Lactation in horses.

Lactation is the most energetically demanding phase of life for a mare. The foal relies entirely on the mare’s milk for approximately the first month of life, after which the foal begins to consume other food sources. Under human management, lactation in horses corresponds to a short period of time relative to gestation—mares lactate for 3 to 6 mo, or approximately 25 to 50% of their 11-mo gestation. In the wild, mares may lactate for over a year. Lactation is greatest 30 to 60 d after foaling, where daily milk production can range from 12 to 15 liters, representing consumption of 21 to 25% of the foal’s body weight (Morresey, 2012). Postnatal development of the mammary glands begins during puberty, pauses, and is completed during the last 2 mo of gestation. The mammary tissue develops into lobes (groups) of alveoli, small sacs that synthesize milk (Figure 1). The alveoli drain into a duct system, which collects the milk and provides a route for the milk to exit the udder during suckling. Unique to the mare, each half of her udder contains two separate mammary gland complexes, which drain into separate teat cisterns and distinct teat canals. The size of the cisterns in the mare are similar to that of sheep and goats, but smaller than cows (Dzidic et al., 2002; Dzidic, 2003). Each of her two teats has two separate openings, thus, the milk produced in each mammary gland remains separate until it reaches the foal’s mouth. This is distinct from the cow, ewe, and goat, where milk from have a single teat canal and orifice. The colostrum and milk produced by the mare are critical for survival of the foal. Colostrum is the first milk secretion following parturition and is vital to the foal’s early growth, development, and immune function. It is rich in nutrients, providing the foal with essential amino acids, bioactive proteins, immunological factors, and antioxidants. Bioactive components of colostrum include lipases and proteinases to aid in the digestion of fats and proteins, respectively, and antioxidants like catalase and peroxidases that protect the neonate against oxidation of proteins, lipids, and DNA. However, the most widely studied component of colostrum are the immunoglobulins. Colostrum is critical for the transfer of passive immunity and proper development of the immune system in the foal. At birth, the foal’s immune system is not mature, so colostral immunoglobulins are necessary to protect the neonate from environmental pathogens and support development of the immune system. Immunoglobulins account for approximately 60% of the protein in colostrum. Immunoglobulin G (IgG) is the predominant immunoglobulin in equine colostrum, with IgA occurring at a lower concentration. Cattle and other ungulates have a similar IgG/IgA colostrum profile, however, in humans, IgA is predominant with little IgG present. Unlike humans, the structure of the equine placenta limits its permeability to immune modulating proteins like immunoglobulins. Thus, unlike babies which are born with circulating IgG concentrations similar to their mothers because these pass through the human placenta (reviewed in Palmeira et al., 2012), foals are born with no immunoglobulins present in circulation and limited ability to resist disease. Since the transfer of immunoglobulins to foals does not occur in utero, immunoglobulins must be transferred through colostrum. In horses, the transfer of IgG from maternal circulation to the accumulating colostrum begins 2 wk before parturition (Peaker et al., 1979). Colostrum immunoglobulin concentrations decrease around 12 h postpartum (Pasquini et al., 2005), and remains low for the remainder of lactation, which is likely an adaptation as the foal is only able to absorb the immunoglobulins for the first 6 to 12 h of life. Implications