Pregnancy rates of Holstein cows after postinsemination treatment with a progesterone-releasing intravaginal device.

Our objective was to determine whether pregnancy rates following first services would be improved by supplementing lactating dairy cows with exogenous progesterone via a progesterone-releasing intravaginal device. Estrus was induced in cows with PGF2 alpha (3-wk clusters of cows ranging from 42 to 63 d postpartum) following a positive milk test for high progesterone. Cows were inseminated at estrus (d 0), and progesterone was administered via the device from d 5 to 13 or d 13 to 21 following estrus; untreated inseminated cows served as controls. Blood was collected from each cow on d 5, 13, and 21 to determine concentrations of progesterone in blood serum. A fourth group of inseminated cows not receiving PGF2 alpha served as an additional untreated, and nonbled, control group. Pregnancy rates at first services, which were followed by insertion of the device, were unaffected by treatments. Concentrations of progesterone in serum were higher in pregnant than nonpregnant cows on d 13 but similar in cows regardless of postinsemination treatment. By d 21, when concentrations of progesterone remained consistently higher as a result of pregnancy, cows given progesterone from d 13 to 21 had higher progesterone in their blood serum, regardless of pregnancy status, than cows receiving progesterone from d 5 to 13. Only serum progesterone in nonpregnant controls on d 21 was less than that of nonpregnant cows given progesterone on d 13 to 21. Pregnancy rates of cows returning to estrus 21 to 27 d after their first service tended to be higher (60 versus 39%) in those cows previously treated with exogenous progesterone than in controls. Furthermore, the proportion returning to estrus 17 to 27 d after first service was reduced in cows previously supplemented with exogenous progesterone (27 versus 49%). Days from calving to conception were 19 d fewer in cows previously treated with progesterone than controls, because average intervals to first service were decreased by 13 d relative to controls. We conclude that supplementing cows with progesterone after insemination initially failed to improve pregnancy rates but appeared to increase pregnancy rate at the subsequent service.