Minimal progesterone concentration required for embryo survival after embryo transfer in lactating Holstein cows

Objectives were to determine progesterone concentration (P4) from days 4 to 28 relative to presumptive estrus necessary for maintenance of pregnancy in lactating Holstein cows. Cows were assigned to the low P4 (LowP4, n=28) or control (n=153) treatments. All cows were presynchronized with two injections of PGF(2α) (14d apart) and an ovulation synchronization protocol (11d later; GnRH on day -10, PGF(2α) on day -3; and GnRH on day 0=presumptive estrus). Cows in the Low P4 treatment received 2 injections of prostaglandin F(2α) on days 4 and 5 (day 0=presumptive estrus) and a new CIDR insert on day 5 that was replaced every 7d until day 28. Blood was sampled on days -9, -2, 0, 4, 7,14, 21, and 28. Ovaries were examined with ultrasound on days -9, -3, and 7 and cows bearing a corpus luteum ≥20mm on day 7 received an embryo. On days 0, 4 and 7 P4 did not differ (P≥0.27) but control cows had greater (P<0.01) P4 on days 14, 21, and 28. Progesterone concentration fold change from day 0 to 7 was not (P=0.14) affected by treatment, but P4 concentration fold change from day 7 to 14 was (P<0.01) greater for control cows compared with LowP4 cows. No LowP4 cows became pregnant after embryo transfer, whereas 35.7, 25.5, and 21.4% of control cows were pregnant on day 28, 42, and 63, respectively. Progesterone concentration fold changes from day 0 to 7 (P=0.03) and from day 7 to 14 (P=0.05) were associated with pregnancy outcomes on day 63. Among cows that were pregnant on day 63, the minimum P4 concentration fold changes from day 0 to 7 and from day 7 to 14 were 2.71 and 1.48, respectively. Interestingly, cows with P4 concentration <5ng/mL on day 14 were (P=0.01) and tended to be (P=0.07) more likely to lose pregnancy from day 28 to 42 and from day 28 and 63, respectively. Faster rise in P4 concentration during the metestrus and early diestrus are associated with pregnancy establishment following embryo transfer, which suggests that early rise in P4 concentration has an indirect effect on embryo development through modulation of uterine environment and secretion of histotroph. Furthermore, the positive effects of early rise in P4 concentration appear to go beyond the phase of maternal recognition of pregnancy through adhesion and placentation stages.