Effects of increased milking frequency on metabolism and mammary cell proliferation in Holstein dairy cows.

Results of previous studies have shown that increased milking frequency (IMF) during early lactation results in increased milk yield not only during the period of IMF but also after cows have returned to a decreased milking frequency. The cellular mechanisms underpinning this increased milk yield and the overall effects of IMF on metabolism have not been well characterized. The objective of this study was to determine the effects of IMF on metabolism and mammary epithelial cell proliferation in dairy cows. Thirty primiparous and 30 multiparous Holstein cows were assigned randomly at calving to 1 of 2 treatments. The control group was milked twice daily (2x) for 119 d, whereas the IMF group was milked 4 times daily (4x) from d 2 postcalving until d 21 and then 2x from d 22 until d 119. Overall milk yield did not differ between treatments throughout the 119 d monitored; however, the interaction of treatment by week was significant in that IMF cows yielded 4.8kg/d more milk than control cows during wk 2 and 3 and had similar levels of milk yield during the remainder of the study period. Reanalysis of data excluding data from cows subjected to mammary biopsy suggested that the mammary biopsy procedure contributed to the lack of overall responses of milk yield, but that responses overall to IMF were greater in primiparous cows compared with multiparous cows. Plasma nonesterified fatty acid concentrations were elevated in multiparous cows subjected to IMF during the period of IMF, but were not influenced by treatment in primiparous cows. Plasma beta-hydroxybutyrate concentrations were not affected by treatment. Mammary tissue was collected by biopsy in a subset of cows (n=8 cows per parity and treatment) at calving and at d 21 and 75 postpartum and used for immunohistochemical localization of the cell proliferation antigen, Ki67. Effects of treatment on mammary epithelial cell proliferation were not significant, suggesting that other mechanisms must be responsible for carryover effects of IMF on lactational performance.