Some of these answers depend on your herd’s level of milk production. Assuming your herd is between 18,000 to 20,000 pound rolling herd average (8,000 to 9,000 kg), the optimal calving interval is between 12 to 13 months. To achieve this, you will need to have between 85 and 115 days open assuming Holstein cattle with 280-day gestation length. Services per pregnancy should always be two or less. A good goal would be between 1.5 and 1.8 services per pregnancy.
Three factors will determine your herd average days open: 1) your voluntary waiting period (typically 45 to 60 days), 2) your service rate (the proportion of cows serviced within each 21-day heat cycle after the voluntary waiting period), and 3) your conception rate. We used to think of service rate as the heat detection rate but have changed that philosophy due to synchronization programs and timed artificial insemination.
Achieving these goals assures that you will always have your herd in high milk production (typically, average days in milk between 160 to 180). This will also allow you to have ample replacement heifers assuming minimal death loss and average age at first calving of 22 to 24 months.
A Web-based spreadsheet will soon available to calculate some of the economic aspects of these goals. This program also allows you to compare your current situation and perform a sensitivity analysis of your reproductive program. Check back in a few months to see our progress.
Dick Wallace
Dairy Extension Veterinarian – Illini DairyNET