The Effect of Month and District on 100-Day In-Calf Rate in Year-Round Calving Dairy Herds

SIMPLE SUMMARY: The 100-day in-calf rate is a key parameter used to monitor herd reproductive performance in year-round calving dairy herds and is defined as the percentage of lactating cows that become pregnant within 100 days of calving. When 100-day in-calf rate is determined for all current lactating cows over 100 days in milk, the parameter effectively becomes a 7-month rolling average because dairy cows are typically dried off 7 months after conception. Year-round calving dairy herds typically need to achieve a 100-day in-calf rate of 40% to maintain an average calving interval of thirteen months. The objective of this study was to determine the effect of month on 100-day in-calf rate on ten dairy farms in New South Wales, Australia. The 100-day in-calf rate conformed to a seasonal fluctuation, with the lowest 100-day in-calf rates in late autumn and the highest 100-day in-calf rates in late spring. Farms located 15–140 km from the coast that experience higher maximum temperatures in summer experienced a greater fall in 100-day in-calf rate in summer and autumn compared to farms located less than 10 km from the coast. The authors propose that an assessment of herd reproductive performance against a fluctuating 100-day in-calf rate target that considers the time of year is an appropriate alternative to a constant target. ABSTRACT: Monitoring 100-day in-calf rate (100DICR) is an integral part of the assessment of reproductive performance in year-round calving dairy herds. The objective of this study was to investigate the effect of month on 100DICR in year-round calving herds in New South Wales (NSW), Australia and determine whether a fluctuating 100DICR target is an appropriate alternative to a constant 100DICR target. The 100DICR is defined as the percentage of all current lactating cows over 100 days in milk (DIM) that conceive on or before 100 DIM. As dairy cows are typically dried off 7 months after conception, 100DICR was an approximate 7-month rolling average. Mean monthly 100DICRs were calculated with a generalised linear model for six NSW north coast herds located 15–140 km from the coast and four NSW south coast herds located less than 10 km from the coast, over a two-year period. The mean 100DICR was lowest in May at 28.62% (95%CI 28.31–28.93) and increased during winter and spring, peaking in December at 34.74% (95%CI 34.32–35.15). The observed trend was similar for north and south coast herds, although north coast herds experienced a greater change in 100DICR from the peak to a nadir of 27.58% (95%CI 27.18–27.98), a 7.15-point difference, compared to south coast herds with a nadir of 30.18% (95%CI 29.69–30.67), a 4.67-point difference between the peak and nadir. In conclusion, 100DICR is affected by month with the lowest 100DICRs observed in late autumn and the highest 100DICRs observed in late spring and early summer. Therefore, a fluctuating target 100DICR is an appropriate alternative to a constant target when assessing reproductive performance in year-round calving herds. While the district does not affect mean 100DICR per se, the district does affect the difference between peak and nadir 100DICR.