I used to wish you could just ask the old girl if it would be “tonight,” or maybe the night after? But then I became the expectant mother myself and guess what! Overdue and full as a house, I didn’t have a clue when my own actual blessed moment would come. This comes under the Mother Nature category. Or does it?
The average gestation period of most cows is between 279 and 289 days. But the actual length of the gestation period depends on many factors, including:
A cow’s breed and genetic background. Her age.
Parity — whether this is her first calf or not (first-calvers normally have a shorter gestation).
The cow’s size — smaller cows have shorter gestations, on average. Her fertility — cows with poorer fertility often have longer gestation periods. By the same token, late-maturing cows will have a longer gestation than early-maturing cows. What shape she’s in — fat cows may have a longer gestation than normal-conditioned cows.
The sex of the calf — bull calves are carried longer, on average, than heifer calves.
Single or multiple birth — twins come up to four or five days early, on average.
The genetics of the calf’s sire. Fetal stress factors (less space, high C02 levels, cow stress, etc.)
Environmental factors, including nutrition, ambient temperature and the season of the year have the smallest influence, according to the MERCK VETERINARY MANUAL.
Scientifically, parturition (calving) is induced by the fetus. It is initiated by rising cortisol levels in the fetus, which provokes a cascade of endocrine activity in the mother. Heightened fetus cortisol levels are a result of stressors such as deprivation of oxygen or exposure to high carbon dioxide levels. A deformed brain may not give that needed signal.
Nevertheless, the breed of cattle has the greatest influence on gestation length. Holstein-Friesian cows are at one end of the spectrum, with gestation lengths closer to 279 days, while Continental beef breeds such as Charolais, Limousin, and Simmentals are at the other end, averaging 289 days. British breeds, such as Angus, Hereford and Shorthorns fall neatly in between at 281 to 285 days.
Now, sure, it would be good to know when a top show heifer or cow will calve so her progress can be monitored. But why else should anyone care about gestation length? Really, if the window of opportunity for change is a maximum of 10 days, what is to gain? The answer is calving ease and for most cattlemen, that’s not a convenience trait.
The Gelbvieh associations know that best. Both Canadian and American associations of the breed added a Gestation Length (GL) EPD (expected progeny difference) to its roster of EPDs, along with Calving Ease Direct and Calving Ease Daughter EPDs, according to Susan Knights Willmon, director of breed improvement for the American Gelbvieh Association (AGA). Gestation Length information is also used within Calving Ease EPD calculations, as it is in most beef breeds. The Canadian Limousin Association is the only other breed association that lists a dedicated Gestation Length EPD, however.
The result for the Gelbvieh association has been remarkable. Calving Ease EPDs have improved by 17 points in the past 15 years, according to Sean McGrath of RAK, the company which oversees the breed’s genetic evaluations.
Selecting for smaller birth weights alone may have inadvertently influenced gestation length. Wendy Belcher, general manager of the Canadian Gelbvieh Association (CGA), recently researched gestation length statistics and found that the Gelbvieh breed had an average gestation of 287 days when it first came to North America. Today, however, that number is closer to 284 days, which is comparable to the average gestation length for British breeds.
Not only is that good for calving ease, says Belcher, but shorter gestations also lead to longer recovery times before rebreeding, especially for first-calving heifers.
The GL EPD range between Gelbvieh bulls, from the shortest to the longest gestation, is -5.4 to 2.8. This accounts for only a two-to four-day difference, says Willmon. The EPD range is much more narrow than that of birth weight or weaning weight.
That’s why no genetic trend is produced for this EPD, for either the Gelbvieh or Limousin associations. “It is not something that is selected for extensively at breeding time,” says Willmon.
“The tricky part is, there isn’t a whole lot of change we can affect on gestation length,” says Karin Schmid, breed development co-ordinator with the Canadian Hereford Association. “It is simply not biologically possible to decrease gestation by a month, for example.”
Schmid says gestation can, in fact, be shifted by two to four days, on average, but the accuracy and availability of breeding information needed to do so would be too low. Realistically, she says, the number of cattle bred artificially pales to the number bred naturally.
Going forward, Schmid says that associations are also trying to stay away from stand-alone EPDs. The traits added must remain economically important and relevant to all participants in the evaluation, and they haven’t seen a lot of interest for following that isolated trait from other countries.
That’s not the case in the New Zealand dairy industry, however, where milk production is seasonal. If a first-lactation heifer misses her conception window she must be culled. The industry’s solution is to lengthen her postpartum interval by breeding virgin heifers to beef bulls with known short gestation periods. New Zealand dairy heifers are also being bred artificially to yaks, whose natural gestation length is 250 to 260 days.
Heritability for gestation length is considered moderate, at about 10 per cent, according to Brian Van Doormaal, president of Holstein Canada and general manager of the Canadian Dairy Network (CDN). CDN traces gestation length in dairy sires.
Is a shorter gestation length actually better?
Van Doormaal says no one has determined if it is or not. Even the dairy industry does not deliberately publish GL data. It makes it available to New Zealand breeders only, in response to demand there.
CDN genetic evaluations account for calving ease and calf survival, instead. Gestation length data are factored into those estimates. Van Doormaal guesses that both beef and dairy industries would be best to avoid extremes where selecting for gestation length is concerned. Consistently shorter gestation length selection might manifest an increased number of stillbirths, for example.
“Intentional selection for either shorter or longer gestation length is not recommended without consideration of its possible effect on other dependent traits (e. g. calving ease and stillbirth),” according to a 2009 study on gestation length in dairy cattle, published in the Journal of Dairy Science. No comparable studies were found in the realm of beef cattle research.
If calving ease is the object, either purebred producers or those who synchronize heifer groups for A.I. breeding, might choose to use sexed semen to select for heifer calves which come with the added benefit of a shorter gestation length, according to Dr. Steve Miller, associate professor and director for the Centre for Genetic Improvement of Livestock (CGIL) in Guelph, Ontario. The average difference between bull calf and heifer calf birth weight is about 10 pounds, says Miller. That leaves sufficient opportunity to improve on calving ease, if the sex alone can be influenced.
Published genetic trends across all popular North American beef breeds show tremendous strides toward lowering average birth weights while increasing genetic propensity for higher weaning and yearling weights.
When analyzing the data, remember that the units of measurement are also important. GL (gestation length) represents days, while BW (birth weight), WW (weaning weight) and YW (yearling weight) EPDs are represented in pounds. CE (calving ease) reflects per cent unassisted, according to McGrath.
“GL is used in the estimation of calving ease in the Charolais (breed). In other breeds, its effects are probably limited to its indirect influence (from) birth weight, which is then used in the calculation of calving ease.”
So for now, forget gestation length stats. Birth weight and calving ease data have it covered.