While there are many factors which can impact fertility and pregnancy in the mare, nutrition plays a fundamental role from conception through to foaling.
Bodyweight and Condition
Research now tells us that a mare’s nutritional requirement is elevated from conception, not just the third trimester (where the majority of foetal weight gain occurs). While the rate of foetal development fluctuates during pregnancy, certain developmental landmarks are associated with increased nutritional requirement.
Achieving optimal body condition (amount of body fat carried) throughout pregnancy is vital. Conception rates have been shown to be reduced in mares that are significantly over or underweight, with mares entering the breeding season in optimum condition being shown to cycle earlier than those in poor condition. Leading up to conception, on a 9-point scale a body condition score of 5-5.5 (moderate to fleshy) is thought to be optimal. Preparation is key, changes should be made 6-8 weeks in advance particularly when letting down out of competition or training.
The optimal body condition score for a pregnant mare at any stage of gestation has not been established. But anecdotally, a body condition score of 6 (out of 9) is recommended, providing ample buffer should weight loss occur in late gestation. This is of particular importance in the management of maiden mares or where breeding history and trends in condition are unknown.
Obesity during late gestation offers no advantages to mare or foal. Excessive body fat caused by over-feeding during late pregnancy has not been shown to affect a foal’s birthweight but has been associated with higher risk of dystocia (difficulties foaling). Maternal obesity has also been associated with decreased insulin sensitivity and increased risk of osteochondrosis dissecans (OCD) in foals. Mares should however be seen to gain overall bodyweight in the last three months of gestation (owing to the rapid increase in size of the foetus during this period), the absence of this may suggest she is utilising existing body stores to support foetal growth which may put her at a disadvantage during lactation.
Regular weight monitoring and body condition scoring is essential for the broodmare and can provide valuable feedback on the ration and its suitability, particularly with regard to calorie provision. As a guide, over the length of the pregnancy, expected weight gain should be around 10-15% of pre-pregnancy weight (foal birth weight typically accounting for just under 10% of this).
Meeting Macronutrient and Energy Requirement
Macronutrients encompass nutrients which are required by the horse in greater quantities and include fibre and protein. From conception through to mid-gestation the foetus is still small (<2% of the mare’s bodyweight) and requirement for energy/calories and protein can often be met through provision of ample (2-2.5% bodyweight/day) good quality forage.
From month five, requirement for protein and energy is seen to increase beyond maintenance. Key milestones in foetal development at this point include the development of placental tissue, internal organs, and hair. From this point until term, requirement for energy and protein continue to increase.
During late pregnancy, the foetus will gain roughly 80% of its birth weight. Reflecting this rapid growth, the mare’s energy (calories), protein, vitamin and mineral requirement increases significantly. During late gestation ensuring adequate forage consumption can be a challenge. As the foal starts to take up more space within the abdominal cavity, the mare’s appetite and ultimately capacity for feed intake decreases. This can mean that she is physically unable to consume sufficient forage to meet her energy and nutrient requirements. This can be further compounded if the forage provided is of low quality. To counter this, provision of higher energy and nutrient dense feeds in small frequent meals is essential.
Meeting Micronutrient Requirement
Micronutrients are nutrients that are required in much smaller quantities such as vitamins and minerals. While macronutrient requirements, as we’ve discussed, are often easily met in early pregnancy, micronutrient deficiency throughout pregnancy is more common and can have a significant impact on the health of the mare and unborn foal.
In the first two trimesters, it is common for the broodmare to be deficient in several key minerals including copper, zinc, selenium, calcium, and phosphorus. Minerals such as copper and zinc and vitamins A and D are essential in the development of internal organs (such as the heart) and the placenta during this period, therefore ensuring nutrient requirements are met is key.
During months 8-11, around 90% of the calcium and phosphorus required for skeletal development is deposited, along with around 50% of the copper, zinc and manganese, which are all required for elastic connective tissue and collagen formation (key component of bone matrix). It is during this stage that correct maternal nutrition enables the foetus to fortify its liver with trace minerals. The foal then utilises these mineral stores during the first two months of life to support skeletal growth and buffer mineral deficiencies in the mare’s milk. Consequently, if a foal’s liver stores are inadequate, he may be predisposed to growth disorders.
