Interspecies variation in milk composition among horses, zebras and asses (Perissodactyla: Equidae).

Milk samples of four species of wild equids (onager, Equus hemionus onager; mountain zebra, E. zebra hartmannae; plains zebra, E. burchelli; Przewalski horse, E. caballus przewalskii) and two domesticated equids (ass, E. asinus; pony, E. caballus) were analysed. At mid to late lactation the milks of all species were very similar, containing on average 10-12% total solids, 1-2% fat, 1.6-1.8% true protein, 6-7% 'lactose', 0.3-0.5% ash, 0.08-0.12% calcium, 0.04-0.07% phosphorus and a calculated energy content of 2.0-2.4 kJ/g. Milk samples collected in the first 2 weeks after birth showed elevated levels of total solids and protein, and some had reduced 'lactose' levels, but there were no observable trends in milk composition during mid to late lactation (1-12 months post partum). It was concluded that these closely related species produce milks that are nearly identical in gross composition and that the domestic horse is a representative model for the study of equid lactation.