Understanding Equine Reproduction Success Rates: A Closer Look with Dr. Sarah Malenchak
It can be frustrating and discouraging when you have a mare that is not getting pregnant. Breeding success in horses is influenced by many factors including the mare's age, reproductive history, and overall health. We had Westhills own Dr. Sarah Malenchak dive deeper into each category to better understand why success rates vary - including young maiden mares, older maiden mares, barren mares and mares with a foal at side.
Young Maiden Mare (<7 years old): Highest Success Rates
Young mares who have never been bred before are at their reproductive peak. Their reproductive systems are usually healthy, and there’s minimal risk of age-related conditions that can affect fertility. With proper management and timing, conception rates are often 70-80% per cycle.
What factors can decrease the success rates, even in younger mares?
Poor vulva conformation preventing a protective vaginal seal can lead to environmental contaminants and bacteria entering the uterus. Performance horses with poor conformation can lead to windsucking and decrease breeding success rates.
Suboptimal nutrition or weight: Underweight or overweight mares may have hormonal imbalances that affect fertility. Obese horses can experience altered levels of hormones like insulin and leptin, leading to insulin resistance. This can disrupt the mare’s reproductive hormone cycle affecting ovulation, oocyte quality and overall reproductive health.
Tip: Ensuring mares are in peak body condition, while providing optimal nutrition including a high quality protein present in their diet increase their chances. High-quality protein and essential amino acids support ovarian function, embryo development, and uterine health.
Older Maiden Mares (>7 years old): Lower success rates than younger mares
As mares age, their reproductive system undergoes changes, including decreased ovarian function and more fibrotic uterine and cervical tissue, which can reduce conception rates. Older maiden mares may also have subtle reproductive tract abnormalities due to age. As a result, success rates usually fall to 50-60% per cycle.
What commonly causes low success rates in older mares?
Cervix Fibrosis: As mares age, fibrous tissue prevents proper dilation of the cervix, which can decrease uterine clearance and fluid accumulates in the uterus. This creates a hostile environment and embryonic death.
Decreased ovarian function: Older mares may have lower-quality eggs or blocked oviducts which can reduce fertility.
Delayed uterine clearance: Older mares may have decrease uterine muscle tone which slows uterine clearance, which can cause fluid accumulation and inflammation, making conception more difficult.
Tip: Specialized care, including advanced reproductive diagnostics such as uterine biopsies, cultures and biofilm treatments, can improve outcomes for older mares.
Barren Mares: Variable Success Rates A barren mare may be defined as one that comes in heat regularly, is bred at each heat period, but does not get in foal; a mare that may come in heat at irregular intervals, is bred at each heat period, and does not get in foal; or a mare that does not come in heat at all. Barren mares issues can be due to undiagnosed uterine health issues such as hormonal imbalances, infections, or trauma to the cervix or uterus. Their success rates depend largely on identifying and treating the underlying cause, leading to a range of 40-50% per cycle.
Why is success variable?
Undiagnosed infections: Subclinical uterine infections can persist without noticeable symptoms and prevent pregnancy.
Hormonal imbalances: Barren mares may experience irregular estrus cycles or insufficient progesterone levels to maintain pregnancy.
Uterine or cervix trauma: Trauma from previous foaling injuries can prevent the cervix from functioning normally or can hinder embryo attachment.
Tip: Thorough diagnostic workups and customized treatment plans are essential to increase the chances of conception.
Mares with Foals at Side: Similar to young, maiden mares
Mares that have recently foaled tend to have relatively high conception rates, around 60-70% per cycle, especially if they’ve recovered well and are in good body condition. Their recent successful pregnancy suggests their reproductive system is functioning well. However, nutritional demands and physical stress from nursing may slightly affect fertility.
What can cause low success rates in a mare that just had a foal the previous year?
Postpartum recovery issues: If the mare had a difficult foaling, she might have uterine trauma or a retained placenta, reducing fertility in the subsequent breeding cycle.
Nutritional demands from nursing: Lactating mares require significant energy, and if they are in a negative energy balance, their reproductive system may may not be able to support a pregnancy.
Stress from foal management: Some mares find being a mom very stressful, especially first time moms, which affects hormone balance and may have difficulty conceiving again.
Tip: Proper postpartum care and nutrition are crucial to maintaining reproductive health and increasing conception chances.
At Westhills Equine, we are proud to specialize in customized reproductive care to give every mare the best chance for breeding success. Please don’t hesitate to get in touch with our reproduction department with any further questions!
