The Phantom Cow Syndrome is a major impediment to achieving compact conception patterns in Victorian herds. Some inseminated cows fail to return for a further insemination even though they are not pregnant because the corpus luteum formed following insemination is maintained for an extended period. Five years of records were obtained from a herd of Holstein cows in Northern Victoria. Each cow was fitted with a hock tag that combined electronic identification with pedometer monitoring for oestrus. It was the only detection method used within a herd of up to 550 cows that had three AB programs of 6 to 8 weeks in each year. There were periods when cows detected in oestrus would be inseminated and others when they would not. Records were available for 2777 inseminations. The return intervals following these inseminations were classified as: Short (<18 days); Normal (18 to 24 days); or Long (>24 days). The overall conception rate for the 2777 inseminations made from 41 days to 160 days post-calving was 41.7%. The 417 Long intervals represented 15% of all inseminations and 25.8% of cows not confirmed pregnant to a preceding insemination. Only 54 of 1459 oestrus events (3.7%) not associated with an insemination were followed by a Long cycle (P<0.001). The proportion of Long returns declined linearly from 19.9% among the 738 cows inseminated from 41 to 60 days post-calving to 7.4% among the 189 cows inseminated from 141 to 160 days post-calving. These results demonstrated that the percentage of Long return intervals increased significantly following insemination. The magnitude of this increase declined with an increase in the interval from calving to insemination.
Proceedings of the New Zealand Society of Animal Production, Volume 66, Napier, 325-328, 2006
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