Effects of calving season and milk yield on pregnancy risk and income in North Carolina Holstein cows.

Effects of season of calving and milk yield and their potential interaction on days from calving to last breeding were investigated using survival analysis and an economic model in 2000 Holstein cows that calved during 1989 and 1990. The final Cox proportional hazards model included lactation number, calving season, and herdmate deviation FCM. The interval from calving to last breeding ranged from 40 to 570 d. Compared with cows that calved in fall, cows that calved in summer were two-thirds as likely to become pregnant. Conversely, cows calving in winter or spring were more likely to become pregnant. Milk yields beyond approximately 8025 kg lowered the risk of pregnancy. The interaction of season and yield was nonsignificant, suggesting that these factors may act independently to affect reproduction. Lower pregnancy rates associated with high yield were detected earlier postpartum than were lower rates associated with calving in summer. Within each season, higher yield offset the lower income over feed costs associated with poorer reproductive performance. Nevertheless, summer calving lowered income over feed costs per cow per year by $98, $2, $176, and $68 for low, medium to low, medium to high, and high yielding cows, respectively.