Gestation is the time period from conception until the offspring(s) is born. Gestation length averages about 283 days in cattle, but can range based on breed. For example, it has been identified in Simmental cattle that the average gestation length is 284.3 +/- 5.52 days, where gestation length was 1.9 days longer for calves born to mature cows than heifers. Furthermore, male calves often result in longer gestation lengths for the dam than heifer calves (e.g., 1.5 days in Simmental). Therefore, it is useful to identify the primary breeds influencing a producer's herd and their average gestation lengths, as reported.
Calving date. Each calf’s birth date is also its dam’s calving date.
Breeding date. Conception date is the breeding date and is known based on records of:
- bull mount (visual, patches, or detection software),
- artificial insemination date, and/or
- fetal age based on pregnancy diagnosis using ultrasonography.
Gestation length (GL) is then calculated as the number of days between the breeding date and the calving date.
Genetic Evaluation & Usage
Predictions of genetic merit are not generated on gestation length as it requires detailed pregnancy diagnosis that is not readily available in all breeds or across breeds. Even so, monitoring gestation length in a herd can be economically beneficial to producers. For example, cows with longer gestation periods must have a shorter post-calving interval in which to conceive. This can cause challenges if they are to stay on a yearly calving schedule (e.g., see Carpenter and Sprott, 2007).
- Andersen, H. and M. Plum. 1965. Gestation length and birth weight in cattle and buffaloes: A review. J. Dairy Sci. 48(9):1224-1235. 
- King, K.K., G.E. Seidel, Jr., and R.P. Elsden. 1985. Bovine embryo transfer pregnancies. II. Lengths of gestation. J. Anim. Sci. 61(4):758-762. 
- Wray, N.R., R.L. Quaas, and E.J. Pollak. 1987. Analysis of gestation length in American Simmental cattle. J. Anim. Sci. 65(4): 970-974. 
- Carpenter, B. and L.R. Sprott. 2007. Long calving seasons: Problems and solutions. Texas AgriLife Extension B-1443.