Studies on plasma progesterone concentrations and fertility in Friesland dairy cows during the post-partum period

SUMMARY Plasma progesterone concentrations were monitored in 69 lactating Friesland dairy cows during the post-partum period. Cyclic ovarian activity resumed in most cows within 40 days of calving. Forty-eight per cent of cows remained pregnant to their first insemination, 41% returned to oestrus before day 30 after insemination, and a further 11% returned to oestrus between days 30 and 88 after insemination. The mean of progesterone values on days 11 to 4 before insemination tended to be higher in cows that conceived than in cows that did not conceive. These differences were signficant on day –8 (P <0·05) and on days –8, –7, –6 and –5 combined (P <0·01). Cows in which the concentration of progesterone in plasma did not rise above 4·5 ng/ml during this period did not conceive. After insemination, mean progesterone concentrations between pregnant and non-pregnant cows diverged after day +8. Relatively low concentrations of plasma progesterone (< 1 ng/ml) up to day +8 after insemination did not preclude conception. There was no indication that embryo loss between days +30 and +88 after insemination was related to an insufficiency of ovarian progesterone during early pregnancy.