Serum progesterone concentrations in cows receiving embryo transfers.

Progesterone concentrations in systemic blood were determined by radioimmunoassay in crossbred cattle used as recipients in an embryo transfer programme. An embryo was transferred surgically to the uterine horn of 528 females which were in oestrus within one half-day of the donor. Jugular blood was obtained at the time embryos were transferred (3-7 days after oestrus) and again from most females between Days 9 and 14. Pregnancy was determined by rectal palpation 45-65 days after oestrus. There were no significant differences between serum progesterone levels of females which remained pregnant and those which did not. Out of 177 pregnant recipients, none had serum progesterone levels less than 0.5 ng/ml on Days 10, 11, or 12 but in 8, values were less than 1.0 ng/ml. Blood samples were also taken on Days 20, 21, or 22 from 113 of these recipients. The mean +/- s.e.m. concentration of progesterone in the pregnant females (5.14 +/- 0.34 ng/ml) was significantly higher (P less than 0.001) than in the non-pregnant females (1.17 +/- 0.25 ng/ml). The correlation coefficients between progesterone levels on Days 3, 4, 5 or 6 and 10-12 ranged from 0.18 to 0.37 (all P less than 0.02). Progesterone levels were not related to length of the previous cycle, the time of day an animal was first noticed in oestrus or the side of the corpus luteum. However, cows with a short oestrus had higher progesterone levels on Days 3-7 (P less than 0.01) than those in oestrus for a longer time.