Just like mares, stallions exhibit seasonal variations in sexual behaviour, hormones and fertility. Sex hormones, semen volume, sperm count, motility and quality increase in spring / summer. Nutrition plays a critical role by supporting both general wellbeing and long-term health and supports the balance of hormones. There is a solid body of research investigating nutritional supplements to improve sperm quality and fertility. Key nutrients essential for healthy formation and function of equine sperm omega 3 fatty acids are incorporated into the plasma membrane( Figure 1.), enabling them to be flexible, compressible, deformable and elastic, as well as increasing viability and velocity. Linseed oil supplementation improves sperm acrosome integrity, while selenium and vitamin E given with omega 3 DHA increase total sperm count, longevity during storage and progressive motility. However if the diet omega 3 enrichment was done with whole foods, rather than with oils, no improvement was seen in sperm motility or morphology. Other nutritional supplements found to impact upon stallion sperm quality and function include L-carnitine which supports spermatogenesis and maturation, especially in sub-fertile stallions. Carnitine is formed from lysine and methionine, but this capacity is limited in some horses and dietary L-carnitine supplementation is beneficial.
Age has much less effect on stallions’ than on mares’ fertility. Although signs of aging may appear around 15 years of age, stallion fertility is not usually decreased until 20 – 25 years of age. Mare fertility decreases and the risk of pregnancy loss increases from 10 – 15 years of age.
As for other mammals, mares are born with a set number of primordial follicles, that varies between mares and the number of available follicles declines with aging Ovarian activity is reduced after 15 years of age and is not observed anymore in around 17 % of mares over 20 years – and by 25 years it is thought the mare has exhausted her oocyte stocks. Fertilization however isn’ t affected by age as the number of embryos collected via oviductal flushing at Day 2 post ovulation is similar between young, fertile mares and old, sub-fertile mares. And, when looking at embryo recipient mares, age does not affect initial pregnancy rates until 15-18years but embryo loss is higher at day 50: 20.5 % in mares over 10 years compared to 13.3 % in younger mares. This is thought to be due to endometrial aging.
Fertilization and early embryo development take place in the oviducts, which have a higher incidence of fibrosis in older mares. After 5-6 days in the oviduct, the embryo induces its own movement down into the uterus. Cysts in the uterus- more common in older mares- could prevent embryo movements, fixation and implantation which are essential for the uterus to recognise and respond to the pregnancy. Uterine clearance of fluid is impaired in older mares and age-related lengthening of the vulva allows a larger entrance for bacteria. Endometritis is more common in old mares, partly due to poorer conformation and delayed uterine clearance of fluid and debris after foaling. However, the placenta is smaller and less efficient in maiden mares and the foals are smaller. Foals from young and maiden mares also have a more immature regulation of glucose – making it even more important that the mare’ s are not overfed and that the diet is low in starch and sugar.
Obesity and insulin dysregulation( ID) have been associated with subfertility in the mare. There has been a substantial increase in the prevalence of overweight and obese horses in recent years( 45- 50 % now, up from 4.5 % in 1998). Some of these mares, with a history of good sport performance, enter breeding programs with high expectations of embryo production or pregnancy. Without losing perspective of the reproductive problems associated with ageing and obesity, it is very important to consider metabolic problems( e. g., equine metabolic syndrome( EMS) and pituitary pars intermedia dysfunction( PPID; i. e., equine Cushing’ s disease ]) that may directly impact fertility. A large proportion of ID and obese mares continue to cycle over winter, have a longer period between ovulations and less ovulations. Once identified it ' s imperative that dietary errors are avoided. These mares should be managed with a low calorie( ie sugar and starch) diet, appropriate exercise and, if necessary, medication.
One often over-looked aspect is the necessity for specific essential amino-acids for the production of reproductive hormones. Regardless of the percent protein of the feed and the amount of protein in the diet, if essential amino acids are not present in the correct amount, hormone synthesis in the ovaries is impaired. The risk of amino acid deficiencies is much greater in mares fed teff, cereal and pasture hays and in those on bran / pollard / wheat / barley and corn-based manufactured feeds. Foal growth during lactation is heavily influenced by the amino acid quality of the mare’ s diet. The dietary protein intake of the lactating brood-mare should therefore contain sufficient high-quality protein to prevent depletion of her body reserves and consequently a loss in body mass – which is a risk factor for early embryonic loss. In addition, the growth of their foals can be impaired by up to 25 %. Foals that are weaned with a low body mass are more likely to have periods of catch-up growth after weaning and are highly susceptible to epiphysitis and other developmental bone diseases.
Advancing maternal age as the single most significant factor to consider with the rate of fertility. Similar to stallions, the extent to which diet can affect mare fertility depends on the amino acid, anti-oxidant and mineral levels in the feed and the duration of feeding. Certainly improvements in reproductive efficiency have been demonstrated when a correctly balanced feed is fed for a minimum of 2 months before breeding. Grain-based feeds and diets have alter oocyte metabolism and functions in ways that are only just being understood, but that are detrimental to fertility 2.
Stress Based on studies in other species, effects of stress on the oestrous cycle in mares have been suggested. It is common practise to transport mares during the periovulatory period and owners often question whether transport shortly before and after ovulation, repeated gynaecological examination and exposure of the mares to a novel environment in the new stable may have negative effects on ovulation and conception. Although these events cause elevations in the stress hormone( cortisol) which last for several hours, no differences in duration of oestrus, time of ovulation, pregnancy rate or early embryonic loss between transported mares and non-transported mares. Stress may affect progesterone( a hormone necessary to maintain the pregnancy) levels in mares with marginal levels and your treating veterinarian is best-placed to discuss whether your mare requires progesterone.
Puberty can occur as soon 14-15 months of age and peak fertility in mares is from 4 to 6 years of agedecreaing gradually to become almost zero at 25years of age. Many mares in show jumping, dressage and other disciplines commonly compete past 15 years of age- only then beginning their reproductive career. Thanks to ultrasonography, the newly available hormones and our increased knowledge of reproductive physiology, semen handling and pathology, foaling rates have improved over recent decades. In many breeds, the use of the artificial insemination has also been a major contribution.
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2025 AQHA SEPTEMBER / OCTOBER ISSUE