Pregnancy – Foaling – Lactation | The Truth About Mare’s Milk

Have you ever wondered how it’s possible for a foal to nearly double its weight within the first month? The answer lies in the unique composition of equine milk—a precise blend of energy, highly digestible proteins, and minerals that differs drastically from the milk of other species. In my latest article, I break down this "liquid gold" into its core components while offering advice on how a mare’s diet can realistically influence the quality of the nourishment that builds a future champion.

Mineral Composition of Mare’s Milk: Copper, Zinc, and Iron During Lactation

Mare’s milk is characterized by a relatively low content of certain microelements compared to the demands of a rapidly growing foal. In particular, it is widely considered that mare’s milk is deficient in copper and zinc—it naturally contains too few of these trace elements to fully cover the needs of a developing foal. Below is an analysis of copper (Cu), zinc (Zn), and iron (Fe) concentrations in mare’s milk at various stages of lactation (colostrum, transitional milk, mature milk) based on scientific research data. Furthermore, these values are compared with the Cu, Zn, and Fe content in the milk of other species (cows and humans) to evaluate the true mineral status of mare's milk.

Changes in Cu, Zn, and Fe Content During Equine Lactation

Colostrum (First Days Postpartum)

Colostrum, the milk produced in the first hours and days after foaling, contains the highest concentrations of many nutrients, including microelements. Research indicates that zinc and copper levels in colostrum are elevated compared to later stages. For example, mare’s colostrum contains approximately 2.95 mg Zn/L, 0.606 mg Cu/L, and 0.996 mg Fe/L. Some sources suggest that Zn and Cu levels may even slightly increase over the first few days, reaching a peak around day 5 of lactation before beginning to decline. Iron content in colostrum is also among the highest recorded throughout lactation.

Transitional Milk (1st Week Post-Foaling)

Transitional milk occurs during the shift from colostrum to mature milk. By the end of the first week of lactation, zinc and copper content reach their peak and then begin a systematic decline. In a study by Schryver et al. (1986), it was measured that on day 7 of lactation, mare’s milk contained an average of 3.1 mg Zn/L and 0.85 mg Cu/L, while by week 4, these values dropped to approx. 2.2 mg Zn/L and 0.55 mg Cu/L. Iron levels during this period do not follow such a clear trend, showing some fluctuations (ranging from 0.8–1.2 mg/L).

Mature Milk (Subsequent Months of Lactation)

As time passes, the microelement content in mare’s milk decreases further. After 2–4 months of lactation (mature milk), zinc and copper concentrations drop to relatively low levels. By week 8, values recorded were approximately 1.9 mg Zn/L and 0.29 mg Cu/L, and by week 16, they reached 1.8 mg Zn/L and 0.28 mg Cu/L. This means that compared to the period immediately after birth, Zn concentration is halved, while Cu drops to about one-third of its initial value. Iron remains stable at around 1 mg/L.

Comparison with Cow’s and Human Milk

In terms of trace element content, mare’s milk presents a different profile compared to bovine or human milk. It is particularly poor in zinc (especially compared to cow’s milk) and has a low copper content.

Note: Values for mature milk.

Conclusions and Breeding Implications

Analysis of mare’s milk composition confirms that it is relatively poor in zinc and copper throughout the entire lactation period. Without additional sources, a growing foal receives limited amounts of these elements. While iron levels in mare’s milk are slightly higher than in cows or humans, they still do not fully meet total requirements. However, foals are born with iron reserves stored in their bodies, which, combined with the high bioavailability of milk-borne iron, helps prevent anemia in the first months of life.

In summary:

• Mare’s milk contains low amounts of zinc and copper, which justifies the common practice of supplementing mares during late pregnancy and lactation.

• Since milk (regardless of species) does not provide large amounts of Fe, foals rely on prenatal stores and the early introduction of solid creep feed.

• Due to the rapid decline of Cu and Zn levels in later lactation stages, mineral supplementation via other routes is essential for the healthy development of young horses.

Sources:

Data on the composition of mare's milk and other species were drawn from: Schryver et al. 1986 (Journal of Nutrition), Csapó et al. 1995 (Int. Dairy Journal), Summer et al. 2004 (Livest. Prod. Sci.), and human milk data from a review in Nutrients 2020. All numerical values are supported by the cited literature.

Lactation in the horse: the mineral composition of mare milk - PubMed

https://pubmed.ncbi.nlm.nih.gov/3794822/

Lactation in the Horse: The Mineral Composition of Mare Milk - ScienceDirect

https://www.sciencedirect.com/science/article/abs/pii/S0022316623079841