The gestational length of cows is commonly believed to be 283 days. The topic of gestation length often comes up among cattle breeders during calving season. These conversations often pertain to calving artificially inseminated cows or recips carrying embryo transfer calves because exact breeding, implant and expected calving dates are known. It is the observation of many that we have shortened the gestation length of modern beef cattle over the generations.
An article from the Journal of Animal Science in 1952 reported significant differences in the average gestation length of Angus (281.7 days), Hereford (286.1 days) and Shorthorn (284.3 days). Also, bull calves tended to be carried longer, and are heavier at birth, than heifer calves. This study also reported a significant positive relationship between birth weight and gestation length.
A 2024 abstract from the Journal of Animal Science analyzed a total of 10,037 records obtained from reproductive data collected from Fall of 2011 through Spring of 2018, from 14 herds from the Virginia Department of Corrections beef cattle operations and the Virginia Tech Beef Center research herd. Cows used to measure gestation length were all pregnant to artificial insemination (AI) and only pregnancies that resulted in a live birth were included in the dataset. The objective of this study was to perform an analysis to determine gestational length and evaluate cow and calf parameters that influence gestational length of Bos Taurus beef cows. The average age of cows in this study was 5.5 years, with a minimum of 2 years and a maximum of 18 years of age. The average gestation length was 280.2 days. The shortest and longest gestation lengths recorded were 253 days and 300 days, respectively. Gestation length was affected by calving season with a significantly shorter gestation length Fall calvers (279.1 days) than Spring calvers (281.1 days). Calf sex also had an impact on gestation length. Bull calves were gestated almost a day longer (280.8 days) than heifers (279.6 days). Correlation coefficients (r values) were calculated between gestation length and other variables. Cow age (r = 0.2136), calf birth weight (r = 0.3834), calf weaning weight (r = 0.1438) and calf adjusted weaning weights (r = 0.0253) were correlated with gestation length of beef cows. In conclusion, the average gestation length of Bos Taurus beef cows was 280 days, which is shorter than the commonly used 283 days, and it was affected by calving season (fall or spring) and calf sex. Further, while correlations exist among gestation length, cow age and calf traits, these correlations were weak to moderate.
References: https://academic.oup.com/jas/article/102/Supplement_1/2/7617956
https://academic.oup.com/jas/article-abstract/11/1/34/4761198
