Interrelationships of parturition problems, production of subsequent lactation, reproduction, and age at first calving.

Data of 22,691 observations on parturition related problems (dystocia, calf mortality, milk fever, retained placenta, and mastitis) were collected from 27 cooperating California dairy herds. Age at first calving did not affect calving difficulty; thus, an optimal age for first calving was not identified. Decisions on breeding virgin heifers should be based on size of cow because of the effect of size of cow on difficulty at first parity. Small calves experienced greatest mortality at parities greater than one; however, large calves had greater mortality at first parity. Greater mortality for large calves at first parity was partially due to the increased difficulty associated with larger calves. Calving difficulty resulted in impaired reproductive performance (more days open, more services, and more days to first breeding) and decreased 30-day milk production but had no effect on 90-day milk production and 305-day mature-equivalent milk production. Dystocia, milk fever, and retained placenta occurred as a complex. Incidence of dystocia and calf mortality decreased with parity, and all other problems increased with parity.