Colostrum is the first milk produced by female mammals, including mares, immediately after giving birth. It is a thick, yellowish fluid that is rich in essential nutrients, growth factors, antibodies known as immunoglobulins, and other immune-boosting substances.

Mares only produce small quantities of colostrum, but this highly concentrated substance is essential for the health of foals during their early days of life. Colostrum builds the foal’s immune system, provides nutrients for growth and development, and promotes development of the gut. [1]

Newborn foals must consume sufficient colostrum within the first 24 hours after birth to absorb vital antibodies. During this period the foal’s gut is permeable, allowing immunoglobulins in colostrum to pass directly into the bloodstream.

However, the quantity, quality and antibody content of colostrum can vary among mares. Some mares may produce colostrum that lacks sufficient antibodies to adequately protect the foal. Other foals may not be able to nurse their mare for various reasons including illness or loss of the mare, rejection by the mare, or prematurity of the foal. [1]

In such cases, donor colostrum ensures that a foal receives necessary immune support, even if colostrum is insufficient or unavailable from its mother. Breeders can collect and store their own donor colostrum or use the services of a colostrum bank. [4]

Colostrum for Foals

During late pregnancy and in the immediate hours following foaling, the mare’s mammary glands produce a thick, golden-yellow fluid called colostrum. This first milk is rich in specialized proteins called immunoglobulins, which help to establish the immune system of the newborn foal. [1]

In the initial hours after birth, a foal’s mother gradually shifts from producing colostrum to mature milk. Mare’s milk has a distinct nutritional composition with lower antibody levels compared to colostrum. [1]

Newborn foals must consume enough colostrum within the first 24 hours of life, as this is when their gastrointestinal tract is most permeable to the large immunoglobulin molecules.

After this period, the foal’s intestines undergo gut closure, significantly reducing the foal’s ability to absorb these antibodies into the bloodstream. [1]


The concentrations of antibodies in colostrum are 100 times greater than those in mature milk. These antibodies, or immunoglobulins, provide newborn foals with early protection from harmful bacteria and contagious diseases.

Immunoglobulins are large bioactive protein molecules that are produced by white blood cells in the horse’s body. They recognize and bind to antigens, such as bacteria and viruses, and trigger an immune responses that help to neutralize or eliminate invading pathogens from the body. [1]

The immunoglobulins found in colostrum mainly consist of these three types:

  • Immunoglobulin G (IgG)
  • Immunoglobulin A (IgA)
  • Immunoglobulin M (IgM)

Immunoglobulin G (IgG) is the most prevalent antibody, accounting for over 80% of immunoglobulins in the horse’s colostrum and blood. [5]

Newborn foals have special cells in their digestive tract that enables them to absorb the immunoglobulins in colostrum. [6][7] Once immunoglobulins are absorbed, they travel through the bloodstream and aid the immune system in identifying and destroying pathogens. [1]

This is particularly important for foals because they are born with an immature immune system. Newborn foals rely on the passive transfer of immunity from the mare through colostrum to protect them during the early stages of life when they are most vulnerable to infections.

Other Components

In addition to antibodies, colostrum is also rich in nutrients such as proteins, fats, sugars, vitamins, and minerals. These nutrients supports the initial growth and energy requirements of the foal.

Colostrum is also a source of hormones, growth factors, enzymes, white blood cells, and antimicrobial factors, which play various roles in the foal’s early development and immune function.

Besides providing passive immunity, colostrum contains factors that promote the maturation of the foal’s gastrointestinal tract. These factors promote gut closure, preventing the absorption of large molecules and pathogens that could harm the foal. [6]

Components in colostrum that support immunity and gut health include the following: [8][9][10]

  • Insulin-like growth factor
  • Lysozymes
  • Lactoperoxidase
  • Lactoferrin
  • Oligosaccharides
  • Essential fatty acids
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