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Subfertility can be complex

Subfertility can be complex, multifaceted and affect either or both mare and stallion. For successful conception many different parameters must be met for example adequate hormone production, a healthy uterus, adequate number of progressively motile sperm cells, a healthy oocyte (egg), and the meeting of sperm and oocyte in the correct window of time is required.

So what key can a breeder do to optimise chances of conception and full term pregnancy?

Start with a healthy, happy horse (mare and stallion) 

Overall health must be optimal to maximise the reproductive efficiency, welfare and longevity of equines. Prevention is preferable than cure. Provide core and risk-based vaccinations, deworming and teeth care. Create a suitable health plan in consultation with the vet. Screen for metabolic diseases as recommended by the vet. Evaluate dietary needs periodically and ensure nutrient requirements are being met. Remember that pain or inflammation anywhere in the body can negatively impact reproductive success.

Eliminate as many stress factors as possible. Stress negatively impacts fertility. Minimise transport, minimise change of social group (familar herd mates), and maintain a consistent routine. 

Start the season with a breeding soundness examination

This assesses the mare’s ability to reproduce and identifies or predicts any special care she might need. This should include at a minimum a rectal palpation and ultrasound exam of the uterus, cervix and ovaries; vaginal examination; and uterine culture and cytology.

 The protective barriers of the vulva lips, vestibular fold and cervix protect the uterus from contaminants and where compromised place pregnant mares at risk of placental inflammation and potentially abortion. Fluid accumulation in the uterus can be associated with endometritis (inflammation / infection of the uterus lining) negatively affecting ability to conceive and carry a foal to term. Ensure the uterus is healthy. 

Determine if the mare is cycling normally 

This can be achieved by the vet examining the ovaries and their follicles and investigating hormone levels. If there is a history of subfertility additional diagnostics may be required.

With stallions the most straightforward measure of fertility is his overall pregnancy rate. Breeding soundness exams can also be performed on stallions where subfertility is suspected. An integral part of that exam is the collection and analysis of semen which can provide valuable information on semen volume, concentration, motility, and morphology all of which can influence pregnancy rates.

Address reproductive issues and freedom from disease before the first breeding attempt 

Tackle issues such as endometritis; pooling of urine; or unbalanced hormones to avert unsuccessful breeding efforts. A Caslicks procedure might be needed to avoid further uterine contamination after breeding. 

Assess freedom from diseases including Contagious Equine Metritis, Equine Viral Arteritis and Equine Infectious Anaemia Read more

Communicate in time with the semen provider when using chilled semen 

Good and timely communication with the agent or stud farm selling semen is critical. The time required for transport and the stage of the mare’s oestrus cycle must be synchronised. Find out which days of the week semen is collected and how much notice is required for semen to be sent. Find out if transport costs are included or additional. Notify the provider on the first day of oestrus and again request to transport at least 24 hours in advance of desired insemination time. The use of ovulatory stimulants i.e Human Chorionic Gonadotrophin (hCG) or Deslorelin can also be used to improve synchrony between arrival of shipped semen and time of ovulation. Discuss with your vet.

Ensure adequate quality and quantity of semen 

For conception to occur a healthy sperm must meet a healthy oocyte at just the right time. To achieve the goals of both mare and stallion owner it is necessary for each dose of semen to contain sufficient numbers of functionally competent sperm to maximise the probability of conception.  A mare must be inseminated with at least 500 million progressively motile sperm 12-18 hours before ovulation using chilled semen/ at least 300 million progressively motile sperm (after thawing) within six hours of ovulation using frozen semen, with acceptable motility. But, sperm motility does not necessarily equal fertility. A sperm that is not motile is likely not fertile, but a sperm that is motile may or may not be fertile. In addition to progressive motility, a fertile sperm must possess acceptable morphological characteristics, intact plasma and membranes in order to bind to and penetrate the oocyte and a whole host of other known and unknown functional attributes.Ensure the semen dose is adequate, and the semen are healthy.

Have the mare inseminated within the appropriate window of time to ovulation (chilled 12-18 hours before ovulation ; frozen – within 6 hours of ovulation).

Evaluate the pros and cons of using frozen semen if that is the plan 

In theory freezing semen provides infinite sperm longevity as long as the straws are stored, handled and thawed correctly. The reality is pregnancy rates are lower than those achieved with fresh and chilled semen. When used with less than fertile mares, those that are repeatedly barren or have reduced pregnancy rate, older mares (>15 years), and when collected from some stallions for varying reasons the likelihood of success diminishes. Correct handling of semen is also critical. The window of opportunity relative to ovulation is tighter, within six hours, increasing management time required but semen can be readily available in storage on site where insemination is to be performed removing the concerns about availability that can be associated with chilled semen.

Study the terms and conditions under which semen is being sold before investing. Purchasing frozen semen per dose without any guarantee of fertility or minimum semen quality is a risky business. A safer approach is to purchase frozen semen with a contract for pregnancy.

Address persistent mating-induced endometritis promptly

Most mares experience mild transient inflammation of the uterus after breeding that resolves on its own and doesn’t affect conception rates. However, there are mares who experience persistent mating-induced endometritis and prove difficult to get in foal. These are often older (>14 years) mares that have bred several foals and with a history of accumulating, and failing to clear, fluid within the uterus. If the fluid isn’t cleared by the fifth day after ovulation when the embryo descends from the oviduct into the uterus, the pregnancy is at risk. Clearing any ‘persistent inflammation’ without delay is essential. Treatment consists of uterine lavage as early as six hours after breeding, oxytocin injection to promote uterus contraction and clearance of fluid and inflammatory products; and in some cases additional medications.

Scan the uterus to check for pregnancy 

Ultrasound scan at 14-16 days after ovulation enables early detection of a pregnancy and early detection of twins before they become fixed in the uterus. This ensures a plan can be made for re-breeding if the mare failed to become pregnant or an attempt can be made to reduce one of the twins in that instance.

Take action if failure to achieve pregnancy after two oestrus cycles – a thorough breeding soundness exam (if not already performed) may be necessary to determine if the mare is at fault. If semen quality is suspect breeding to another stallion may be an option for consideration. To avoid the expense of repeat breeding try to select a stallion that has been proven to achieve good pregnancy rates with cooled / frozen semen whichever is the case. To be successful mare owners, stallion owners/ agents and veterinarians must work together as a team to optimise the chance of conception from each breeding.


The theory is relatively simple, one sperm must meet one oocyte to produce an embryo. In practice, many factors must align – timing, semen quality, and uterine health not least among them – to establish a pregnancy. Creating a plan to optimise health and fertility is merited for successful outcomes.

Note: Ensure to familiarise yourself with the permission of the breed society regarding any assisted reproductive technology used. i.e. Thoroughbreds can only be bred using live cover, though the use of ovulatory drugs is permitted.