Let’s get pregnant: Feeding for Fertility

Of all domesticated animals, horses have the lowest fertility rate. This is often due to selective breeding for performance and conformation rather than fertility. Equine fertility is influenced by a complex mix of environmental, management, and individual animal factors.

Age and Fertility

Puberty can occur as early as 14-15 months of age, and peak fertility in mares is from 4 to 6 years of age, decreasing gradually to almost zero at 25 years of age. Many mares in show jumping, dressage and other disciplines commonly compete past 15 years of age - only then beginning their reproductive career.

• Stallion fertility usually remains high until 20–25 years of age.

• Mare fertility decreases from 10–15 years, with increased risk of pregnancy loss. Mares are born with a set number of follicles, which decline with age. Ovarian activity reduces after 15 years, and by 25 years, most mares have exhausted their oocyte stocks.

Fertility challenges in older mares

• Oviduct and uterus: Older mares have higher rates of fibrosis and cysts, which can impede embryo movement and implantation.

• Endometritis: More common due to poor conformation and delayed uterine clearance.

• Placenta efficiency: Smaller and less efficient in maiden mares, leading to smaller foals.

• Foal growth: Young and maiden mares’ foals have immature glucose regulation, so diets should be low in starch and sugar.

Amino Acids and Protein Quality

One of the most overlooked factors in equine fertility is the role of essential amino acids in reproductive hormone production. It’s not just about how much protein is in the diet; it’s about the quality. If the right amino acids aren’t present in sufficient amounts, hormone synthesis in the ovaries can be compromised, even with a high-protein feed.

Mares fed teff, cereal and pasture hays, or bran/pollard/wheat/barley/corn-based manufactured feeds are particularly at risk of amino acid deficiencies.

Why it matters for foals

During lactation, the mare’s diet directly influences foal growth. If her protein intake lacks essential amino acids, she may lose muscle mass, an established risk factor for early embryonic loss. Foals can also suffer, with growth rates reduced by up to 25%. Those weaned with low body mass often experience catch-up growth, which increases the risk of epiphysitis and other developmental bone disorders.

Age, diet, and fertility

Maternal age is the most significant factor affecting fertility. However, diet plays a crucial role too, especially when it comes to amino acids, antioxidants, and minerals. Studies show that feeding a balanced diet for at least two months before breeding can significantly improve reproductive outcomes.

Grain-based feeds, in particular, have been shown to alter oocyte metabolism in ways that are only beginning to be understood, but these changes are often detrimental to fertility.

Supplementation and sperm quality

Sperm cells require oxygen to function, but oxygen metabolism produces reactive oxygen species (ROS), oxidants that can damage sperm membranes. This damage reduces motility, longevity, and viability, all of which are critical for fertility. Fortunately, antioxidant supplementation can help protect sperm cells and improve reproductive outcomes.

Key nutrients for stallion fertility

• Omega-3 oils
  Found in fresh grass and flaxseed, omega-3s improve sperm motility and concentration and reduce abnormalities. In contrast, grains and oils such as corn, sunflower and safflower are typically low in omega-3 and high in omega-6, which can disrupt the balance.

• Timing matters: It takes ~60 days to produce sperm, but studies show omega-3 supplementation should continue for at least 126 days for optimal results.

• Linseed and canola oils are particularly beneficial, improving acrosome integrity, which is vital for fertilisation.

• Maca (Lepidium meyenii)
  A traditional Andean root with potent antioxidant properties. Feeding 20g daily for 60 days has been shown to enhance sperm motility and structural integrity.

• Vitamin E
  Deficiency can lead to abnormal sperm and poor motility. Supplementation improves both.

• Vitamin A
  Essential for spermatogenesis and maintaining healthy sperm motility.

• Zinc & Selenium
  When combined with Vitamin E, these minerals enhance sperm motility, viability, and morphology.

• Vitamin C, Copper, and Manganese
  These trace elements support overall sperm health and antioxidant defence.

• L-carnitine

  Particularly useful for sub-fertile stallions, L-carnitine supports sperm production and energy metabolism.

Stallion Management

Participation in competitions does not negatively affect semen quality; in fact, it may improve motility and sperm count.

Seasonal Variations

Stallions, like mares, show seasonal variations in fertility, with higher hormone levels and semen quality in spring/summer.

Obesity, Insulin Dysregulation and Fertility

Obesity and insulin dysregulation (ID) are increasingly recognised as contributors to subfertility in mares. Alarmingly, the prevalence of overweight and obese horses has surged from just 4.5% in 1998 to 45-50% today.

Many of these mares have had successful athletic careers and enter breeding programs with high expectations for embryo production or pregnancy. However, it's essential to consider the metabolic challenges that may accompany ageing and obesity, particularly Equine Metabolic Syndrome (EMS) and Pituitary Pars Intermedia Dysfunction (PPID), also known as equine Cushing’s disease. These conditions can directly impair reproductive function.

How metabolic issues affect reproduction

• Obese and insulin-dysregulated mares often continue to cycle through winter.

• They tend to have longer intervals between ovulations and fewer ovulations overall.

• These disruptions can significantly reduce fertility.

Management strategies

Once identified, it’s critical to avoid dietary errors. These mares should be managed with:

• A low-calorie diet, specifically low in sugar and starch.

• Regular, appropriate exercise to support metabolic health.

• Medication, if necessary, under veterinary guidance.

Stress and Reproduction 

Studies in other species have suggested that stress may influence the oestrous cycle in mares, raising concerns among owners and breeders. It’s common practice to transport mares during the periovulatory period, and many wonder whether this, along with repeated gynaecological examinations and exposure to unfamiliar environments, could negatively affect ovulation or conception.

These activities do cause temporary elevations in cortisol, the body’s primary stress hormone, which can remain elevated for several hours. However, research has shown no significant differences in the duration of oestrus, timing of ovulation, pregnancy rates, or early embryonic loss between mares that are transported and those that are not.

That said, stress may still play a role in mares with borderline progesterone levels. Progesterone is essential for maintaining pregnancy, and in mares with marginal levels, stress could potentially tip the balance. In such cases, it’s best to consult your veterinarian, who can assess whether progesterone supplementation is appropriate for your mare.

Modern Advances

Over recent decades, foaling rates have steadily improved, thanks to significant advancements in reproductive science and technology. The widespread use of ultrasonography, newly available hormonal therapies, and a deeper understanding of reproductive physiology, semen handling, and pathology have all contributed to better outcomes.

In many breeds, the adoption of artificial insemination has played a major role in enhancing fertility rates and expanding breeding possibilities, allowing for more precise timing and genetic selection.

References

Robles et al (2017) Maternal Nutrition during Pregnancy Affects Testicular and Bone Development, Glucose Metabolism and Response to Overnutrition in Weaned Horses Up to Two Years. PLoS ONE 12(1): e0169295. doi:10.1371/journal.pone.0169295 https://pubmed.ncbi.nlm.nih.gov/28081146/

Oocyte metabolic function, lipid composition, and developmental potential are altered by diet in older mares. Catandi et al (2022) Reproduction 2022;163(4):183–198. doi: 10.1530/REP-21-0351 https://pmc.ncbi.nlm.nih.gov/articles/PMC8942336/

Dr Jennifer Stewart
BVSc BSc PhD Equine Veterinarian and Consultant Nutritionist