Short Estrous Cycles in New Zealand Dairy Cattle

Abstract

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 re- turn intervals occurred from 8 to 10 days after first insemination. Because nonreturn rates to second insemination following these service intervals were equal to non- return rates for first inseminations, it is probable that some cows were experiencing short estrous cycles of about 9 days dura- tion. The incidence of these cycles in- creased 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 vasec- tomised 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.