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).
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