Optimizing Nutrition for Pregnant Mares

Updated February 23, 2024

Having a healthy foal starts with prioritizing the health of your mare throughout pregnancy. The minute an egg is fertilized in your mare, it begins dividing rapidly. That single cell becomes the approximately 100 trillion cells in a newborn foal. Each cell has a special purpose, and the building blocks for producing these cells come from your mare’s diet.

Keep reading to understand what aspects of nutrition are most important for your mare throughout pregnancy for the optimal health and development of her foal.

Equine Gestation 101

A mare’s pregnancy typically lasts 11 months or 330 days but can range from 320 to 380 days in total. This time frame is also referred to as gestation.

Early on in gestation, most mares are sufficient on the same nutritional program as before becoming pregnant – eating a complete and balanced diet at a rate of 1-2% of her body weight daily based on high-quality forage. Of course, fresh, clean water and loose salt should always be available.

The foal grows slowly in the early stages, but during the last several months of gestation, growth accelerates. The National Research Council (NRC) currently recommends making dietary changes for your mare during her 5th month of gestation [5].

Nutritional Requirements and Considerations for Pregnant Mares

By the 5th month, your foal is about the size of a Beagle! This is a great time to implement some dietary and management changes to ensure both mare and foal are receiving their essential nutrients.

The NRC provides a website where you can input the basic details of your mare’s diet and her stage in gestation, and it will calculate her minimum daily nutritional requirements. To try this for your own mare, follow the link to use the Nutrient Requirements of Horses online tool.

Keep in mind that changes to your mare’s diet should be made gradually. Your veterinarian and equine nutritionist are excellent resources for creating a safe plan that considers the unique needs of your mare and foal.

Maintain Optimal Body Condition

Aim to keep her in good flesh, but not too heavy (between a 5 and a 6 on the Henneke Body Condition Scoring scale, where 1 = emaciated and 9 = obese). Many mares gain 9-12% of their body weight (about 140-200 pounds) during pregnancy. Familiarize yourself with how to body condition score and monitor her throughout gestation. Being both too thin or overweight can have negative effects on her and the foal’s health.

As the pregnancy progresses, your growing foal will begin to take up more space in the abdomen, leaving less space for the digestive tract. This can decrease her appetite, just when she should be eating more. Having a little extra weight during early pregnancy can help provide calories for times when she might not finish her meals later in gestation. Try to divide her feedings into as many meals as possible throughout the day.

Protein Requirements

Research has shown that consuming high-quality protein rich in essential amino acids in early pregnancy is linked to better progesterone levels and fewer early losses of pregnancy in mares [7]. In addition to helping the foal grow, protein is needed for the placenta, placental fluid, and for creating the extra blood volume necessary during gestation. The natural breeding season also coincides with early spring grass, which is often high in protein (20% or more).

Protein is critical and your mare’s requirements for it increase as the pregnancy progresses. Adult horses in maintenance, as well as mares in early gestation, only need 630 grams of crude protein (CP) per day. But by the 8th month, when the foal is building lots of tissue, crude protein will need to be upped to 759 grams and increasing throughout the third trimester.

Keep in mind, many grain mixes have 2-3x the calories of hay but not 2-3x the protein. Therefore, feeding a lot of grain may lower your mare’s overall protein intake. You may want to consider feeding a fortified grain that is specifically formulated for broodmares. Also, swapping some of her grass hay with alfalfa is one way to increase the amount of protein in her diet.

Vitamins and Supplements

First, ensure that your mare’s forage is high-quality and nutritious. Then, consider any gaps in the essential vitamins A, C, D, and E to avoid deficiencies. A vitamin and mineral supplement made especially for gestating mares may be a smart choice to cover all her needs.

Be sure to refer to the NRC online tool to find exact daily servings for your mare depending on her stage of gestation. Some of the essential vitamins she will need include:

• Vitamin C: Critical in the formation of bone, cartilage, and connective tissues like tendons and ligaments. Horses create some vitamin C from glucose, however, typically not a daily dose.
• Vitamin E: Can improve immunoglobin levels in the colostrum (the mare’s first milk) after foaling, as well as in the foal [1]. This passive transfer of immunoglobulins can have positive effects on the foal’s immune health.
• Vitamin A: Plays an important role in growth and remodeling of bone. If you’re feeding her hay that’s older than 6 months, a vitamin A supplement may be beneficial.
• Vitamin D: Improves the utilization of calcium and phosphorus in the diet [3]. Horses synthesize vitamin D in their skin when exposed to sunlight, and vitamin D is also present in sun-cured hay (but at variable levels).

