ALL breeders hold hopes and dreams for the foals they produce, and many things will influence their future athletic performance and success. Of course, choosing the best sire for your mare will give that foal a great start genetically, but did you know that genetics is only responsible for less than 25 % of performance?

Lots of extrinsic factors such as diet, training, rider and medications will affect performance in addition to intrinsic factors such as temperament, health and metabolism.

All of these intrinsic factors can be influenced by the environment around the time of conception and pregnancy or both.

In the female, the eggs that will eventually be fertilised by a sperm to create an embryo are in the ovary since that female is born. As such, the environment that the female lives in, the diet she eats and the lifestyle she has, will be incorporated into that egg and be potentially passed down to her offspring.

In humans, we know that an unhealthy maternal lifestyle and diet during the preconception period and pregnancy, not only increases health risks for the mother but also has detrimental effects on children such as increased risk of obesity, cognitive dysfunction and incidence of diseases such as asthma and diabetes.

Males are not exempt from this phenomenon and in humans, paternal obesity is linked to increased pregnancy loss and compromised offspring health. In stallions, new sperm are made all the time, and it takes a sperm 57 days to mature.

As such, those two months prior to ejaculation are especially important in relation to diet and lifestyle. During pregnancy, we know that the mare is unique in that the uterine size determines the size of the foal due to the type of placenta.

This means that regardless of the genetics of the foal, size will be influenced by the uterus.

The mechanism underpinning these changes and offspring effects is known as ‘developmental programming’ and is attributed to epigenetic* modifications, damage to DNA and even changes in the ejaculation fluid in males.

In horses we are just at the beginning of our understanding on how these mechanisms of developmental programming might be at play in our mares, stallions and foals.

Overall, we as an equine breeding industry should keep this in mind and shift focus from maximising pregnancy rates to adopting practices that maximise and safeguard the future health of the offspring. Below is a summary of what we know already and the, as yet, unanswered questions.

Mare nutrition - overfeeding

Obesity is a growing concern in our domestic equine population particularly in native breeds and leisure horses. The negative effects of obesity on overall health and welfare are well established and we are beginning to understand the specific effects on reproductive function and offspring health. Researchers in Colorado State University and INRAe in France have been instrumental in adding to our body of knowledge on the subject in recent years.

To give a short summary, approximately 50% of leisure horses in the UK are reported to be obese which rises to 70% when considering native breeds such as the Connemara pony in Ireland.

Furthermore, researchers in UCD have shown that owners are not very accurate when evaluating the body condition score (BCS) of their own horses. In humans, detrimental effects of obesity on fertility are well documented, however the effects in horses appear more nuanced or as yet undescribed.

Despite many obesity-related changes at the molecular level in the egg, follicle cells, uterus and embryo, the fertility rate does not seem to be lowered in high BCS mares. However, this does not mean there is not a problem. A recent study showed that if the mare was obese at the time of artificial insemination (AI), the foals had an increased incidence of joint disease (Osteochondrosis dissecans; OCD).

When the mare is pregnant, the effects of obesity and overfeeding are well described. Foals born to obese dams have more inflammation, altered metabolism and higher risk of OCD (up to eight times the incidence in some studies).

Overfeeding can also induce changes in fat composition in the milk and changes to the placenta. Work carried out last summer by our group in UCD and funded by DAFM showed that similar issues are observed in obese Connemara ponies here in Ireland.

Additionally, if a mare has chronic laminitis during pregnancy, a common side effect of obesity in horses, it causes blood vessel changes in the placenta which results in lighter foals with a shorter gestation length. Long term studies on the foals born to obese dams are still needed to ascertain the clinical and performance related relevance.

Underfeeding

While of course obesity is detrimental, so too is severe undernutrition. Not only does undernutrition result in lowered pregnancy rates but if they do get pregnant, foals are lighter, the placenta is lighter, canon bones are thinner, and puberty is delayed.

Mare parity

Maiden mares have lighter foals with lighter placentas compared to mares that have had previous foals.

Researchers in INRAe in France showed that parity also influenced performance.

Foals born to mares that had a foal previously have higher performance ratings in show jumping than those born to maidens.

This is a very interesting finding and should be considered in the context of recipient mare selection in embryo transfer programmes.

Thought should also be given to potentially allowing mares destined for athletic careers to have a foal early in life rather than having their first foal after retirement.

The age of the dam also affects foal growth and performance, therefore the ‘old maiden mare’ is a situation to be avoided, not least of all as those mares also have reduced fertility.

There is also emerging evidence that if a mare is lactating (i.e. has a foal at foot) at the time of conception, she has reduced milk yield in the subsequent lactation.

What this means for breeders is that particular attention should be paid to mares that are having foals every year and a close eye should be kept on the growth rates of those foals and nutrition supplemented if necessary.

Early life events

Developmental programming doesn’t stop at birth.We know from other species that mothering behaviour, stress and diet can continue to shape how the genes work in later life.

Another way adult health can be influenced is via the population of bacteria in the gut, commonly known as the “microbiome”.

Researchers in Nottingham University recently showed that racehorse performance was positively linked to the diversity (increased numbers of different types of bacteria) of the foal’s microbiome in early life.

They went on to further show that lowered diversity was correlated to higher rates of bone and soft tissue injury as well as respiratory disease whilst in training.

Importantly for the breeder, this microbiome diversity was negatively affected if the foal had received antibiotics as a young foal.

While antibiotics can be lifesaving medicines, the rise in resistance and side effects on the microbiome means they should be reserved for true bacterial infections, which is why vets are increasingly trying to avoid them unless specifically indicated.

The stallion

Research from University of Limerick and Australia has shown that in racehorses, offspring performance gets lower as sire age increases i.e. on average, foals from old stallions do not perform as well as those from young stallions.

The reason for this in stallions is still under investigation.

However we know that in other species DNA damage and altered metabolism can be involved.

The role and effects of the ejaculation fluid (seminal plasma) is another area coming to light that may be a “major player” in passing on information to offspring.

The seminal plasma, like blood, can change in response to metabolic state and diet but unlike blood, it is not sterile and has its own “microbiome”.

As yet, the effects of stallion lifestyle on developmental programming is unknown.

However studies are currently underway in this area in our group in UCD and other universities and results should become available in the next few years so watch this space.

Niamh Lewis (BVM&S DACT DECAR PhD) is Assistant Professor in Equine Reproduction at the UCD School of Veterinary

Take home message for breeders

In summary, this is an emerging and exciting field but, if in doubt, nature knows best! We should be aiming to keep our broodmares and stallions healthy so that they produce foals that will not only perform but will have a long and healthy life and thus maximise positive welfare in our equine population.

Key points:

*Epigenetic = Alterations in how a gene works without altering the DNA itself