin E deficiency (Crandell, 2000; Kane, 2008).

There is no signs of vitamin E toxicity (Duren and Crandell, 2000), nevertheless, the NRC set an upper safe limit to 20UI per kilogram of bodyweight per day.


Vitamin E can be found in abundant quantities in green lush pasture, especially alfalfa, and requirements are mostly attained if horses are grazing fresh green grass (Duren and Crandell, 2000; Kane, 2008). Supplementation of this vitamin is especially important for stabled equids relying solely on stored forage as 30 to 85% of the alpha-tocopherol present in grass is lost during harvest and storage of hay (Duren and Crandell, 2000; Kane, 2008).
Some vegetable oils are relatively high is vitamin E, such as corn or soybean oils (Kane, 2008). Oils pressed from sprouted grains also contain great amounts of vitamin E as it is abundant in germs of grains (Kane, 2008).

Natural vitamin E is transported quicker and retained twice as long as synthetic vitamin E, also when supplemented a water soluble, absorption is greatly enhanced (Kane, 2008). Natural sources of vitamin E, RRR-alpha-tocopherol or d-alpha-tocopherol, possess a 36% greater biological activity than synthetic sources, also, when horses are fed natural sources of vitamin E, they have higher plasma levels than those fed synthetic one, and similarly if a mare is fed a natural form rather that a synthetic one, milk transfer of alpha-tocopherol is greatly improved (Kane, 2008). Synthetic sources of vitamin E are a mixture of 8 chemically different sources of alpha-tocopherol of which only one is d-alpha-tocopherol (Waldridge, 2013). Water soluble, unesterified alpha-tocopherol is better utilised than acetate esters, and thus the most efficient source of vitamin E is water-soluble d-alpha-tocopherol (Kane, 2008; Waldridge, 2013). Also, absorption of vitamin E supplement is better when naturally consumed with grain, rather than as an oral paste or stomach tube (Crandell, 2000).

Body stores can also be an intrinsic source of vitamin E and are able to cover 4 months of inadequate alpha-tocopherol intake (Duren and Crandell, 2000).


Levels of alpha-tocopherol can be assessed by doing blood measurements, low blood levels are the primary sign of deficiency, below 2 micrograms per millilitre is insufficient, above 4 is adequate (Kane, 2008).
Requirements are higher if enhanced immunity is expected or in the cases of new-born foals, breeding, and performance animals (Kane, 2008).
The NRC set requirements for horses to 50UI per kilogram of dry matter of diet (or 1UI per kilogram of bodyweight per day), and specific requirements for young growing foals, pregnant or lactating mares to 80IU per kilogram of dry matter fed. Even though there are no signs of vitamin E toxicity, the NRC set an upper safe limit to 20IU per kilogram of bodyweight per day, or 1000 IU per kilogram of dry matter fed.


The diffuse epitheliochorial placenta does not allow fat soluble vitamins to cross in appreciable amount, therefore neonates are highly susceptible to vitamin E deficiencies, and they rely heavily on their dam’s colostrum and milk as a source of vitamin E (Schweigert and Gotwald, 1999). It is necessary to feed mares higher levels of vitamin E in late pregnancy and early lactation to make sure there will be adequate levels in colostrum and milk, as foals born from depleted mares have little to no reserves, therefore are more susceptible to diseases (Duren and Crandell, 2000; Kane, 2008). Supplementing twice the amount of vitamin E for one month prior and post foaling is beneficial to mares known to have poor quality colostrum or that previously has foals with failure of passive immunity transfer (Harper, 2002). Mare fed lower quality hay should be supplemented at least one month prior and post foaling to ensure sufficient levels (Harper, 2002). Mare’s serum and colostrum concentration can be increased as soon as 7 days after starting supplementation (Drury et al., 2009), and supplementation 21 days prior foaling showed higher plasma alpha-tocopherol in mare, higher plasma alpha-tocopherol 12 to 36 hours post suckling, and a higher transfer of vitamin E (Kane, 2008).

