THE ONSET OF OVARIAN ACTIVITY IN DAIRY-CO WS

The onset of ovarian activity post partum was investigated, by measuri ng progesterone concentrations in milk samples, in two dairy heard con sisting of 118 cows with an average milk yield of 8340 kg FCM. Samples were taken three times a week till 50 days post partum. In 17 cows (1 4.4 %) anoestrus occurred. The daily milk yield in this group was 2.65 kg FMC higher than the average yield in the group returning to oestru s before day 50 post partum. In cows returning to oestrus within 50 da ys post partum the first rise in progesterone was detected on average 27.6 days after calving. In first calvers (31.4 +/- 10.2) and in multi parous cows in the winter period (26.9 +/- 9.4) the onset of ovarian a ctivity was delayed compared to start of ovarian activity in the summe r period. In the first cycle only 28% of the cows had a normal luteal phase (12 - 17 days), 36% of the cows had a shortened luteal phase (le ss than 6 days), and 24% of the cows had a short luteal phase (6 - 11 days). In 12% of the cows the luteal phase was longer than 17 days. In the second cycle 56% of the cows had a normal luteal phase while 17% had a shortened luteal phase, and 17% had a short luteal phase. Pregna ncy rates after first insemination in cows with a short dioestrus (10 - 25 days) were higher than in cows with a prolonged dioestrus (26 - 5 0 days). On the basis of these result it might be expected that postpo ning the first insemination until the second or even the third cycle i n high-yielding cows will have only a marginal effect on the number of open days and a large effect on the number of inseminations per pregn ancy.