Seventy-four heifers in four separate breeding groups were allocated into two treatment groups for oestrus synchronisation. Group 1 was given a combination of an initial injection of norgestomet and oestradiol valerate together with a norgestomet ear implant left in place for nine days. Group 2 was given two injections of the synthetic prostaglandin luprostiol 10 days apart. The animals in group 1 were artificially inseminated once 48 hours after the removal of the implant and those in group 2 were inseminated once 72 hours after the second injection of luprostiol. Subsequently any returns were rebred by either AI or natural service. There was no significant difference between the numbers of animals in the two treatment groups which were diagnosed pregnant 33 to 35 days after insemination, although the implant treatment gave a higher overall proportion of pregnancies (70 per cent, 51 per cent). It also gave significantly smaller numbers of 'open' days over the whole of the breeding period. A study of individual animals by progesterone assay and investigation of ovarian structures by real-time ultrasound showed that some of them had unusual progesterone profiles but nevertheless became pregnant. It would appear that a corpus luteum may be responsive to prostaglandins even though it is secreting only a low level of progesterone.
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