Colostrum levels of vitamin E is related to maternal intake (Drury et al., 2009), and colostrum quality is better, with higher levels of vitamin E, and antibodies (igG, igA, and igM), when mare is supplemented, independently of quantity of supplementation, those colostrum levels mirror mare and suckling-foal’s serum levels, and postpartum milk levels (Hoffmann et al., 1999; Lawrence, 2008; Drury et al., 2009). Also, greater passive transfer of antibodies from mare to foal through colostrum means enhanced immune system for the foal (Kane, 2008).
Vitamin E levels tend to decrease in mare after foaling (Gay et al., 2004). Supplementing mares with 150UI per kilogram of dry matter fed could reduce the incidence of equine degenerative myelopathy (Kane, 2008), and young foals showing signs of incoordination appeared normal after 2 years of supplementation of 6000IU per day (Blythe and Craig, 1993).
For all those reasons, early vitamin E supplementation may be an important management consideration (Drury et al., 2009).


Selenium is needed for the synthesis of glutathione peroxidase, that breaks down peroxides, which it is part of (Stowe, 1998; Waldridge, 2013).


Selenium (through glutathione peroxidase), acts as a cytosolic antioxidant, helping to protect cell membranes and organelles from oxidative damage (Combs, 1998; Stowe, 1998; Hintz, 2000). Therefore, selenium plays a role in maintaining muscular, vascular and immune cell system and cell membranes integrity, which gives it a huge role in immune response, growth and reproduction (Reilly, 1996; Stowe, 1998; Hintz, 2000; Huntington, 2012; Waldridge, 2013). It also acts as an anticarcinogen, and helps prevent cystic fibrosis, age related macular degeneration, and intrahepatic cholestasis of pregnancy (Hintz, 2000; Reilly, 1996; Combs 1998).


Deficiencies of selenium may cause white muscle disease, tying up (sporadic or not), masseter muscle myopathy, and in most severe cases sudden death (Hintz, 2000; Waldridge, 2013). In mares, deficiency can result in reduced reproductive performances and fertility (also applicable to stallions), as well as retained placenta (Hintz, 2000; Waldridge, 2013). In the case of foals, neonates can show white muscle disease, characterised by weakness, impaired locomotion, difficulty suckling and swallowing, respiratory distress, and impaired cardiac function (potentially leading to heart failure), but also generalised steatitis (Dill and Rebhun, 1985; Waldridge, 2013).

Toxicity can result in alkali disease or Kasin-Beck disease, blind staggers or blindness, head pressing, cracked and very sore hooves or sloughing of hooves, hair loss and impaired development (particularly mane and tail), joint erosion, lameness, ataxia, and respiratory failure (Stowe, 1998; Hintz, 2000).


Selenium content of grain and forage is determined by the selenium content of the soil its grown into (Hintz, 2000).
In terms of supplementation, selenium retention and efficiency are greater when organic selenium yeast is fed rather than inorganic sodium selenite or selenate (Hintz, 2000; Huntington, 2012; Thunes, 2022).
Injectable selenium should be very carefully used as it may cause anaphylactic reaction, generalized oedema, or apparent blindness (Stowe, 1998).


Vitamin E and selenium roles being interdependent, low vitamin E levels influence the need for selenium supplementation, and vice-versa (Hintz, 2000)
Selenium levels can be assessed by plasma or whole blood measurement of selenium or glutathione peroxidase content (Huntington, 2012).
The NRC set the maximum tolerable level for selenium to 2 milligrams per kilogram of dry matter fed, and a toxic level between 5 and 40 ppm (mg/kg of dry matter).
According to the NRC, requirements are between 0,1 and 0,3ppm, pregnant mares requirement being closer to 0,3ppm. Intake of 0,1ppm is considered sufficient to prevent white muscle disease and maintain glutathione peroxidase levels (Stowe, 1998; Roneus and Lindholm, 1983).


Selenium is necessary for development of acquired immune system, greater antibodies levels have been found in foals of which mares received higher levels of selenium, also higher antibodies levels in plasma, colostrum and milk were detected when dams were fed organic selenium rather than an inorganic form (Hintz, 2000; Janicki et al., 2001; Huntington, 2012).
Neonate selenium deficiencies are related to a deficient maternal intake (Huntington, 2012).
Supplementing lactating mares with selenium show increased igG concentrations, and therefore helps their foal’s immune system during the first few months of life when they are more susceptible to diseases (Janicki et al., 2000).


