Plasma progesterone assays in superovulated cattle.

Plasma progesterone was measured in 14 normally cycling heifers and cows subjected to non-surgical recoveries of embryos. A radioimmunoassay (RIA) method was used for progesterone determination. The average progesterone concentration increased from 7.5 to 11.6 ng/ml in 8 of the animals following treatment with PMSG on day 8–12. Six animals had a decrease from 5.0 ± 2.1 to 3.9 ± 2.5 ng/ml. The overall increase was from 6.4 ± 2.7 ng/ml to 8.3 ± 4.8 ng/ ml. Prostaglandin F2a-analogue (cloprostenol) treatment resulted in a sharp decrease in plasma progesterone followed by a rapid increase to an average of 46.8 ng/ml on day 16. A high degree of variability in this peak value was observed, and it was not correlated with the number of corpora lutea. The superovulatory cycle was generally prolonged. The heat following the superovulatory treatment was silent, and a typical ovarian resting period was observed during which the progesterone concentration remained low and the ovaries small.