EVALUATION OF THE EFFECTS OF CONTINUOUS MILKING, BOVINE SOMATOTROPIN, AND PROSTAGLANDIN E2 ON SUBSEQUENT MILK PRODUCTION, MILK COMPOSITION, MAMMARY GENE EXPRESSION, AND MAMMARY CELL TURNOVER IN DAIRY CATTLE.

Abstract

Previous research has shown that dairy cows require a dry period of at least 40 d for maximal milk yield in the subsequent lactation. Reducing the dry period requirement could prove beneficial to animal health and dairy profitability if subsequent milk yield was not reduced. Studies were conducted to evaluate the effects of continuous milking (CM) and hormonal treatments on milk yield, mammary epithelial cell (MEC) turnover during late gestation and early lactation, and mammary gene expression in dairy cows. A commercial trial using primiparous and multiparous cows demonstrated equal milk yields in bST-supplemented, CM and 60-d dry (CTL) multiparous cows, but lower milk yields in bST-supplemented CM, primiparous cows treated with bST. Subsequent experiments evaluated mammary development requirements during the dry period in primiparous cows and methods of rescuing milk yield. MEC growth was lower in CM glands during most of late gestation. Maintenance of lactation in CM glands resulted in a marked reduction in the MEC turnover process that occurs in the early dry period. In the last 20 d of gestation, MEC growth remained reduced in CM glands. By the last week of gestation, MEC growth was 50% less in CM tissue vs. CTL tissue. MEC apoptosis was unaffected by CM during the last 20 d of gestation, but a premature decrease in early lactation apoptosis occurred in CM glands at 7 d postpartum. Mammary gene expression demonstrated bax and insulin-like growth factor binding protein 5 are involved in apoptosis and cyclin D1, CCAT/enhancer binding protein-β, and bcl2 are involved in mammary development. Ultrastructure of CM tissue revealed large populations of resting or involuting alveoli by d 20 postpartum, whereas CTL glands had a homogenous population of secretory alveoli. Collectively, these data suggest that a 40-53% reduction in milk yield in CM glands is caused by reductions in MEC renewal and reduced secretory capacity. Treatments (bST, prostaglandin E2) to stimulate milk synthesis or MEC growth in CM primiparous glands were unsuccessful. In conclusion, primiparous cows continue to require a 60-d dry period, but multiparous cows are good candidates for short dry periods, and potentially no dry period.