I noticed when I add more barns to divide the herd - more calves are born. It may take a year or two to have the birth rate increased. I had four barns with 10 cows each (herd max size).
I have a return of 8 cow birth each year.
But I find birth rate is more efficient if 2 cows are slaughtered (manually) in each barn, each year (in this example, depends how many you’ve got). It produces more food and ensure calves are born.
Finally, if you haven’t place your grazing in a higher percentage field of graze , do that as it helps during winter months and with their health.