Short estrous cycles in New Zealand dairy cattle.

Over 18% of cows returning to service within 49 days of first insemination were remated within the first 17 days. A distinct peak representing 31.4% of all short return intervals occurred from 8 to 10 days after first insemination. Because nonreturn rates to second insemination following these service intervals were equal to nonreturn rates for first inseminations, it is probable that some cows were experiencing short estrous cycles of about 9 days duration. The incidence of these cycles increased with increasing herd size and was most common among second calving 3 year-old cows. As this same phenomenon was also apparent where entire or vasectomised bulls ran with cows in large herds, these 8 to 10 day return intervals were not the consequence of behavioral sub-estrus. However, since cows were not examined, occurrence of ovulation at each estrus could only be presumed.