While nutrition is often one of several factors behind developmental orthopaedic disease (DODs) in foals, research suggests that foals born to mares that were given supplemental copper had lower incidence of osteochondrosis. This emphasises the importance of ensuring the mare’s diet provides sufficient trace minerals that are correctly balanced to ensure optimal absorption and utilisation.
Both vitamin A and E intakes are elevated above maintenance requirements throughout pregnancy and are often lacking, particularly for mares during the winter breeding season or those without access to fresh forage (grass).
For vitamin A, key functions of note include its role in vision; gene expression; immune response and fertility. When considering natural dietary sources of vitamin A for the horse, it should be noted that these do not contain vitamin A directly, but carotenoids which are precursors for vitamin A, the most important of which is Beta-Carotene. Within the horse, precursors are converted to vitamin A which is then absorbed in the small intestine and stored in the liver.
Sources rich in beta-carotene include fresh growing grass and legumes, with content in the leaves thought to be greater than within the stem, making leaf to steam ratio an important consideration. It follows that conserved forages (hay and haylage) are much poorer sources, suffering losses in the preservation process. For broodmares where the primary forage is conserved (hay or haylage) or limited winter grazing, it is likely that vitamin A intakes are low and may be inadequate to support beta-carotene stores within the ovaries.
Vitamin A is vital for ovulation and pregnancy maintenance. One of the body stores for beta-carotene is the ovary (specifically the corpus luteum), where it assists in the role of progesterone control and secretion. With this in mind, appropriate supplementation may have positive implications for conception rates and application for embryo transfer, recipient mares and ICSI (Intracytoplasmic Sperm Injection). Supplementation of beta-carotene and vitamin E provided to the mare has been shown to increase the concentration of these in colostrum and milk and plasma of the foals which may infer immune function, growth, and health benefits.
Folic acid belongs to the B complex group of vitamins; an umbrella term for a collection of vitamins primarily manufactured by the microbial population in the hind gut. Therefore, any disturbance to hindgut function, for example excessively high starch or low fibre diets, can reduce production of B vitamins leading to deficiency. As water-soluble vitamins, they cannot be stored in the body and therefore are reliant upon regular supply.
Specific requirements for most B complex vitamins including folic acid are unknown for the horse, and folate status throughout pregnancy has not yet been adequately assessed. However, maintenance of folate status throughout pregnancy has been identified as a significant factor in the health of human pregnancies. Furthermore, there is literature to suggest that mares with access to fresh forage during pregnancy and lactation are likely to maintain folate status better than those that do not implying potential benefit to supplementation for these individuals.
The most significant source of antioxidants in the horse’s diet is fresh forage (grass). Subsequently, supplementation is often required where the sole forage source is hay or haylage.
The main function of vitamin E is as an antioxidant forming part of the non-enzymatic defence in conjunction with antioxidative enzymes. During gestation and lactation, requirements for vitamin E are thought to double compared to maintenance. Increased vitamin E provision (2-3 times maintenance requirement) has been shown in research to increase the concentration found in colostrum, milk, and plasma. Although this does not assure improved health, it may improve resistance to systemic infections and respiratory diseases.
Selenium is required to support normal musculoskeletal development and works together with vitamin E as an antioxidant. Without these two vital nutrients, a foal may be born with (or later develop) white muscle disease (a disorder resulting in weakness/stiffness in young animals which can affect the skeletal and heart muscles).
Omega 3 fatty acids, specifically DHA is thought to have many benefits for the broodmare. More recent research also indicates potential benefit of DHA supplementation on improving conception rates during the foal heat. Omega 3 fatty acids could also help to improve the mare’s reproductive efficiency. Research indicates that they help inhibit prostaglandin production thereby prolonging the maintenance of corpus luteum, which may be particularly beneficial for mares with a history of abortion. Furthermore, supplementation of the mare with omega 3 has been shown to transfer to the foetus increasing immunity potential in young foals.
