The time to resumption of ovarian cycles was determined in 27 post partum beef suckler cows by twice weekly milk progesterone analysis and was 24.5 +/ 1.6 (SEM) days. Treatment of 87 cows with a double injection of 500 micrograms cloprostenol at an interval of 11 days resulted in 55 cows (63.2 per cent) responding by a fall in progesterone levels and ovulation. Thirty-eight of these cows (43.7 per cent of total) subsequently conceived to a timed double insemination. Eight (9.2 per cent) had high progesterone levels at the second injection had failed to respond; three (3.4 per cent) had a prolonged period of low progesterone levels after the first injection which were still low when the second was given. A further 21 cows (24.1 per cent) had low progesterone levels at the time of the second injection but could not be categorised further with certainty because of lack of previous samples. These cows were treated at a significantly earlier stage post partum than the rest and in fact were treated earlier than the recommended day 42. However, four of the 21 cows conceived to the timed insemination resulting in a total conception rate of 48.3 per cent.
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