Temporary alterations to milking frequency, immediately post-partum, modifies expression of milk synthesis and apoptosis genes in the mammary glands of grazing dairy cows
Temporary changes to post-partum milking frequency can elicit lactation-long changes in milk production. The hypothesis tested in this experiment was that the immediate and long-term milk yield response to altered milking frequency would correspond to gene expression changes associated with mammary cell secretory activity and/or number. Multiparous, grazing Holstein-Friesian cows (n = 150) were randomly assigned to one of five groups at calving: milked once daily (1X) for 3 or 6 wk and twice daily (2X) thereafter; milked 2X for the entire lactation (control); or milked thrice daily (3X) for 3 or 6 wk and 2X thereafter. Milk yields were recorded daily and milk composition weekly. Mammary tissue was collected at 3, 6 and 9 wk post-partum (n=12 cows/trt), and gene expression measured using RT-qPCR. Data were analyzed using mixed models fitted with REML in GenStat including: treatment and contrasts to test milking frequency (1X, 2X, 3X), duration (3 wk, 6 wk), and their interaction as fixed effects, and cow as a random effect. Immediate (P < 0.001) and lactation-long (P < 0.01) decreases in milk and energy-corrected (ECM) production were recorded in cows milked 1X post-partum relative to 2X; however, cows milked 3X only produced greater (P < 0.05) milk volumes during the treatment period and did not differ (P > 0.05) in ECM production. Transcript levels from genes involved in milk fat (ACACA, FASN), protein (CSN1S1, CSN2), and lactose (LALBA, B4GALT1) synthesis were not altered in cows milked 3X, but were down-regulated (P < 0.05) at 3 and 6 wk post-partum in cows milked 1X. Decreased (P < 0.05) expression of these genes was maintained after 1X cows were switched to 2X milking. Furthermore, at 9 wk post-partum, cows milked 1X for 3 wk had lower (P < 0.05) expression of genes involved in fat and lactose synthesis than cows milked 1X for 6 wk. In contrast, apoptotic genes (PYCARD, FAS) were up-regulated (P < 0.05) in cows milked 1X. This effect was still apparent at 9 wk (P < 0.01), indicating that greater mammary cell death was maintained post-treatment. In conclusion, greater milk volumes during 3X milking were not associated with altered expression of genes involved in milk synthesis or mammary cell death. However, changes in expression of genes involved in these processes may underpin the long-term reduction in milk and ECM yields in cows milked 1X post-partum.