Sources of variation in milk osmolality in dairy cows

This is a descriptive longitudinal pilot study aimed at investigating the individual variation in milk osmolality, within and between cows, both diurnally and over one lactation. Milk samples from 21 Swedish Holstein (n = 9) and Swedish Red Breed (n = 12) dairy cows on one farm were collected 5-44 days after calving, then monthly for 12 months. During the first month, samples were also obtained every second hour (during the day) for 4 days. Milk samples were collected by hand milking in 10 ml tubes and analysed in a Fiske osmometer. The sample collection time was measured, and individual variation in milk osmolality was investigated within both days and months. In addition, bulk-tank milk samples (including milk from additional 230 cows) were collected at the end of each sampling day, with osmolality varying between 286 and 305 mOsm/kg among days. Individual osmolality ranged from 279 to 317 mOsm/kg, and there were significant variations in the mean milk osmolality between cows (2% variation). The individual osmolality varied significantly within days (up to 8%) and over months (up to a 13% variation). Swedish Red Breed cows had higher (1%, P = 0.02) osmolality (301 ± 1 mOsm/kg) than Swedish Holstein cows (299 ± 1 mOsm/kg). The observed differences range from 1% to 9% with variations between breeds being the smallest and those between months/lactation stages and individuals being the largest. It is concluded that there are variations in milk osmolality between individuals, days and breeds that exceed what can be interpreted as thirst-stimulating (+1-2%). Since the variation among clinically healthy animals and over time is considerable, there is potentially a challenge to the practical use of milk osmolality as an indicator of hydration state to be of practical use. Further studies are needed to understand the variations in milk osmolality when cows are water deprived.