Equine reproduction: Mare nutrition and its impact on fetal growth

The nutritional health of the mare is among the top priorities during her pregnancy. If the mare is lacking her nutritional demands, the results have potential to be detrimental to the developing fetus. The mare in foal should be kept as healthy as possible and fed all necessary supplements with her grain if her pasture is lacking the nutritional requirements. Pasture quality can be tested in a lab by sending well-mixed samples from various places where grazing occurs in the mare’s pasture. The most important nutrients to be conscious of during pregnancy (and for upkeep in general) are protein, energy, vitamins and minerals, and should be fed according to the need of the individual (based on exercise, stage of gestation, or lactating vs. non-lactating mares). Feed must be of good quality and the horse should be provided with adequate forage to aid digestion and prevent boredom-related vices.

Pregnant mares with a foal at foot and pregnant mares previously barren are best kept separated for feeding because of differences in nutritional demands. During early pregnancy weight gain should be kept at a minimum, supporting a body condition score of 5 as closely as possible. The non-lactating mare’s requirements do not significantly change until the final three months of gestation when they should be sufficient to allow a 14.5% increase in body weight. This percent helps guarantee that it is the growth of the fetus rather than internal body fat which could interfere with the efficiency of the reproductive tract, especially in obese mares. It is desirable for some weight gain during late pregnancy to act as an emergency supply for the fetus. If too much weight is gained, it can cause pressure on the internal organs making parturition unnecessarily painful. Underweight mares who fail to gain in the final stage risk reduction of energy supplied to the growing fetus. However, most healthy mares are able to compensate for low nutritional intake and still give birth to a normal birth-weight foal.

As gestation progresses, the nutritional demands significantly increase in the mare. Although most mares (55%) seem capable of low nutritional demands, the added stress is unnecessary and should be avoided. To begin, crude proteins, from early gestation to the last 90 days of pregnancy, should increase from 656 g per day to 866g per day. The increases in levels are best administered according to the more significant stages of development of the fetus. For crude protein specifically, this can be achieved by feeding fresher grasses or legume hay, which even after being dried maintains higher protein levels. Dried grass or hay has a tendency to lose levels of protein during the drying process. If such feeds are inaccessible or unavailable in desirable quantities, the proteins can be supplemented by animal or plant products. Animal by-products such as fish-meal, bone-meal etc, are usually more expensive, and even banned in some countries due to the cattle disease, bovine spongiform encephalitis. Plant products, however, tend to be cheaper and more popular and include soybean meal, linseed meal etc. Soybean meal is especially handy not only because of the quantity of protein, but the quality. It provides all the essential amino acids necessary to the fetus and therefore is very useful in late pregnancy. Linseed and barley oats, despite the high total protein has limited use for late pregnancies because they are lacking in some essential amino acids which are otherwise needed by the unborn foal.

Energy demands are of significant importance, so much that deficiency is implicated as a cause of embryo loss. In early pregnancy, energy demands can be met simply by feeding good quality forage. 16.4 Mcal per day should be sufficient at this stage unless the mare is a poor doer or is being exercised regularly thus should be supplemented appropriately. In early pregnancy, it can be achieved by increase in hay, but as the fetus grows, it begins to limit the digestive tract. In late pregnancy where 19 Mcal per day is required, and in consideration to her digestive tract’s limits at this stage, the energy demands can be met be reducing forage and partly replacing that quantity of feed with energy-rich concentrates making sure that the required protein intake is maintained. Too much excess forage at this time can add strain on the mare’s digestive system and add unnecessary fat around the reproductive tract which can make parturition of the foal more painful.

The specific effects of many vitamins and minerals, classified as micronutrients, and their deficiencies in the pregnant mare are unknown except for calcium (Ca), phosphorous (P), and vitamin A. Calcium and phosphorous are demanded not only by the mare, but by the fetus as well. If the pregnant mare is lacking a supplementation of Ca or P, the fetus will take from the mare what it needs, mobilizing her storage to satisfy its own needs. If the demand is great, as it is during late pregnancy, then the mare’s storage in her bones will gradually lessen, leaving her bones brittle and possibly even too weak to bare the weight of pregnancy or the stress of parturition. However, excess P can have the same effect as Ca deficiency because it interferes with Ca absorption in the mare. Legume hay is a good source of both of these micronutrients, and any pregnant mare fed this hay shouldn’t require supplementation of Ca or P. If the mare is fed grains late in her pregnancy, Ca should be supplemented, possibly in the form of bone-meal (not common), ground limestone flour or milk pellets (more common), because grain is usually high in P. Bran should be fed sparingly in small quantities as a laxative because of its high concentration of phosphorous. Vitamin A is important in reproductive function, cell regeneration and development, because it is a component of epithelium. Satisfactory levels of vitamin A are best reached by feeding fresh green forage. Supplementation is required, however, if mares have no access to fresh pasture. Supplementation of pregnant mares must be monitored regularly to ensure better health of the fetus, because excess and deficiency can both lead to abnormalities related to bone growth, which include developmental orthopaedic disease, angular limb deformities and epiphysitis.

A lactating pregnant mare has more nutritional demands than any other horse. With a foal, the pregnant mare not only has to support herself and the fetus, but also have enough nourishment that her milk is of good calibre for the nursing foal. The efficiency of the transfer of vitamins and minerals through the milk is far less than that of the placenta, so much, that the mare’s nutritional demands increase by 70-75% directly after parturition and by 50% later on in lactation. A mare can produce up to 3% of her body weight as milk during the climax of lactation.
Protein is an essential demand. If protein supply is deficient, she will not have enough for milk supply and thus cause milk production to decline. If there is low energy in the mare’s diet, she will use her own reserves in the milk and eventually become emaciated. Some loss in weight is expected during lactation but should be prevented by a proper diet. Ca and P are still very important to the lactating mare’s diet because the foal must obtain appropriate levels of them for proper bone and tendon growth and development. Approximately 56-78g of Ca per day are required while only 35-50g of P should be fed per day. Excess P causes Ca to drain from the bones of the mare.

Vitamins A and D are important during lactation. Vitamin A, from fresh green forage, and vitamin D, from sunlight, are required because deficiency can cause lack-lustre and ill thrift in the mare and foal. Deficiency of vitamins A and D also causes inefficient use of other vitamins and minerals.
Water, the often over-looked necessity of life, is of utmost importance. The non-lactating pregnant mare needs approximately 50 l of clean, fresh water per day in small frequent amounts, depending on DM content. Free access to a clean, fresh water supply is also required by the lactating mare with a foal at foot. After all, milk is made up of 90% water.

As discussed, nutritional demands should be met at all times by the lactating/non-lactating pregnant mare or health risks for mare, foal and/or fetus are increased. Special attention should be paid while the mare enters the final trimester of gestation when all of her nutritional needs rise with the fetus’ growth rapidizing, as well as the increased demands again post-parturition when the foal is suckling and the milk provides less efficiency for nutrient transfer. After a short time, the foal will be able to graze and find food beyond the mare’s milk, but attention must be paid to ensure a healthy life for both mare and foal.

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