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Are Genetics Enough to Breed an Equine Superstar?

September 2015 by Dr John Chopin, BVSc, MANZCVS (Equine Medicine), PhD, FANZCVS, Registered Specialist in Equine Reproduction

As Registered Equine Reproduction Specialist Dr John Chopin explains, the single most limiting factor affecting a horse’s athletic potential might be the 11 months spent in utero.

  • Did you choose the best bloodlines? 
  • Are you feeding your equine athlete all the recommended feeds and micronutrients? 
  • Have you got the best trainer? 
  • Do you get them regularly massaged or acupunctured? 
  • Are you looking for something else to maximise the potential of a horse that, genetically, should be a superstar, but is not quite living up to expectations?

Unfortunately, if you didn't consider the 11 months spent in utero, it might be too late for your equine superstar.

What is foetal nutrition? 

An individual that is not living up to genetic expectation might already have had setbacks before it was born. These setbacks will rule its entire life and it will never live up to genetic expectation, despite all the supplements and training it receives. These setbacks can be a result of changes to metabolism that are locked-in for life and relate to the structural make-up of bones, muscles, lungs and heart.

They are all dictated and laid down during pregnancy. There is good scientific evidence that events during pregnancy can be influenced to change the metabolism and structure of individuals as they develop. This article will help explain why this might be the case, so future individuals might have their full genetic potential exploited.

Nutrition drives reproduction. Good nutrition makes mares cycle earlier in the season, cycle more regularly and increase the chance of going in foal.

Low-protein diets or low-energy diets result in a higher embryonic loss rate in mares. If a pregnant mare is restricted from eating, she immediately prepares to abort with the hormones of labour increasing in her bloodstream. This is an important consideration with any pregnant mare that is not eating due to illness, or is having food withheld for a medical or surgical procedure.

A developing pregnancy changes from an embryo to a foetus when all the rudimentary organs have formed. Although the term foetal nutrition will be used in this article, embryonic nutrition is also critical.

Foetal nutrition is the balance between molecules the foetus needs to grow (oxygen, sugars, proteins, electrolytes) and also the removal of harmful waste products (carbon dioxide and other waste products of metabolism).

The delivery and removal of these products occurs through communication between maternal and foetal bloodflow. This occurs at the border between the uterus (maternal bloodflow) and foetal membranes (foetal bloodflow). The combination of these two powerful units is called the placenta.

Foetal nutrition is not only dependent on what is delivered or removed, but also how much. Changes in bloodflow from either the maternal or foetal side can change foetal nutrition. So, dietary intake of the mare is not the only factor involved. Her overall health is important.

  • Is she in good condition?
  • Is she feeling well?
  • If she has a period of illness or is exposed to toxins from the environment, these might influence foetal nutrition.

One last consideration is the older mare that might have damage and scarring to the uterus. This might change the bloodflow to the placenta and also influence foetal nutrition.

There are a wide range of factors that can influence foetal nutrition. Some of these are temporary and can be treated (mare illness); others are permanent and might not be able to be accommodated (uterine damage or scarring). If the pregnant mare undergoes any change in her state of wellbeing, consideration should always be given to supporting the other individual inside her.

Metabolic preprogramming

In the hours to days after fertilisation, metabolic preprogramming occurs.

Metabolic preprogramming is where the young embryo reads information about the external environment from signals supplied from the mother and adapts its metabolism to optimise its chances of survival. These metabolic decisions are locked in for life.

There are thought to be two types of metabolism:

  • Thrifty
  • Non-thrifty

Thrifty metabolism results when the embryo decides that life outside of the uterus looks tough and makes major changes to its metabolism in order to increase survival after birth.

The classic description of a thrifty metabolism is a couch potato that would rather watch sport than participate in it. In humans, these people are insulin resistant (prone to diabetes), have altered exercise behavior (will be sedentary to conserve resources), have altered feeding patterns (will prefer and select energy-dense foods), have reduced skeletal muscle mass (less exercise), central fat deposition (saving energy for a rainy day) and also have other alterations in autonomic control (one being high blood pressure). In humans, this can be as a result of low oxygen, low nutrition or low protein during early pregnancy.

The opposite of this metabolism is the non-thrifty individual that is very active, eats whatever they want and doesn’t seem to be overweight. These individuals developed where there was no indication to the young embryo there was a need to spare resources as environmental resources appeared to be in plentiful supply.


SSS or Size, Strength, Scope are characteristics most Thoroughbred farm managers evaluate when a foal is born. This gives them feedback to give to the mare owner, so a decision can be made on who to mate the mare back to. If it was favourable, the mare owner might send the mare back to the same stallion. If unfavourable, the mare owner might change stallions.

