ESTROGEN ADMINISTERED AT FINAL MILK REMOVAL ACCELERATES INVOLUTION OFBOVINE MAMMARY-GLAND

To evaluate whether estrogen hastened involution of mammary tissue, Ho lstein cows were injected with 4 ml of ethanol excipient (n = 21) or 1 5 mg of estradiol-17 beta (n = 23) on each of the 4 d that preceded fi nal 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. D uring the dry period, each mammary quarter within the cow was sampled once to collect secretions on dates that corresponded to d 0, 3, 11, a nd 25 or 1, 7, 18, and 30 of the dry period. Milk synthesis and secret ion declined abruptly because of treatment. The decreased concentratio ns of alpha-lactalbumin, lactose, citrate, and potassium in secretions of controls, as well as the increased somatic cells, protein, lactofe rrin, and sodium, occurred earlier in secretions from treated cows. Th ese shifts of approximately 6 d, relative to days dry, suggested that exogenous estradiol increased the involution rate of mammary tissue.