Genetic and environmental factors that might affect gestation length (GL) were investigated. Data included information from >11 million parturitions from 1999 through 2006 for 7 US dairy breeds. Effects examined were year, herd-year, month, and age within parity of conception; parturition code (sex and multiple-birth status); lactation length and standardized milk yield of cow; service sire; cow sire; and cow. All effects were fixed except for service sire, cow sire, and cow. Mean GL for heifers and cows, respectively, were 277.8 and 279.4 d for Holsteins, 278.4 and 280.0 d for Jerseys, 279.3 and 281.1 d for Milking Shorthorns, 281.6 and 281.7 d for Ayrshires, 284.8 and 285.7 d for Guernseys, and 287.2 and 287.5 d for Brown Swiss. Estimated standard deviations of GL were greatly affected by data restrictions but generally were approximately 5 to 6 d. Year effects on GL were extremely small, but month effects were moderate. For Holstein cows, GL was 2.0 d shorter for October conceptions than for January and February conceptions; 4.7 and 5.6 d shorter for multiple births of the same sex than for single-birth females and males, respectively; 0.8 d longer for lactations of < or =250 d than for lactations of > or =501 d; and 0.6 d shorter for standardized yield of < or =8,000 kg than for yield of > or =14,001 kg. Estimates for GL heritability from parities 2 to 5 were 33 to 36% for service sire and 7 to 12% for cow sire; corresponding estimates from parity 1 were 46 to 47% and 10 to 12%. Estimates of genetic correlations between effects of service sire and cow sire on GL were 0.70 to 0.85 for Brown Swiss, Holsteins, and Jerseys, which indicates that those traits likely are controlled by many of the same genes and can be used to evaluate each other. More accurate prediction of calving dates can help dairy producers to meet management requirements of pregnant animals and to administer better health care during high-risk phases of animals' lives. However, intentional selection for either shorter or longer GL is not recommended without consideration of its possible effect on other dependent traits (e.g., calving ease and stillbirth).

Reproductive information since 1995 from the USDA national dairy database was used to calculate yearly Holstein and Jersey means for days to first breeding after calving (DFB), 70-d nonreturn rate, conception rate (CR), number of breedings per lactation (NB), interval between first and last breedings during the lactation, days to last breeding after calving (DLB), pregnancy rate (PR), calving interval (CI), and interval between consecutive breedings. Data were from nearly 20 million breedings during >8 million lactations of >5 million cows in >23,000 herds. Means were also calculated for some traits by parity and breeding number for both breeds and by geographical region and synchronization status for Holsteins. The DFB declined for Holsteins from 92 d in 1996 to 85 d in 2007; the trend in yearly differences was not as consistent for Jerseys. First-and all-breeding 70-d nonreturn rate declined 5 to 9 percentage units over time. First-and all-breeding CR declined 2 to 4 percentage units. The DFB were longer for later parities of Holsteins than for early parities. Second-and third-breeding CR were sometimes 1 to 2 percentage units above first-breeding CR for Holsteins but lower (1 to 7 percentage units) for Jerseys. The CR within breeding number declined across parities for both breeds. The NB increased by 0.3 to 0.4 breedings over time but remained constant (2.5 or 2.6 breedings) across parities for Holsteins and increased (from 2.2 to 2.4 breedings) for Jerseys. Holstein DFB were fewest in the Northwest (78 d) and greatest in the Mountain region (92 d). Regional CR was highest for the Northeast and Southwest (33%) and lowest for the Southeast (26%); NB was fewest for the Northeast (2.3) and greatest for the Southeast (2.7). Mean DLB was fewest for the Southwest (127 d) and greatest for the Mountain region (157 d); CI was shortest for the Southwest (406 d) and longest for the Mideast (434 d). Mean PR was highest for the Southwest (28.3%) and lowest for the Mideast and Southeast (22.2%). Use of timed artificial insemination following synchronized estrus appears to have reduced DFB, lowered CR, and increased NB while reducing DLB and CI. However, synchronized breeding was not a primary cause of Holstein regional differences for reproductive traits. Since 2002, phenotypic performance for CR, DLB, and CI as well as genetic merit for daughter PR have stopped their historical declines and started to improve.

Trends since 1980 for calving age and calving interval, 2 factors that influence herd life, were examined by parity for 5 breeds of US dairy cattle. Calving data were from cows with records that passed edits for USDA genetic evaluations and were in herds that remained on Dairy Herd Improvement test. First-calf heifers calved at progressively younger ages over time, but the age decline was less for later parities because of longer calving intervals. Breed differences for calving age were evident for all parities; current mean age at first calving ranged from 24 mo for Jerseys to 28 mo for Ayrshires. Mean calving age across all parities declined over time for all breeds, primarily because of increased turnover rate, and ranged from 48 mo for Holsteins to 54 mo for Ayrshires. Across parity, annual increase in calving interval was reasonably consistent (0.90 to 1.07 d/yr) for all breeds except Jersey (0.49 d/yr). Within parity, regressions of calving interval on year were generally similar to overall breed trend. Breed means for first calving interval across time ranged from 390 d for Jerseys to 407 d for Brown Swiss.