Table 1: Vitamin function, requirement and implications of deficiency or excess in breeding stock.
Requirement at maintenance
Feed management or reproductive status increasing requirement
Risk of excess intake/toxicity
Vitamin A (retinol)
Immune response, fertility, vision, gene expression, bone and tissue integrity and development, metabolism of fats and carbohydrates.
a30IU /Kg BW
(150 IU /Kg BW 0.75)
Leafy plant matter (grass or legume) carrots and fish oils.
Commonly provided in complementary feed.
Mares during late gestation and lactation and growing youngstock have elevated requirements (60IU/kg BW and 45IU/Kg BW respectively), those with no access to fresh forage will be more reliant on vitamin A from supplementary feed.
Toxicity levels are yet to be reported in horses. Signs of deficiency follow the key functions, including night blindness, impaired growth, poor immune function, and fertility.
Folic acid (folacin)
Production of red blood cells. Largely unresearched in the horse.
Fresh plant matter. Synthesised by the microbial population in the hind gut.
Low forage intake over a prolonged period.
Excess in B vitamins have not been described in horses.
Vitamin E (tocopherol)
Antioxidant properties, muscle development and function, immune function, and fertility.
1IU/kg BW (5mg/Kg BW0.75)
Fresh forage (grass or legume) and cereal germs.
Commonly provided in complementary feed.
Horses in moderate to hard work, gestation, lactation, and growth or on a fat enriched have elevated requirements of 10mg/kg BW0.75per day increasing further to 20mg/kg BW0.75per day for those in intense training and those susceptible to ERS.
Excess is not common in the horse and even with high intakes, toxicity has not been reported.
a Limited evidence-based information to inform requirements.
Feeding for the Future
A high rate of tissue turnover during gestation and early life results in a period very sensitive to change. It is well-established in human research that maternal nutritional environment can have life-long effects on health and this is now recognised in horses; The mare’s diet effectively providing information to the developing foetus on how metabolism should be adapted for the environment into which it is born.
The importance of ensuring the right balance of vitamins and minerals for the broodmare cannot be overstated. What we feed broodmares from conception has a profound impact not only on her own health but that of the unborn foal. Although more research is required to better understand the impact of diet on the unborn foal, simple changes to reflect forage and management can improve the chances of producing a healthy foal.
Take home points:
Feeding for reproductive performance is key from conception through to lactation.
Broodmares that are more likely to require additional vitamin supplementation include mares bred out of season or those fed solely on conserved forage (hay or haylage).
Additional practical application of tailored micronutrient support for the broodmare lies within reproductive technologies such as recipient mares for embryo transfer.
Bondo, T., Jensen, S.K. (2011) Administration of RRR-α-tocopherol to pregnant mares stimulates maternal IgG and IgM production in colostrum and enhances vitamin E and IgM status in foals. J Anim Physiol Anim Nutr, 95(2):214-22.
Jacobs, R. D., Ealy, A. D., Pennington, P. M., Pukazhenthi, Budhan S., Warren, L. K., Wagner, A. L., Johnson, A. K., Hess, T. M., Knight, J. W., and Splan, R. K. (2018) Dietary Supplementation of Algae-derived Omega-3 Fatty Acids Influences Endometrial and Conceptus Transcript Profiles in Mares. Journal of Equine Veterinary Science, 62: 66–75.
Kuhl, J., Aurich, J.E., Wulf, M., Hurtienne, A., Schweigert, F.J., Aurich, C. (2012) Effects of oral supplementation with β-carotene on concentrations of β-carotene, vitamin A and α-tocopherol in plasma, colostrum and milk of mares and plasma of their foals and on fertility in mares. J Anim Physiol Anim Nutr, 96(3):376-84.
Robles, M., Hammer, C., Staniar, B., Chavatte-Palmer, P. (2021) Nutrition of Broodmares. Vet Clin North Am Equine Pract. 37(1):177-205.