Size and strength is determined by genetics, so evaluating this is incredibly important. However, other factors are involved, which might confound the decision about ideal genetic matching.

Maiden mares (a mare that has not had any previous foals) will have smaller foals than mares that have had foals before. Maiden mares should, therefore, have their foals compared to foals from other maiden mares.

This is because there is a change in the placental unit between the first and subsequent pregnancies. The first pregnancy is almost like a priming run to optimise the function of the placenta and, from the second pregnancy, the placenta then works at full capacity.

As the mare has subsequent foals, the uterus can become damaged and the placenta starts to deteriorate, and suboptimal foals can be produced towards the end of a mare’s breeding career.

Bones and muscles are not the only structures that can have their development influenced by events in gestation. If there is an interruption in foetal nutrition, any organ that is developing at that time can have its structure and function impaired. If the interruption is severe and long enough, intra uterine growth retardation (runting), or foetal death and abortion might be the result.

Fertility of the offspring is one of the potential systems that can be damaged. Undernutrition of sheep in early pregnancy reduces ovarian function of the female offspring. Undernutirition of pregnant guinea pigs in the first trimester can result in a 30% reduction in muscle fibres in the offspring. Overnutrition of pigs during the stage of foetal muscle development can increase muscle mass.

When the foetus is in the pregnant uterus, it is important it has enough space to move and stretch its limbs. This is determined by the amount of fluid that is in the uterus.

Acute dietary restriction in mid pregnancy reduced the size of the lungs of newborn lambs. This was a result of a reduction in fluid, due to the dietary restriction of the pregnant ewe. The reduced fluid restricted the volume the lamb had to move in. The lamb had to curl up, which caused the abdominal contents to put pressure on the diaphragm, which reduced the chest space and the smaller lungs developed in a smaller chest space.

There was a recent report on foals born with contracted limbs that showed there were reductions in the circumference of the foetal membranes. This might have reduced the volume these foals were in and they were not able to periodically stretch out their limbs, and so, might have led to the foals developing flexural contractions.

Neurology - coordination and the ability to learn

Coordination, strength and the ability to learn need a neurological system that is working at full capacity.

The development of the nervous system can be influenced by foetal nutrition. Subclinical cobalt deficiency in early pregnancy can produce lambs that are slow to stand and suck. This made them prone to colostrum deficiency and infection.

Human babies that are affected by Sudden Infant Death Syndrome (SIDS) have reduction in their diaphragm muscles and a reduction in lung capacity, but they also have reductions in the brain and the nerves that drive the diaphragm and respiration.

Disease risk

There are a number of diseases present during growth and as an adult that are related to intra uterine growth retardation (or runting).

On top of skeletal muscle development and impaired athletic performance, the list includes:

  • Hyperlipaemia - a fatty blood disease affecting multiple organs, including the liver
  • Allergic conditions
  • Chronic obstructive pulmonary disease (COPD) - the horse equivalent of asthma,
  • Osteochondrosis - a bone developmental problem that can lead to deformities, bone cysts, joint chips and arthritis.


In the mare, the presence of adequate to excess body condition during cyclicity, pregnancy and lactation does not appear to have any detrimental effects. In comparison, poor body condition has measurable detrimental effects on reproduction and the foetus.

Inadequate nutrition during equine pregnancy can limit the development of foetal organs and alter metabolism permanently. In the equine, the single most limiting factor affecting athletic potential might be the 11 months spent in utero.

ABOUT THE AUTHOR: Registered Specialist in Equine Reproduction, John Chopin, grew up in south-west Queensland and graduated from The University of Queensland Veterinary School in 1990. After 18 months in general mixed practice, John completed a residency in equine medicine and surgery under the tutelage of Dr Dick Wright. John then completed a PhD with Dr Kerry Dowsett in equine frozen semen and the endocrinology of ovulation in the mare. A theriogenology residency followed, mentored by Dr David Pascoe and Dr Reg Pascoe at Oakey Veterinary Hospital. This included a teaching sabbatical at The University of California Davis under the direction of Dr Barry Ball. From here, John completed his Fellowship and joined Coolmore Australia in the Hunter Valley as an equine reproductive specialist. John then worked for Tamarang Veterinary Breeding Services and established his own practice: Chopin Equine Reproduction Services in Murrundi, New South Wales. He currently works at WestVETS Animal Hospital and Equine Reproduction Centre in Marburg, Queensland.