Environmental effects on ovulation rate in four spring-calving dairy herds with above-average twin calving rate

Abstract

Relationships between five environmental factors and ovulation rate were investigated in four spring-calving dairy herds, selected for study because of their high twin calving rate. Data were collected over 5 years, comprising 14 herd years, with 2475 ovulation records from cows and 441 records from yearling heifers. Factors studied were age of cow, previous calving date, last mating date, the interval from calving to conception, and the number of heat periods between calving and conception. The overall mean ovulation rate from cows was 1.083. Dry cows, carried over from the previous season, had a mean ovulation rate of 1.085. Ovulation rates for yearling heifers, 2-year-olds, 3-to 7-year-olds, and older cows averaged 1.04, 1.02, 1.08, and 1.16 respectively (P < 0.01). There was a tendency for early calvers to have higher ovulation rates than late calvers, though this was not significant. Cows ovulating late in the season had higher ovulation rates (by 0.025 ±0.007 per 21-day cycle) than those ovulating earlier (P < 0.01). A positive relationship was also found between ovulation rate and postpartum interval to conception: 0.020 ±0.005 more ovulations for each 21-day increase in interval (P < 0.01). There was no significant difference in ovulation rate among cows with 1-3 detected heat periods between calving and conception.