Mammary epithelial cell exfoliation increases as milk yield declines, lactation progresses, and parity increases.

Milk yield (MY) naturally declines after peak lactation. One approach to improving production is to increase lactation persistency, or the length of time a cow maintains lactation. To do so, cows must maintain functional secretory mammary epithelial cells (MEC), which synthesize milk. Some viable MEC are exfoliated from the udder into milk. The objective of this experiment was to identify the magnitude and direction of association between exfoliated MEC and MY, days in milk (DIM), and parity. Our hypothesis was that MY and the quantity of exfoliated MEC were negatively correlated. Therefore, milk from low-yielding, late-lactation, and primiparous cows would contain more MEC. Thirty-six multiparous and 12 primiparous Holstein cows were sampled once during lactation. Flow cytometry was used to identify hematopoietic cells and MEC in milk using an antibody specific for cluster of differentiation-45 (CD45) and an antibody against butyrophilin 1A1 (BTN), respectively. Nucleated cells were stained with Hoechst dye, and propidium iodide was used to identify dead cells. For statistical analysis, we analyzed the effect of parity, MY, and DIM, and DIM on the number and concentration of both total and live exfoliated MEC. Primiparous cows exfoliated fewer MEC and had a lower concentration of MEC in milk than multiparous cows regardless of lactation stage. This could reflect continued mammary growth in first lactation. Cows shed an average of 9.80 × 10 ± 6.90 MEC (LSM ± SEM) with a concentration of 4,582 ± 3,574 cells/mL in one milking. Milk yield was inversely related to concentration of both total and live MEC in milk. Similarly, cows in late lactation, which is associated with MY decline, exfoliated a greater number of MEC into milk. Primiparous cows had a lower MY and shed fewer total and live MEC in this study. The relationship of MY and DIM with MEC losses in milk may also contribute to declining MEC number in the mammary gland found in postpeak lactation. Understanding MEC exfoliation regulation could help develop husbandry practices that improve secretory cell retention in the mammary gland and increase lactation persistency.