Department of Animal Science, Cornell University, Ithaca, NY 14853, USA.
J Dairy Sci. 2010 Feb;93(2):565-73. doi: 10.3168/jds.2009-2345.
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.
先前的研究结果表明，在泌乳早期增加挤奶频率（IMF）不仅可以增加 IMF 期间的产奶量，还可以增加奶牛恢复到较低挤奶频率后的产奶量。支撑这种产奶量增加的细胞机制以及 IMF 对新陈代谢的整体影响尚未得到很好的描述。本研究的目的是确定 IMF 对奶牛新陈代谢和乳腺上皮细胞增殖的影响。30 头初产和 30 头经产荷斯坦奶牛在分娩时随机分配到 2 种处理中的 1 种。对照组每天挤奶 2 次（2x），共 119 天，而 IMF 组从产后第 2 天开始每天挤奶 4 次（4x），持续 21 天，然后从第 22 天到第 119 天每天挤奶 2 次。在监测的 119 天内，两种处理方式的总产奶量没有差异；然而，处理与周的交互作用显著，即在第 2 和第 3 周，IMF 奶牛比对照组奶牛多产奶 4.8kg/d，在研究的其余时间内，产奶量水平相似。排除接受乳腺活检的奶牛数据的重新分析表明，乳腺活检程序导致了产奶量整体反应的缺乏，但 IMF 对初产奶牛的整体反应大于经产奶牛。在 IMF 期间，IMF 处理的经产奶牛的血浆非酯化脂肪酸浓度升高，但在初产奶牛中不受处理的影响。血浆β-羟丁酸浓度不受处理的影响。在产后第 21 天和 75 天，从一部分奶牛（每头奶牛 8 头，每头奶牛和处理）中采集乳腺组织，用于细胞增殖抗原 Ki67 的免疫组织化学定位。处理对乳腺上皮细胞增殖的影响不显著，这表明 IMF 对泌乳性能的持续影响必须归因于其他机制。
