When will she calve?
Cows generally calve about nine months and seven days after conception. Average length of pregnancy is about 283 days, but this figure is just an average and very few cows calve exactly on their estimated "due date." Gestation length is partly t matter of genetics; some breeds and family lines within breeds tend to have longer or shorter gestation lengths, and this is a factor in calving ease. Cattle with shorter gestation lengths generally give birth to smaller calves, since the fetus is growing fastest at the end of gestation. A 278-day gestation usually results in a smaller, lighter calf (in the same breed and type of animal) than if a calf is carried 288 days. One study showed that for each extra day of gestation, there is about a pound of increase in the size of the calf.
One reason bull calves are often larger and heavier than heifer calves is that they tend to be carried longer (partly due to hormonal factors). A cow that calves a few days before her estimated due date often gives birth to a heifer, whereas a cow that goes overdue often has a bull calf. But "due date" can vary, depending on the gestation length of certain individuals; some cows always tend to calve earlier or later than their projected "due date" and some bulls sire calves that are always born earlier or later.
Fetal development and rate of maturation (determining when the calf reaches full term and triggers the beginning of labor) is influenced by the genetics of both sire and dam. One reason a "low birth-weight" bull tends to sire smaller calves is that his calves inherit a shorter gestation length than average. If both the sire and dam tend to have short gestation lengths, the calf will be born earlier than the average "due date". If either the sire or the dam has genetics for a longer gestation, however, the results may be mixed, depending on which trait is inherited by the calf. Even if you have breeding dates on the cow, she may calve a few days ahead or a few days later than her projected due date and it can sometimes be a challenge to predict the day/night she will calve.
There are some fairly reliable clues, however. As a cow or heifer nears the end of gestation, her body makes changes to aid the birth process. One of the first signs of approaching calving is development of her udder. It may begin enlarging as early as six weeks before she calves (especially in heifers) or may suddenly fill during the last few days of gestation. Some cows and heifers have so much udder development that you think calving is imminent, but they go many more days before the actual event, often becoming miserably uncomfortable from the udder swelling. Others may "bag up" overnight and can fool you; they may calve before you realize they are ready.
One clue that calving will take place within about 24 hours is the teats filling. Even if the udder has been large for many days, the teats themselves often don't become full and distended until the cow is nearly ready to calve. Occasionally the plug in the end of the teat will start to come out, showing a bit of secretion on the end of the teat. Teats filling, and secretion on the end of the full teat, are clues that the cow will soon calve.
Other signs of impending labor include mucus discharge from the vulva as the cervical plug/seal softens and is expelled. A long string of mucus may hang from the vulva. The tissues around the birth canal become soft; the vulva is enlarged and flabby so the calf can more easily push through. The floppy vulva is a sign that the cow or heifer is approaching her calving date, but she can still keep you guessing awhile because some individuals become loose and floppy several weeks before calving.
The pelvic ligaments also relax. If you look closely at the back end of the cow or heifer you'll notice that the area between her tail head and the point of the buttock (pin bone) on each side of the tail is relaxed, appearing somewhat loose and sunken. Labor will usually begin about 12 hours after complete relaxation of those ligaments.
If your cows are gentle enough to let you walk up and scratch them (as some cows enjoy), you can feel the pelvic ligaments next to the tail head. This is one of the surest ways to predict calving. These ligaments are about an inch in diameter, connecting the pin bones to the spine. They attach to the vertebrae just ahead of where the tail starts and are easily visible on most dairy cows and on thin beef cows. On a fleshy beef cow you can't see them but you can feel them. The ligaments are normally very hard and tight, except for a few hours just before labor begins, and just after calving. They loosen up as part of the birth process that enables all parts of the birth canal to expand so the calf can come through.
Several hours before you notice any signs of early labor, these ligaments start to relax and may become so soft you can hardly feel them. If they are soft and spongy, the cow is only a few hours away from calving. If your cows are in a calving area where you can walk through them, and are gentle enough, you can walk among them slowly and patiently while they are eating, and assess these ligaments. On a gentle cow that knows and trusts you, you can walk up behind her (be sure to let her know you are there, so as not to startle her) and rub the tail head. This is a way to check any cow you think might be close to calving, to see if she'll calve within the next 12 hours.
If you rub your hand alongside the tail head, most cows enjoy this, since you are scratching an area that's hard for them to reach. This allows you to assess the tightness (firmness) or softness of the ligament. The cow has to be standing up for you to adequately feel the ligament. It won't give you an adequate assessment if she's just gotten up; it takes a few minutes of standing before the ligament assumes its normal tightness or looseness.
If you can check a cow a couple times a day as you walk through the herd, you'll know whether or not to worry about checking her more often. If the ligaments still feel tight at your evening check, before you go to bed, you'll know that she probably won't calve that night. And if you detect that the ligaments are becoming loose, you'll know you'd better watch her closely, even if you were not expecting her to calve so soon.
If you have breeding dates on a cow (as when breeding by AI, or if you are closely observing the cows during their breeding season to know when they were bred by a bull), you have a good general idea when she will calve, and a projected "due date." But sex of the call age of the cow, season of year (summer calves may come a few days earlier than winter calves), heritability of gestation length, nutrition of the dam, etc. can all be factors in whether calves come ahead of or after their projected due date.
Weather also makes a difference. There is often a flurry of calving just before a storm. The calves that might have arrived over a several day period will all come at once. When the barometer is falling, you can expect a lot of calves. Many cows that are ready to calve will begin labor in a low pressure period rather than when the barometer is rising. It is very common to have an abnormally high number of cows calve just before a storm and then less calving activity in the herd for the next 48 hours.
Some stockmen try to time their cows' calving with feeding manipulation, hoping for a higher percent of calves born during daylight hours instead of at night. Some claim that feeding most of the hay ration late at night makes for less calves born at night. This may tend to be the case if the night ration is lower quality hay that is eaten slowly rather than quickly, with cows spending much of the night eating. Others who have tried it say it doesn't make much difference. It never did work very well in our herd. About 2/3 of our calves always arrived at night. But calving in January at our latitude, you have to consider that 2/3 of the 24 hour period is "night." Our daylight hours at that time of year are few.
HEATHER SMITH THOMAS