We investigated changes in the composition and quality of mare’s milk throughout lactation. Milk samples were obtained from fourteen mares immediately after foaling, and then once weekly from the first week of lactation up until the second through twelfth week depending on the foaling date of each mare. Samples averaging 3 mL for colostrum samples, 3 mL for weekly sampling thereafter, and 2 oz. for DHI plastic snap top vials, were collected after each teat was disinfected with a cotton ball that was moistened with 70% ethanol. Each 3 mL sample was examined for microbial growth via the application of approximately 0.1 mL milk sample on ¼ of a blood agar culture plate which was then incubated for 24 to 48 hours before being analyzed. Each 2 oz. sample was analyzed for fat, protein, lactose, milk urea nitrogen, and somatic cell count.

The concentrations of fat, protein, and somatic cell counts decreased as a whole throughout lactation, while those for lactose and milk urea nitrogen increased. However, somatic cell counts and milk urea nitrogen did not do so in a linear fashion. The averages for fat, protein, lactose, milk urea nitrogen, and somatic cell count were 1.70%, 1.94%, 6.65%, 26.37%, and 34,000 cells/ml respectively for the collection period. No bacterial infections were found on the culture plates. A California Mastitis Test (CMT) was also conducted, of which no inflammatory results were found. All mares maintained good condition throughout lactation, and one mare lost her foal during the fifth week of lactation. This resulted in rapid milk composition changes associated with involution (decreased lactose and increased somatic cell count). Overall, composition was similar to other studies with horses showing excellent mammary health and milk quality.