Post-partum and post-conceptional ovarian activity of dairy cows: evaluation based on progesterone profiles

The progesterone level of milk was measured daily in 20 Icelandic dairy 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 post-partum 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.