The concept of the developmental origins of health and disease, also known as “developmental programming”, was put forward by David Barker. It is based on a series of human studies supporting the hypothesis that undernutrition in utero and during infancy results in permanent changes to physiology and metabolism, increasing the risk of coronary heart disease and stroke in later life (Barker and Osmond, 1986; Hales and Barker, 1992; Barker, 1994). Developmental programming encompasses the foetal and neonatal periods.
What is foetal programming?
Foetal programming is the process by which the foetus adapts to the uterine environment. These adaptations are likely to have evolved to “prepare” the foetus for the extrauterine environment (Satterfield et al., 2010). Maternal nutrition is one of the potential triggers for these adaptations.
Organs and tissues have critical “windows” of development when they are sensitive to environmental triggers, resulting in changes in the structure and function of a particular organ or tissue (Rossdale and Ousey, 2002; Fowden et al., 2013). A trigger can have different outcomes based on whether it occurs in early, mid- or late pregnancy.
| Robles et al. (2021) provide an excellent summary of current knowledge on timing of organ development in the equine embryo and foetus together with a comprehensive review of broodmare nutrition. |
The role of nutrition in foetal programming is an active area of research in human medicine, production animal science and, to a lesser extent, equine medicine. It is important to be aware of the practical implications of recent advances in our knowledge. Research in horses is in its infancy, so when evaluating the relevance of new findings in other species, we need to be conscious of species differences in placental structure, foetal development and timing of critical periods of development.
What is epigenetics?
Epigenetics is the study of stable changes in gene activity that alter the degree of gene expression without changes to the DNA sequence. These changes, called “epigenetic marks”, are passed on when cells divide, and some can be transgenerational.
The most studied epigenetic mechanisms include DNA methylation – when a methyl group is added to DNA – which “turns off” that gene, and histone modification – how tightly the DNA sequence is wound around the histone core. If it is tightly wound, it reduces gene expression and gene expression is increased when it is more loosely wound (Simmonds, 2008; Van Soom et al., 2014).
Nutrition, stress, drugs, toxic chemicals, exercise, obesity and the microbiome can act as epigenetic modulators
Nutrition, stress, drugs, toxic chemicals, exercise, obesity and the microbiome can act as epigenetic modulators. The emergence of this rapidly advancing field of veterinary medicine has provided an explanation for the mechanisms by which developmental programming can affect the health of the offspring in later life (Gluckman et al., 2016).
Broodmare nutrition: what we know about foetal programming
Most research in horses indicates that moderate under- and overnutrition during pregnancy do not significantly affect foal birth weight (Wilsher and Allen, 2006; Ousey et al., 2008; George et al., 2009; Coverdale et al., 2015; Peugnet et al., 2015; Robles et al., 2018). However, moderate under- and overnutrition during pregnancy may affect organ and tissue development and the function of the offspring, including glucose and insulin metabolism, testicular maturation, inflammatory status, bone development and osteochondrosis (Ousey et al., 2008; George et al., 2009; Coverdale et al., 2015; Peugnet et al., 2016a, b; Robles et al., 2017; Robles et al., 2018).
Moderate under- and overnutrition during pregnancy may affect organ and tissue development and the function of the offspring
In humans, many of the effects of developmental programming are only seen in later life or in response to adverse events. However, much of the limited research in horses lacks long-term follow-up, rarely following foals to maturity. Further research is needed to investigate the effects of both maternal and paternal nutrition on developmental programming in more detail. These should look not only at the short-term outcomes but at long-term health and athletic performance as well.
Obesity and overfeeding
Obesity is an increasingly common problem in horses, and broodmares are no exception. Mares can be obese from conception to parturition or may become obese in late gestation associated with overnutrition in the last trimester.
Overfeeding energy to mares (body condition score (BCS) 6/9) during the last trimester has been shown to increase the density and size of islet cells and affect skeletal muscle development, inducing a higher proportion of slow to fast twitch fibres and changes that may compromise the skeletal muscle’s ability to respond to training in later life (Bradbery et al., 2021a,b).
Higher starch (HS) diets during late pregnancy have been reported to affect glucose and insulin dynamics in foals, with foals from HS mares showing a trend to lower insulin sensitivity by 160 days of age (George et al., 2009). One study also reported an increased incidence of radiographic osteochondrosis (OCD) lesions in foals from mares fed higher starch diets (1.7g/kg body weight per meal) (Robles et al., 2017).
