Weather fronts and incidence of calving
'Producers often observe that changes in weather seem to trigger several cows or heifers to go into labor and deliver calves shortly following that change in weather. Foreign research in the 1960’s (Sommer) reported that fewer calvings were observed on days when a front was approaching and barometric pressure was falling, as compared to other days. A scientist for the University of Illinois (Dvorak, 1978; Animal Reprod. Sci. 1:3-7) examined the relationship between barometric pressure and the incidence of calving in 672 calvings over an 11 year period of time. Angus, Hereford and Shorthorn females ranging in age from 2 to 15 years were involved in the study. Atmospheric pressure readings were recorded twice each day at 7 am and 7 pm. The barometric pressure pattern relative to the time of calving all breeds as a group is shown below.
The barometric pressure pattern was that of a decline in the readings from day 3 before calving to day 1 before calving, followed by a rise in pressure. It was suggested that the changing pressure may have stressed dam or fetus sufficiently to stimulate corticoid secretion at a level that triggered the initiation of parturition. The sensitivity of the cow to this subtle climatic stress may be restricted to near-term pregnancies (those that are more than 270 days).'
Source: Dr. Glenn Selk, OSU Extension Cattle Reproduction Specialist