Dynamics of the lactose content and other osmotic agents in milk throughout lactation according to the cow's parity.

Milk lactose content (LC) is known to vary due to metabolic disorders and udder inflammation. To assess its ability to serve as a marker of these 2 disorders, it is important to first understand its dynamics for healthy cows, particularly its relation with milk osmolarity, because it is the main osmotic agent. To assess the dynamics of LC, we studied relations among contents of lactose and other major osmotic agents in milk throughout lactation in different parities. Cows were fed a constant TMR diet throughout lactation. The profile of osmotic agents in morning milk (lactose, K, Na, Cl, and protein) was measured in the milk of 30 cows (17 primiparous and 13 multiparous). To avoid confounding effects due to udder inflammation, data from cows with unhealthy udders (i.e., 2 or more milk samples with SCC >200,000 cells/mL) were excluded (n = 2 primiparous and 3 multiparous). From 12 to 235 DIM, 8 to 9 samples per cow were first taken 2 wk apart, and then taken monthly. A regression model that included cow parity, DIM, their interaction, and a random cow effect was used to illustrate the dynamics of milk osmolarity and each osmotic agent throughout lactation. Milk osmolarity remained stable throughout lactation (279 ± 0.3 mOsm/L), with no significant difference between primiparous and multiparous cows. The contents of lactose, K, Na, Cl, and protein explained 86.4% of milk osmolarity. Milk LC was lower for multiparous compared with primiparous cows over the entire lactation period, except at 12 DIM, when both parity cows had similar LC. For multiparous cows, the lower LC was compensated by higher Na content. Principal component analysis of all osmotic agents explained 82% and 83% of the cumulative variance throughout lactation for primiparous and multiparous cows, respectively. At 12 DIM, multiparous cows had lower LC and Cl and K contents compared with primiparous cows, which exhibited only lower LC compared with the rest of the lactation period. From 40 to 235 DIM, however, milk from the 2 parities had similar dynamics of osmotic agents: K content decreased and protein content increased. This study revealed that in cows with healthy udders, the dynamics of LC and other osmotic agents contents in milk differed between parities and throughout lactation to maintain a constant milk osmolarity.