We’re 1 week into this Farm Dictionary Advent Calendar and today it’s G for Gestation Lengths.
Gestation lengths differ both between species and within them. As I’m sure anyone who’s been pregnant will tell you, there’s no such thing as an exact due date when Mother Nature is involved!
A cow’s gestation, from mating to birth, is around 9 months (279-292 days), similar to humans.
Some breeds native to the UK or the Channel Islands, like Aberdeen Angus cattle, have gestations at the beginning of the bracket, and breeds from the continent, like Limousins, which originate from France, have longer gestations, towards the end of the bracket.
In our own herd of mostly native breed cows bred to a continental bull (a Bazadaise, originally from France), our cows usually calve in around 284-287 days. Having a rough idea means we can plan when to turn the bull out into them based on when we want calving to start, and we can estimate which week each will calve in if we manage to spot the day they’re served- although some take you completely by surprise and either vastly undercook or overcook their offspring!
There were a few cows this year who we knew were due around a certain time but instead calved nearly a week later, producing rather large, overcooked calves. Luckily, they were big, wide cows who still gave birth to them easily, but we’d much rather they calved small ones on time, or even a few days early.
Generally, the breeds with longer gestation lengths produce bigger calves at birth, but that isn’t always the case. You can still get huge calves born at 280 days and tiny ones at 290!
Giving birth at 270 days or under is classed as an abortion or premature calving and has to be reported to the Animal Health Agency, regardless of whether the calf is dead or alive.
Cow’s gestation lengths can also be influenced by the calves they’re carrying and the age of the cow. Cows who are having their first calf tend to have a bit of a shorter gestation than cows who’ve already had a calf. And interestingly, if the cow is carrying a bull calf, they tend to cook them an extra day or so than if they’re carrying a heifer!
Twins arrive, on average, around 6 days earlier than single calves- so our cow due with twins on Christmas Day will hopefully calve the week before!
In sheep, a handy way to remember their gestation length is “5 months less 5 days”. They tend to lamb 143-147 days after mating- if the Rams go in on Bonfire Night, lambing starts around April Fool’s Day. Most breeds of sheep can only breed at certain times of the year, but we’ll get to that when S for Seasonal Breeders rolls around!
With pigs, their gestation length can be remembered as “3 months, 3 weeks and 3 days” or around 115 days, but anything from 113 – 117 days is normal. The piglets trigger the farrowing (giving birth), not the sow, as they outgrow the nutrient supply from the placenta and so stress hormones are released which triggers an avalanche of other hormones which leads to farrowing, so a small litter of piglets might take a few more days to use up all the nutrients- quite amazing really!