Estrogen administered at final milk removal accelerates involution of bovine mammary gland.

To evaluate whether estrogen hastened involution of mammary tissue, Holstein cows were injected with 4 ml of ethanol excipient (n = 21) or 15 mg of estradiol-17 beta (n = 23) on each of the 4 d that preceded final milk removal. Dates of final milk removal (d 0) were designated as 60 d prior to expected dates of calving. Milk volumes were recorded, and samples were collected prior to the first and fourth injections. During the dry period, each mammary quarter within the cow was sampled once to collect secretions on dates that corresponded to d 0, 3, 11, and 25 or 1, 7, 18, and 30 of the dry period. Milk synthesis and secretion declined abruptly because of treatment. The decreased concentrations of alpha-lactalbumin, lactose, citrate, and potassium in secretions of controls, as well as the increased somatic cells, protein, lactoferrin, and sodium, occurred earlier in secretions from treated cows. These shifts of approximately 6 d, relative to days dry, suggested that exogenous estradiol increased the involution rate of mammary tissue.