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The progesterone level of milk was measured daily in 20 Icelandic dairy cows cows for 50–90 days post-partum and for 30–50 days post-conception by radioimmunoassay. In 40% of the cows the 1st post-partum ovarian cycle was shorter and the progesterone level lower than in normal cycles. The average duration of the 1st postpartum luteal phase was significantly shorter and the progesterone values lower than in the 2nd and 3rd phases. The progesterone level rose to a plateau at the onset of luteal function within 5±2 (mean±s.d.) days, whereas the decrease in progesterone at the end of the luteal phase was more rapid i.e. 3±1 days. The progesterone value for a normal interluteal phase was 1.5±1.3 nmol/l. However in 9 of 48 interluteal phases the progesterone values did not decline below 3 nmol/l. The average progesterone concentration was significantly increased during the first 30 days post-conception. In 4 of 20 cows a significant drop in progesterone concentration was found 15–19 days post-conception. The results indicate that the luteal activity of the 1st post-partum ovarian cycle is inferior to that of the following cycles. There is a significant correlation in the intensity of the luteal activity between cycles. Ovarian cyclicity continues in early pregnancy in, at least, 20% of cows. The sampling frequency is of importance in studies concerning the luteal activity.

Post-Partum and Post-Conceptional Ovarian Activity of Dairy Cows: Evaluation Based on Progesterone Profiles