Submitted to: Journal of Dairy Science
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/7/2006
Publication Date: 2/1/2007
Citation: Annen, E.L., Fitzgerald, A.C., Gentry, P.C., Mcguire, M.A., Capuco, A.V., Baumgard, L.H., Collier, R.J. 2007. Effect of continuous milking and bST supplementation on mammary epithelial cell turnover. Journal of Dairy Science. 90:165-183. Interpretive Summary: Reduced days dry between lactations may improve periparturient cow health and profitability. The current study evaluated milk yield and mammary epithelial cell (MEC) turnover in continuously milked (CM) udder-halves. MEC proliferation in late gestation and apoptosis in early lactation were reduced in CM glands. In addition, resting and engorged alveoli appeared prematurely during early lactation in CM glands. Subsequent milk yield was decreased 53% in CM udder-halves. These data support a hypothesis of increased MEC carryover from one lactation to the next in CM glands. These negative effects of CM in primiparous cows were not alleviated by bST treatment.
Technical Abstract: Objectives were to determine effects of continuous milking (CM) and bST administration on 1) mammary epithelial cell (MEC) proliferation, apoptosis, and ultrastructure during late gestation and early lactation, 2) expression of genes associated with proliferation, and apoptosis in mammary epithelial cells, and 3) milk yield and composition. Second gestation, first dry period cows were randomly assigned to either continuous bovine somatotropin (bST) throughout late gestation and early lactation; +bST; n=4) or no bST (-bST; n=4) administration. Within each animal, udder halves were randomly assigned to CM or a 60-d dry period (CTL) treatment. Daily milk yield and weekly milk composition were measured during the last 60 d of gestation in CM halves and from 1 to 30 d postpartum for both halves. Mammary biopsies were obtained at –20 ± 7, -8 ± 3, +1 ± 0, +7 ± 0, and +20 ± 0 d (mean ± SE) relative to parturition. Prepartum half-udder milk yield was greater in +bST cows than -bST cows (9.9 vs. 8.2 kg/d) and postpartum half-udder milk yields were dramatically reduced in CM halves compared to CTL halves (10.6 vs. 22.2 kg/d), regardless of bST treatment. MEC proliferation was reduced in CM halves at d –8 (2.7 vs. 5.4%). MEC apoptosis was elevated during early lactation for d+1d and +7 in CTL halves, but was only increased at d+1 in CM halves. MEC turnover was not affected by bST. Ultrastructure data indicate involution of the CTL half and lactation maintenance in CM glands (d-20). By -8d, CTL tissue contained alveoli in an immature secretory state, but CM tissue contained both lactating and immature alveoli. Postpartum ultrastructure parameters were similar between halves until 20 d when CTL tissue was composed of a homogenous population of lactating alveoli, but CM tissue contained lactating, engorged, and resting alveoli. Expression of CCAAT/enhancer binding protein-' (CEBP-'), cyclin D1, and bcl2 was upregulated (P < 0.05) during late-gestation, but did not differ between CTL and CM halves. Expression of '-lactalbumin was increased (P < 0.05) in CM halves during late-gestation, but was not different in CM and CTL tissue after parturition, when both halves were lactating. Other genes evaluated; bax, insulin-like growth factor binding protein 5, ATP-binding cassette 1, and p27, were not differentially expressed at any timepoints evaluated. Results indicate that CM reduced subsequent half-udder milk yield in primiparous cows through altered MEC turnover and secretory capacity. Negative effects of CM on the subsequent lactation were not alleviated by bST supplementation.