Gestation length and its associations with calf birth weight, calf perinatal mortality, and dystocia in dairy cattle.

The objective of the study was to quantify how nongenetic factors associate with gestation length; also of interest were the associations between gestation length and calving performance. The study comprised 9,750 gestation length records from 4,256 multiparous dairy cows, as few breeding dates were available for heifers. A linear mixed model was used to investigate factors associated with gestation length and logistic regression was used for the analysis of perinatal mortality (including stillbirths), calving assistance, and dystocia; calf birth weight and gestation length EBV were included as covariates in some models. The mean gestation length was 280.6 d (SD = 5.87), with twin births averaging 4.32 d (SE = 0.31 d) shorter than singleton births. Mean gestation length for dam parities 2, 3, 4, 5 to 7, and 8 to 12 were 278.43 d (SE = 0.64 d), 278.94 d (SE = 0.64 d), 279.40 d (SE = 0.64 d), 279.83 d (SE = 0.64 d), and 279.78 d (SE = 0.71 d), respectively. Of the 11 sire breeds represented in the data set, Holstein-Friesian had the shortest gestation length (277.3 d), and Limousin had the longest (282.82 d). The mean calf birth weight was 35.52 kg (SE = 0.44) for gestation lengths of 260 to 274 d, 37.09 kg (SE = 0.43) for 275 to 279 d, 38.27 kg (SE = 0.43) for 280 to 282 d, 39.37 kg (SE = 0.43) for 283 to 287 d, and 40.95 kg (SE = 0.44) for 288 to 305 d. Cows that conceived earlier (<49 DIM) had average gestation lengths 1.5 d longer than those that conceived after 132 DIM. Calves with very short (260 to 274 d) or very long gestations (288 to 305 d) had nearly double the odds of perinatal mortality relative to calves born with a gestation between 280 and 282 d. The need for calving assistance for calves born with longer gestation lengths (≥283 d) was 1.38 times that of calves born with gestation lengths of between 280 and 282 d. Additionally, the odds of dystocia for calves with very long gestations (288 to 305 d) were almost 1.7 times those of calves born with gestation lengths of between 280 and 282 d. In conclusion, the study provides valuable insights for predicting expected calving dates by considering key factors associated with gestation length in cattle, such as sire breed, dam breed, genetic merit for direct gestation length, month of birth, DIM at conception, parity, twin status, and calf sex. Moreover, the study documents the associations between gestation length, calving difficulty, and perinatal mortality, while adjusting for calf birth weight and direct genetic merit for gestation length. Adjusting for these effects affected the risk of calving complications at extreme gestation lengths but did not explain all of the associations, especially for perinatal mortality. Understanding these factors can help explore strategies to optimize gestation length, such as selecting sires with desirable genetic merit for gestation length.