Protein is the most important nutrient for a successful breeding program, and lysine is the first limiting amino acid in equine diet and a required nutrient for growth, therefore this amino-acid demands specific requirements (Duren, 1997; Ott, 2000; Lawrence, 2003; Pagan and Nash, 2008; Pagan, 2003; Brown-Douglas, 2012; Huntington, 2012). Quality of the protein (its amino acids composition) is an important breeding management consideration (Huntington, 2012).


It has recently been shown that amino-acids were involved in cellular and humoral immunity in animals, and lysine has displayed a role in regulation of the immune response (Huang, D. et al., 2021).
Also, lysine supplementation plays an important role in growth, which will be detailed in the “use for breeding” section.


Lysine (and protein) deficiency can lead to early embryonic death and reduced foal size, decreased reproductive efficiency with anovulation (Huntington, 2012).

Feeding excess proteins is not toxic by itself but the induced excess production of urea leads to a horse drinking and urinating more which can be problematic in poorly ventilated stables as the ammonia build-up may induce respiratory problems (Auwerda, 2020).


Soybean meal, alfalfa, and milk protein are feed ingredients high in lysine, with milk being composed of 20 to 25% of protein, when grains and grasses are typically low in lysine (Huntington, 2012; Auwerda, 2020).


Higher lysine is needed in ration of orphan or early weaned foals, and in creep feed especially if mare’s milk is severely restricted (Breuer, 1997), and weanling, yearling, long yearling, and two years old have specific requirements for proteins, especially lysine (Pagan, 2012).
Protein requirements increase into lactation, lactating mare requirement are twice those of a dry mare (Huntington, 2012). In late pregnancy and early and mid-lactation, protein requirements are doubled, and lysine requirement are above 300% higher (Huntington, 2012) Crude protein requirements are given on an energy basis, lysine requirements on a crude protein basis, thus lysine requirements are dependent on the energy intake (Brown-Douglas, 2012).
According to NRC, lysine requirement for mature horses is equal to 3,4% of the crude protein intake, and for pregnant mares, this proportion increases to 4,3%. Weanlings require 2,1g of lysine per megacalories of digestible energy per day, and 50g of crude proteins per megacalories of digestible energy per day. For yearlings, those requirements are respectively 1,9g and 45g. And lysine requirements 1,7g per megacalories of digestible energy per day for two years old. Also, two times more lysine is required for two years old than for a mature horse for the same amount of work (hard work) (Pagan and Nash, 2008).


Lysine is an essential nutrient, and the first limiting amino-acid for growth, thus young growing horses have specific requirements for proteins, especially lysine (Duren, 1997; Pagan and Nash, 2008; Pagan, 2012).
When the mare is supplemented soybean meal two weeks prior and seven post foaling, an increased foal growth rate (10% taller during 7 firsts weeks of life), and a significant increase of milk crude protein content have been noted (Huntington, 2012)
Foals offered lysine supplementation tend to grow at greater rates than those that are not, when a low protein diet is fortified with the two first limiting amin acids, protein is utilized more efficiently for growth and development (Staniar et al., 2001). Lysine to energy ratio is indirectly related to growth rate, and both protein and energy intake reduction can reduce growth rates (Pagan, 2012 ; Ott, 2000). When lysine supplemented with any other source of protein, growth rate improves comparably (Lawrence, 2003).
Supplementing to maximize growth rate should be done with care as rapid growth, particularly by overfeeding energy, may be implicated in the development of Developmental Orthopedic Disease (DOD) and unsoundness (Lawrence, 2003).


All nutrients of this complex have shown common roles and results, which are summarised in the table below.

Function / result

Vitamin E







Immune response




Deficiency leading in reduced fertility








Colostrum and milk quality




*helping to avoid illness or disease induced reduced growth rates

Each of those three particular nutrients show to be essential for the health of any horse, especially vitamin E and selenium of which deficiencies can be fatal.
Supplementing such a nutrient complex is not only important to breeding mares or covering stallions, but especially to young growing horses at work as they would cross both the benefits seen previously but also would help prevent cell damages from exercise induces oxidative-stress damages (Powers and Jackson, 2008).
An increased focus on broodmares nutrition could should be done to boost fertility and produce sound and healthy and athletic young horses (Huntington, 2012).


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