This article was written in collaboration with SDSU Extension Climate Field Specialist Laura Edwards.
Grandpa told dad, and dad told me that more cows will calve when a spring storm is rolling through. But is there any truth to this? Well, at home on the range it does seem to be the case. However is there any science to back this up? Yes, there is, and it turns out that dad and grandpa where right! Changes in weather patterns, in particular the change in barometric pressure, have been associated with increased incidence in labor in term pregnancies. In fact, when a hurricane storms up to shore, women are expected to go into labor. In Africa, the wildebeest typically give birth in a three-week window that starts in conjunction with the spring rains before their great migration.
Barometric Pressure & Calving
There are several theories on factors that will cause the induction of labor. While some seem farfetched, some have true merit. Many producers make reference to the relationship between barometric pressure and the incidence of calving. A study evaluating 672 calvings over an 11-year period showed that a decline in atmospheric pressure between 1 and 3 days prior to calving, followed by a rise in pressure, increased the number of cows calving (Dvorak 1978). It was suggested that the changing pressure may have stressed the dam sufficiently to stimulate corticoid secretion by the calf at a level that triggered the initiation of parturition (See: It's the Calf's Call).
In a recently published paper from the Journal of Animal Science, Troxel and Gadberry reviewed data from 2005 to 2009 in both fall and spring calving cows. They found that fall calving cows were not impacted by changes in barometric pressure as much as temperature, when compared to spring calving cows. However, for spring calving cows, as the barometric pressure fell 1 to 3 days prior and then increased, along with the temperature decreasing, there was an increase in the number of cows that calved. The authors concluded that “Departing storm fronts are often followed by a trend toward more tranquil weather and increasing barometric pressure during the early spring, which may include clearing and colder conditions. This type of weather pattern seems to influence the incidence of beef cows calving in the spring, whereas warmer temperatures may influence calving in the fall.”
Cold Fronts
Cold fronts are the type of weather systems that will create the pressure and temperature changes associated with increased birth rates. On the evening news weather maps, these are shown by blue lines with triangles pointed in the direction of motion.
By definition, a cold front is indicated by:
- A sharp change in temperature over a relatively short distance.
- Changes in the air moisture content (as shown by changes in dew point).
- Shifts in wind direction.
- Clouds and precipitation patterns.
- Pressure and pressure changes.
With a cold front passage, colder and drier air replaces warmer and moister air at ground level. Pressure falls gradually ahead of it, and then sharply rises after the front. Oftentimes there are rain showers or snow flurries as a cold front approaches. This precipitation may intensify for a brief time, and then produce a quick clearing of the clouds after it passes, as colder air is ushered in. Winds typically shift dramatically in speed and direction. When these weather changes occur together, these signs could potentially point towards a cold front passage and an increase in calving.
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