LINDELL BEACH, B.C. — Excessive summer heat may affect more than crops and the water supply. It can be detrimental for pregnant beef cows giving birth in temperatures of 32 C or more and can trigger the early onset of birth.
The research findings by animal scientists at Oklahoma State University’s division of agricultural sciences and Natural Resources found that cows may calve four days earlier than normal when exposed to extreme heat.
In many mammals, the fetus triggers the onset of birth when natural processes come together. But the studies have shown that high temperature can speed this up during the last two weeks of gestation.
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“We propose that greater ambient temperature results in greater body temperatures of the dam and calf, and this initiates the cascade of endocrine events that cause parturition earlier,” said Bob Wettemann, Oklahoma State University’s regents professor and animal science researcher.
“In other words, the greater temperature influences the time clock that initiates birth.”
Earlier this year, Wettemann was named the 2012 recipient of the American Society of Animal Science’s Animal Management Award, which is presented to scientists who make significant contributions to research in animal behaviour, environmental science, economics, or other aspects of biological or production management.
Under high heat conditions, it is critical that newborn calves find shade immediately or they can die, said Wettemann.
“It is amazing how cows and calves find shade to decrease radiant energy during hot days.”
The slightly earlier onset of birth is not necessarily a bad thing because it can have beneficial consequences for the cow, especially if it had been mated to a bull whose genetics favoured lower birth weights. These factors should combine for an easier birth with a healthy calf on the ground.
Given the warnings about global warming and the trend toward hotter, drier summers, some producers are looking at changing to bulls that produce smaller calves or calves with lower birth weights to try to offset birthing problems.
“Many producers select bulls with expected progeny differences (EPD) that identify animals with lighter birth weight,” said Wettemann.
“Selection for lighter birth weights has resulted in bulls that produce offspring that have shorter gestations.”
It is important to use bulls that produce smaller calves when bred with heifers that are smaller or not fully developed.
Not only does summer heat affect the gestation of cows but drought has an impact on spring calves as well.
“The body condition score of the cows (when calves were weaned in August) was about one score less compared with normal,” said Wettemann.
“This is very typical in many drought areas. It will have a major effect on spring calving cows. They will be thinner, have less body energy stores at calving, resulting in a longer interval from calving to the onset of ovarian function.”
This means cows will breed later or will not initiate estrous cycles and become pregnant.Â
Pregnancy rates will be reduced the following spring. Cows will become pregnant later in the breeding season, resulting in fewer, lighter calves to be weaned the following year.
The two calving seasons in Oklahoma are February-April and September-October. Fall calving is common in the southern U.S. and for those producers with calving seasons, one third have cows that calve in the fall.
Wettemann said that cold weather in January and February is a greater problem for survival of calves.
Wettemann said that even though ranchers have not experienced greater mortality with late summer calving, producers may need to change their herd management practices or reduce the number of cows that calve during the heat of summer, given the additional stresses placed on the cow and fetus coupled with the critical need for shade.