It has been confirmed that long-term maternal obesity does not affect the foal’s birth weight or growth rate (Robles et al., 2018). However, it reduces insulin sensitivity and increases the systemic markers of inflammation (serum amyloid A) and the incidence of osteochondrosis in foals.
Studies in broodmares have suggested that the inclusion of docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA) in the diet of obese mares may help mitigate some of the adverse effects on fertility and the uterine environment in early pregnancy (Pennington et al., 2020; Jacobs et al., 2018). However, further research is needed to investigate the effects on foetal programming.
Practical tips on broodmare nutrition
Body condition
A mare’s body condition should be regularly assessed and energy intake adjusted accordingly. Current evidence indicates that BCS 5/9 (Henneke et al.,1983) is ideal, whereas BCS 6/9 is considered overweight and BCS 7/9, obese (Robles et al., 2021).
As the adverse effects of obesity have been documented from preconception, it is important to discuss the BCS of the broodmares with owners and studs prior to the start of the breeding season. However, if mares are obese at the time of conception, it may be worth considering DHA and EPA supplementation (see above).
Stage of gestation and lactation
During pregnancy, nutritional requirements only increase significantly above maintenance during the last trimester and are the greatest in early lactation. For many mares, early pregnancy and peak lactation coincide.
Nutritional requirements only increase significantly above maintenance during the last trimester and are the greatest in early lactation
Health issues
Conditions including equine metabolic syndrome (EMS)/insulin dysregulation, pituitary pars intermedia dysfunction (PPID), laminitis and dental issues should be considered when advising clients on the best way to meet the mare’s nutritional requirements.
Fresh and conserved forage
Season, paddock management, stocking density and prevailing weather conditions will affect grass availability and quality and, thus, whether mares are relying on fresh or conserved forage. When the mare is due to foal affects the quality and availability of fresh grass and the extent to which grazing is likely to meet the increased demands for energy and protein in late pregnancy and early lactation.
When the mare is due to foal affects the quality and availability of fresh grass and the extent to which grazing is likely to meet the increased demands
Regular analysis of fresh grass is very rarely available; however, generalised basic information on grass growth on production animal farms in various regions of the UK is available online. Reliable local knowledge on micronutrient deficiencies, eg selenium, can be useful in the absence of forage analysis. Nutritional analysis of conserved forage, including minerals and trace elements, provides valuable information when selecting suitable hay or haylage.
Protein
Protein quality and amino acid composition are key to broodmare nutrition, so it is crucial to supplement with protein sources rich and balanced in essential amino acids when necessary.
Micronutrients
During periods when access to grazing and sunlight are limited, vitamins A, D and E are likely to need supplementing.
Alongside the quantity, the balance of micronutrients is also important throughout pregnancy and lactation, eg the calcium to phosphorus ratio should be 1.5:1 to 2:1. UK grass and conserved forage is frequently low in copper and zinc. Both selenium and iodine are important but have a narrow optimal range, with both deficiency and excess having adverse effects.
Energy
Energy levels in the diet should be adjusted regularly to ensure the mare remains in good body condition. The need for supplementation or restriction will largely depend on the nutritional quality and availability of forage.
Final thoughts
To provide optimum nutrition based on current evidence, each mare should be considered as an individual. However, on some larger stud farms, broodmares are fed in groups, either in paddocks or when housed in barns. In this situation, grouping mares based on gestation stage and BCS can provide a practical compromise.
| Tables of minimum requirements based on stage of gestation/lactation and body weight are available and provide an excellent starting point (NRC, 2007; Geor et al., 2013). |
It is well established that diet should be forage-based, with hard feed provided to ensure requirements for good-quality protein and micronutrients are met. Additional energy is only added when it is required to maintain the mare at an ideal BCS.
As vets, we should aim to provide independent, current evidence-based advice to help our clients understand the potentially far-reaching effects of broodmare nutrition and BCS from around the time of conception throughout pregnancy and lactation to help maximise the potential of the horses they breed.
Key takeaways:
- Maternal nutrition from around the time of conception may influence the athletic potential and metabolism of mature offspring
- Broodmares should be maintained at an ideal body condition of 5/9 (Henneke et al., 1983)
- Diets should be forage-based, and the inclusion of cereal grains should be minimised
- Diets should be balanced for good-quality protein and micronutrients
- Energy should be fed according to the mare’s current body condition
