Appropriate mineral nutrition during pregnancy and lactation is vital to produce well grown and durable young stock. Whilst the importance of macro minerals (such as calcium and phosphorus) and micro minerals (such as copper) have historically received considerable attention in this regard, other trace minerals of equal relevance, such as selenium, have received relatively less attention. In horses, as in the majority of other mammals, adequate dietary selenium is essential to support the proper function of many body systems. There are also many selenium-containing proteins that function as enzymes e.g., glutathione peroxidases. Glutathione peroxidases are antioxidant enzymes that help protect the body from the effects of reactive chemicals produced during metabolism that can damage cell membranes. Aside from glutathione peroxidises, there are at least 25 known selenium-containing proteins, which collectively are vital for the optimum functioning of the antioxidant, immune and musculoskeletal systems.
Inadequate selenium intake linked to reproductive problems
Selenium containing proteins are intricately involved in several aspects of metabolism and there are many examples of their importance in broodmares throughout the breeding cycle. It is perhaps, therefore, not surprising that mares receiving inadequate selenium have been reported to be more likely to experience a variety of reproductive problems than those where selenium intake was deemed to be adequate.
Throughout the world, there are large areas where the selenium content of soil is reported to be low. This usually affects the natural selenium content of forages, cereals and cereal bi-products, which form the mainstay of a broodmare’s diet. A low baseline level of selenium in feed means that appropriate selenium supplementation, through complementary feeds or supplements, is essential. Currently the NRC (National Research Council) recommends a daily selenium intake of 0.1mg/kg dry matter for broodmares, to prevent deficiency. However, they accept that the requirement for selenium may be higher for functions, such as, to support the immune system.
Total dietary selenium intake in a broodmare is therefore important to maintain the level of these selenoproteins, including enzymes that are essential for the mare’s antioxidant and immune defences. By virtue of these effects, selenium has significance for all aspects of the breeding cycle, including, fertility, foetal growth, parturition, lactation and the immune status of the foal. However, current evidence suggests that the form in which selenium is provided also has a significant effect on the physiological response to selenium supplementation. Historically, sodium selenite has been the main source of supplementary selenium for horses. However, advances in animal nutrition suggest that organic forms of selenium, such as organic selenium produced by Saccharomyces cerevisiae CNCM I-3060 (Alltech Inc.), are more bio-available and are more readily taken up into the tissues. Bio-available selenium contains selenium in the form of seleno-methionine, which is also the form in which it is found in nature. It is a safer and less toxic source of selenium compared to selenite. Research suggests that feeds and supplements containing organic selenium are the best choice for horse breeders.
Endometritis is an important factor involved in infertility in mares. It is characterised by inflammation of the lining of the uterus, which can develop as a result of fungal or bacterial infection. The level of dietary selenium could be very important in preventing this condition, due to the importance of antioxidant status. The activity of glutathione peroxidase is reported to be decreased in mares with endometritis. Additionally, the plasma concentration of malondialdehyde (MDA), which is often used as an indicator of oxidative damage to cellular membranes, is also increased with endometritis in horses. Oxidative damage is a normal biochemical consequence of inflammation, but the body’s ability to manage this will depend on the mare’s antioxidant defences. The activity of glutathione peroxidase, which is known to be adversely affected by reduced dietary selenium intake, is particularly important. The rate of decline in glutathione peroxidase activity; once Se is removed from the diet, is less in horses fed organic selenium produced by Saccharomyces cerevisiae compared to those given sodium selenite.
Retention of placenta is affected by selenium status
Retention of the placenta in broodmares following foaling is another serious issue that can lead to complications such as endometritis, septicaemia and laminitis. This problem is therefore an important consideration for stud managers. Whilst there are many factors that affect the length of time for which the placenta is retained by a mare before its expulsion, selenium status appears to be involved. From a group of 22 in-foal mares, those with higher serum selenium concentration had the shortest placental retention time. Additionally, in those mares where serum selenium concentration was considered to be low, placental retention time was improved as the result of supplementation of the diet with both selenium and vitamin E. There was further improvement in placental expulsion in mares fed organic selenium produced by Saccharomyces cerevisiae.
Whilst the effect of selenium status on mammary health in horses has not yet been investigated, research carried out in other species, including dairy cows, suggests that adequate selenium intake is an important factor in reducing the instance of infections, such as mastitis. This is probably due to the beneficial effects of selenium on overall immune status.
Selenium supports immune function in mare and foal
A mare’s immune status is also a very significant factor affecting the quality of colostrum. Colostrum is the ‘first milk’ that is produced during the last month of gestation and is vital for the establishment of a foal’s immune system. Immunoglobulins primarily IgG, IgM and IgA are concentrated in the udder from the mare’s circulation. The quality of colostrum, in terms of its immunoglobulin content, is very much dependent upon a mare having overall good immune status. This in turn is believed to be influenced by adequate dietary selenium intake, as diets that were deficient in selenium were found to negatively affect the immunity of ponies.
The positive effect of selenium supplementation on selenium status in both mares and their foals is additionally dependent on the selenium source. Supplementation of mares with organic selenium produced by Saccharomyces cerevisiae, at 3mg per day, has been shown to successfully increase the serum selenium concentration in mares post foaling compared to those supplemented with inorganic sodium selenite (1-3mg / day).
Perhaps the most recognised effect of selenium deficiency in foals is the development of white muscle disease. White muscle disease is a form of muscle dysfunction, or myopathy, which results: in weakness and impaired movement; difficulty in suckling; and compromised heart function. The selenium status of the foal is affected by the selenium intake of the mare as selenium can be transferred from mare to foal, both through the placenta and in milk.
An increased serum selenium concentration in foals has been reported in mares fed either organic selenium produced by Saccharomyces cerevisiae or selenite. However, those supplemented with organic selenium produced by Saccharomyces cerevisiae had significantly higher selenium concentrations in both colostrum and milk. In addition, selenium supplementation of the mares was also shown to support subsequent immunity in the foals. Levels of the immunoglobulin IgG were higher in mares fed 3mg/day vs. 1mg/day of selenium. This level of selenium, whilst higher than the current NRC recommendations, may be required to support immune function, particularly in the case of mares and foals.
Growth – an area for the future
The effect of selenium status on foal growth, either in the womb or after birth, has not been greatly studied. However, it is an important area as the selenium dependent enzyme iodothyronine deiodinase, which is responsible for the activation of thyroid hormone, is known to be involved in the regulation of growth. By using the example of a human condition with some similar characteristics, a link has also been proposed between low selenium status and skeletal disorders such as osteochondrosis in foals.
In summary, good immune status and antioxidant function in broodmares are vital to ensure the production of strong healthy foals. Selenium performs a crucial role in this respect and may also have a specific part to play in ensuring normal growth. Furthermore, providing selenium in an organic form, such as organic selenium produced by Saccharomyces cerevisiae, offers improved bio-availability and physiological effect.