Changes in the fat content, fatty acid composition and vitamin content of mares colostrum and milk during the first 45 days of lactation were studied. Milk samples (300-800 cm(3)) from 29 lactating mares were collected daily at the beginning of lactation and weekly from 5 to 45 days post-partum. Colostrum and early milk samples were obtained by hand, without oxytocin administration, while the foal nursed. Later milk samples were from mixed milk of the totally-milked udder. Each sample was analysed for total solids, fat content, fatty acid composition and vitamin content by conventional methods and gas chromatography. The total solids and the fat contents, respectively, of the colostrum and milk were 24.25-26.28% and 2.85-2.93% on the first day of lactation, 12.15-12.78% and 2.05-2.17% on days 2-5 and 10.37-10.61% and 1.04-1.32% on days 8-45 of lactation. The concentrations of octanoic, decanoic, dodecanoic, myristic and palmitoleic acids increased over time while stearic, oleic, linoleic and linolenic acids decreased. The fatty acid composition of mares' milkfat was very different from that of cows milkfat. Mares' milkfat contained octanoic, decanoic, dodecanoic, linoleic, linolenic, stearic, myristic and palmitic acids, respectively, in ratios of approximately 9.6, 3.1, 2.1, 4.4, 224, 0.2, 0.6 and 0.5 times those of the concentrations in cows' milk. On the basis of the differences in fatty acid composition, a new method was developed to determine the amount of cows' milk mixed with mares' milk. Contents of vitamins A, D-3, K-3 and C in colostrum (0.88, 0.0054, 0.043, 23.8 mg kg(-1)) were found to be 1.4-2.6 times the levels in normal milk (0.34, 0.0032, 0.029, 17.2 mg kg(-1)). No significant difference was found between the vitamin E contents of colostrum and milk (1.342 and 1.128 mg kg(-1)). Vitamin contents of mares' milk were very similar to those in cows' milk.
