Gestation in Cows

In most cattle the average gestation in 283 days.

Jersey Cows

The average gestation period for the 1075 cases studied was 278.51 days but there appears to be an equal chance of a cow calving anytime between the 271st and the 285th day.
The age of the dam apparently has no effect on the length of gestation.
Bull calves were carried an average of one day longer than heifer calves, 297 bull calves being carried for an average of 279.48 days and the gestations of 282 heifer calves averaging 278.64 days.
The data are too scant to draw conclusions relative to twin births but the results compare with those obtained by other investigators in that the gestations of twins are usually several days shorter than the average.
Calves resulting from abnormally long gestations seem to have a better chance of living than do those born prematurely.
No correlation could be detected between the length of different gestations in the same individual dam. Cows showing one or more short gestations also exhibited other gestations longer than the average and vice versa.

As of Spring 2017 Jacob Springs farm has observed gestation length to be correlated with breed of the sire

Dexter sire had confirmed gestations of 272, 273 and 273 days in 2 Jersey cows (2nd and 3rd calf) and one American Milking Devon (7th calf)

Jersey sire on Jersey cow (Veronica) had an extremely long gestation of 323 days!1