Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/7/2009
Publication Date: 2/1/2010
Publication URL: http://hdl.handle.net/10113/41208
Citation: Soberon, F., Lukas, J.L., Vanamburgh, M.E., Capuco, A.V., Galton, D.M., Overton, T.R. 2010. The effects of increased milking frequency on metabolism and mammary cell proliferation in holstein dairy cows. Journal of Dairy Science. 93:565-573. Interpretive Summary: Holstein cows were subjected to increased milking frequency (IMF) for the first 21 d of lactation followed by a return to twice-daily milking. Production responses to IMF were more pronounced during the period of IMF and carryover responses were minimal. Elevated concentrations of blood markers for energy balance suggested greater body fat mobilization in multiparous cows subjected to IMF. Results from mammary biopsies suggested that IMF did not affect mammary cell proliferation and that effects of IMF must be mediated through other mechanisms.
Technical Abstract: 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 one of 2 treatments. The control group was milked 2x for 119 d while the IMF group was milked 4x from d 2 postcalving until d 21 and 2x from d 22 until d 119. Milk yield was not significantly different between treatments throughout the 119 d monitored; however, the interaction of treatment by week was significant in that IMF cows yielded 4.8 kg/d more milk than control cows during wk 2 and 3 and similar levels of milk yield during the remainder of the study period. Excluding yield data from cattle receiving mammary biopsies, suggested that the mammary biopsy procedure contributed to the lack of overall responses of milk yield. Plasma nonesterified fatty acid concentrations were elevated in multiparous cows subjected to IMF during the period of IMF, but were not influence by treatment in primiparous cows. Plasma ß-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, 21 d postpartum, and 75 d postpartum and used for immunohistochemical localization of the cell proliferation antigen, Ki-67. Significant difference was found for day but no treatment effects on epithelial cell proliferation were found. Milk yield is a function of number of secretory cells and secretory activity per cell. Determinations of actual rates of proliferation and apoptosis and measures of cell activity are necessary to pinpoint possible mechanisms for the milk yield responses to early lactation IMF.