Omega-3 Fatty Acid

Omega-3 fatty acids have benefits across the whole body, and may also be beneficial for mares, foals, and stallions. One specific omega-3 is DHA (docosahexanoic acid) which is found in marine-based sources. DHA has been found to improve brain function in humans.

A study that involved feeding mares DHA through the last 3 months of pregnancy found that their foals were quicker to stand and nurse compared to a placebo group [9]. This same study showed that DHA supplemented mares had a faster uterine involution rate after giving birth, meaning that the uterus returned to a smaller size post-birth at a faster rate than in the placebo group. This may help the mare be more likely to conceive her next foal once she goes into heat.

Further, stallions with fertility issues have been shown to benefit significantly when being supplemented with DHA [10].

Mineral Requirements and Considerations

Many minerals are involved in making the increased blood volume, placenta and its fluids, and support the developing fetus in numerous ways. Every organ system in your growing foal, from skin to the brain, require minerals, and the recommended amounts change the most during late pregnancy.

• Calcium and Phosphorus: Adequate levels of calcium and phosphorus are essential for creating strong bones. Pregnant and lactating mares, as well as young growing horses under two years old, have higher calcium and phosphorus requirements than an adult horse at maintenance or in light work. Feeding alfalfa hay during late gestation can help provide some additional calcium which your mare needs.
• Copper and Zinc: These are two of the most common deficiencies, so it’s important to ensure you’re giving the right amount. Although there are some cases that involve a genetic component, copper deficiency has been identified as a cause of osteochondrosis (OC) in species like deer and bison [6, 8]. A study found copper deficiency in 7 of 8 Thoroughbred foals with OC located at two farms [2]. Researchers then fed mares three times the normal level of copper and found that significantly reduced the number and severity of OC lesions in the foals [4]. Therefore, this has been adopted by many major feed and supplement manufacturers.
• Selenium: Many areas are deficient in this important antioxidant that supports immune and reproductive health.
• Iron and Manganese: These are needed in increasing amounts during pregnancy but are typically adequate to high in many forages.
• Iodine: While a pregnant mare may need a little more iodine than horses at maintenance, the fetus is extremely sensitive to both too high or low amounts. Feeding incorrect amounts can result in foals being born with an enlarged thyroid gland (referred to as goiter). This has been reported with intakes as low as 25 mg per day. So, be cautious and always check the ingredients list for added iodine or kelp.

Along with nutrition, work with your vet to make sure all your mare’s other health needs, such as parasite control, vaccinations, hoof, and dental care, are being met. Then, best of luck in carefully monitoring your mare until the big day!

Evidence-Based References

1. Bondo T, Jensen SK. 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 (Berl). 2011 Apr;95(2):214-22.
2. Bridges CH et al. Considerations of copper metabolism in osteochondrosis of suckling foals. J Am Vet Med Assoc. 1984 Jul 15;185(2):173-8. PMID: 6746386.
3. Hintz et al. Effect of vitamin D on Ca and P metabolism in ponies. J Anim Sci 1973 37:285.
4. Knight DA et al. The effects of copper supplementation on the prevalence of cartilage lesions in foals. Equine Vet J. 1990 Nov;22(6):426-32.
5. Nutrient Requirements of Horses, sixth revised edition. 2007. National Research Council. National Academies Presses. Washington, D.C.
6. Thompson KG et al. Osteochondrosis associated with copper deficiency in young farmed red deer and wapiti x red deer hybrids. N Z Vet J. 1994 Aug;42(4):137-43.
7. van Niekerk FE, van Niekerk CH. The effect of dietary protein on reproduction in the mare. VI. Serum progestagen concentrations during pregnancy. J S Afr Vet Assoc. 1998 Dec;69(4):143-9.
8. Woodbury MR et al. Osteochondrosis and epiphyseal bone abnormalities associated with copper deficiency in bison calves. Can Vet J. 1999 Dec;40(12):878-80.
9. Adkin, A. Supplementation of broodmares with docosohexaenoic acid and its effects on reproductive performance and foal cognitive development. Univ FL Thesis. 2013.
10. Brinsko, SP. et al. Effect of feeding a DHA-enriched nutriceutical on the quality of fresh, cooled and frozen stallion semen. Theriogenology. 2004.