Effect of continuous milking and prostaglandin E2 on milk production and mammary epithelial cell turnover, ultrastructure, and gene expression.

Mammary epithelial cell (MEC) growth is reduced in continuously milked (CM) mammary glands, and administration of a mammogenic compound such as prostaglandin E(2) (PGE(2)) at parturition might improve MEC growth in CM tissue. The objectives were to 1) compare MEC turnover, ultrastructure, and gene expression in CM and involuting mammary tissue, and 2) evaluate the effects of CM and intramammary infusion of PGE(2) on early lactation MEC turnover, ultrastructure, mammary gene expression, milk yield, and composition. First- and second-lactation cows (n = 8) were used in a half-udder model, in which one-half was dry for 60 d (CTL) and the other was CM. Udder halves (n = 16) were assigned to a postpartum (PP) treatment of PGE(2) (+PGE(2); 875 mug/10 mL of medium-chain triglyceride oil) or no PGE(2) (-PGE(2)) treatment at parturition and at 72 h PP. Biopsies of CM and CTL quarters were obtained during milk stasis (MS) of the CTL half at 3 and 7 d after dry-off of the CTL half (3d-MS; 7d-MS) and postpartum (PP) at 2 and 4 d (2d-PP; 4d-PP). Milk yield was reduced (P < 0.01) in CM udder halves compared with CTL halves (13.2 vs. 22.1 kg/d), but reductions were less in second-lactation cows. The apoptotic index was greater (P < 0.05) in CTL glands than in CM glands (3d-MS, 0.52 vs. 0.11% and 7d-MS, 0.24 vs. 0.12, respectively). Proliferation of MEC was unchanged at 3d-MS, but was increased (P = 0.01) in CTL halves at 7d-MS compared with CM halves (3.10 vs. 0.93%). At 2d-PP, MEC proliferation was increased (P = 0.05) in CM halves compared with CTL halves (1.3 vs. 0.6%), but was unaffected by PGE(2) (P > 0.2). Apoptosis was elevated in early lactation regardless of treatment. Ultrastructure was unchanged by dry period length or PGE(2). In prepartum tissue, involution in CTL halves increased (P < 0.05) the expression of the proapoptotic genes Bcl-2-associated x protein (bax) and IGFBP5 and decreased (P < 0.05) alpha-lactalbumin expression compared with CM tissue. In PP mammary tissue, CTL halves expressed greater (P < 0.05) levels of ATP-binding cassette 1 (ABC1) and IGFBP5. Treatment with PGE(2) did not alter (P > 0.1) gene expression. The results confirm that CM reduced milk yield of cows with a mammary growth requirement. Reduced MEC turnover and milk yield were not alleviated by IMI of PGE(2), which indicates that peripartum PGE(2) concentrations in CM glands are not limiting mammary growth or milk synthesis.

Effect of continuous milking and bovine somatotropin supplementation on mammary epithelial cell turnover.

Objectives were to determine effects of continuous milking (CM) and bovine somatotropin (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 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 (control) 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 +/- standard error) relative to parturition. Prepartum half-udder milk yield was greater in +bST cows than in -bST cows (9.9 vs. 8.2 kg/d) and postpartum half-udder milk yields were dramatically reduced in CM halves compared with control halves (10.6 vs. 22.2 kg/d), regardless of bST treatment. Proliferation of MEC was reduced in CM halves at -8 d (2.7 vs. 5.4%). Apoptosis of MEC was elevated during early lactation for d +1 and +7 in control halves, but was only increased at d +1 in CM halves. Turnover of MEC was not affected by bST. Ultrastructure data indicated complete involution of the control half and lactation maintenance in CM glands (d -20). By d -8, control 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 d 20 when control tissue was composed of a homogeneous population of lactating alveoli, but CM tissue contained lactating, engorged, and resting alveoli. Expression of CCAAT/enhancer binding protein-beta (CEBP-beta), cyclin D1, and bcl(2) were up-regulated during late gestation, but did not differ between control and CM halves. Expression of alpha-lactalbumin was increased in CM halves during late gestation, but was not different in CM and control tissue after parturition. 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.

Effect of modified dry period lengths and bovine somatotropin on yield and composition of milk from dairy cows.

Dry periods of 40 to 60 d have been an industry standard because dry periods <40 d have resulted in reduced milk yields in the subsequent lactation by 10 to 30%. However, recent research has demonstrated no production losses for cows given a 30-d dry period. The current study evaluated milk production effects of shortened or omitted dry periods for cows at mature-equivalent production >12,000 kg of milk and treated with bovine somatotropin (bST). The study used 2 commercial dairies and one university dairy and included 4 treatments. Five multiparous and 5 primiparous cows from each farm were assigned to each treatment: 1) 60-d dry period, label use of bST (60DD); 2) 30-d dry period, label use of bST (30DD); 3) continuous milking, label use of bST (CMLST); and 4) continuous milking with continuous use of bST (CMCST). Per label, bST use started at 57 to 70 d in milk and ended 14 d before drying (60DD and 30DD) or expected calving date (CMLST). In primiparous cows, average milk yields during the first 17 wk of lactation were reduced for cows on treatments 30DD, CMLST, and CMCST vs. the 60DD treatment. (38.3, 35.1, and 37.5 vs. 44.1 +/- 1.3 kg/d, respectively). For multiparous cows, respective milk yields did not differ (46.6, 43.4, 46.5, and 47.7 +/- 2.1 kg/d). Shortened or omitted dry periods may impede mammary growth in primiparous cows, resulting in reduced milk yield in the subsequent lactation. In contrast, a shortened or omitted dry period with either bST protocol did not alter production in multiparous cows treated with bST. Quality aspects of prepartum milk and colostrum require additional characterization. For multiparous cows, milk income generated for short dry periods or for continuous milking might increase their profitability. At 17 wk of the subsequent lactation, estimates of the cumulative net margins of multiparous cows on the 30DD treatment and continuous milking treatments exceeded those of cows on the 60DD treatment by 40 dollars to 60 dollars per cow.