Heather Smith Thomas

There are many things that influence calving ease and length of labor in cows and heifers, and some of these factors can be managed—to help ensure safe and easy delivery of calves. The main cause for dystocia (difficult birth) is a big calf and small pelvic area in the dam, especially in first calf heifers that have not yet attained mature size. Thus it is important to make sure heifers are genetically selected for early maturing, ease of calving and are well grown by calving time—and bred to bulls that typically sire small calves. Some of the many factors that influence birth weight and calving ease include sex of the calf, age of dam, breed and birthweight of both the sire and dam, gestation length, environment and nutrition of the dam.

First calvers generally give birth to smaller calves than do mature cows. A general rule of thumb is that the larger the dam, the bigger her calf. But if bred to a bull that sires large calves, a heifer may have trouble calving, especially if it's a bull calf. Male calves tend to be larger than female calves, partly because they have a huskier build, and also because they tend to be carried longer. A cow that calves a few days before her expected due date often has a heifer calf, whereas a cow that goes overdue often has a bull calf. Since the fetus is growing fastest in the final stages of gestation, those several extra days of growth create a larger calf. One research study showed that for each extra day of gestation, there's about a pound of increase in size of the calf.

Bull calves also cause more calving problems than heifer calves, not just because they are larger, but also because of hormonal influences. Studies in Montana in 1993 showed that cows carrying bull calves have higher testosterone (male hormone) levels than cows carrying female calves. Cows that experience dystocia also have different estrogen and progesterone levels than cows having easy labor. These hormones may influence the degree of relaxation and expansion of the birth canal, as well as helping or hindering the force of labor contractions.

Birth weight and size of the calf is affected by genetics of both the sire and the dam. This is often related to gestation length, since some breeds and family lines within breeds have longer or shorter gestation. Many calving problems can be eliminated or minimized if you select your breeding stock (females and bulls) that were themselves small to medium size at birth. Even if you breed a heifer to an easy-calving bull that sires low birthweight calves, her calf may still be too large for easy delivery if it inherits its term size from her (if she herself was large at birth).

Birthweight tends to be less in hot seasons and more in cold weather. When it's cold, the blood circulation of the dam is concentrated more internally, to conserve heat and keep vital internal organs warm, and thus brings more nutrients to the fetus and it grows faster. By contrast, in warm weather the circulatory system is sending more blood to the outer surface of the body for cooling, with less concentration of blood around the internal organs. The fetus grows a little slower.

Feeding high levels of protein during the last 90 days of gestation can increase the size of calves at birth. Pregnant cows need adequate energy and protein for proper growth and health of the fetus and to keep up their own body condition (cows too thin may have poor uterine contractions during labor and need assistance, and may also be slow to breed after calving) but overfeeding protein may result in big calves and more calving problems. Overfeeding energy (making cows too fat) may create too much fat in the pelvic area, and lead to difficult calving.

When cattle were wild, before domestication, they gave birth quickly and easily, like bison or elk. Calves were small and easily born. Any calves that had trouble coming into the world did not survive to have offspring, and any cow with serious problems died, and did not produce any offspring to perpetuate a problematic trait. Thus mother nature ruthlessly culled the herds and selected for ease of calving. With domestication, however, we have created animals that are bigger, meatier, and faster growing. In selecting for improved beef traits, we have also inadvertently selected for animals that are larger at birth. Hence the domestic beef cow may have more calving problems than her wild ancestors did.

After experiencing some horrible calving wrecks with large calves (especially during the 1970's and 1980's when stockmen in America were experimenting with larger continental breeds and crossing them with smaller British breeds) many cattle raisers realized they also needed to select for smaller birthweights and calving ease. Breeds with large muscle mass also have more calving problems, just because of the extra bulk. Many of those calves have to be pulled or delivered by C-section.

In almost all breeds there are some bulls that sire larger than average calves, or calves with wide hips and shoulders that don't pass through the birth canal easily, creating a higher than usual incidence of calving problems. Some cows always have big calves, and hard births, no matter what bull you breed them to, just because their calves tend to inherit these traits from the dam.

Some breeders claim that birthweight alone does not make for difficult (or easy) calving—that the shape of the calf is just as important. To some degree this is true, since a calf with a very wide head, shoulders and hips (or double muscling) will be slower coming through the birth canal than a streamlined calf. A big streamlined calf may be born just as easily as a smaller, lighter-weight blocky calf. But whenever you have large, long-legged calves, there is more risk for calving problems even if the calf is streamlined. A calf with long legs has more chance for malpresentation—if those legs don't get aimed properly when they straighten out in early labor to head for the birth canal. Ranchers with big, long-limbed calves have often commented on having more incidence of legs turned back, heads turned back, etc. There's just not as much room in the uterus for the calf to properly position himself at the beginning of labor. It may take longer to get started through the birth canal.

In some breeds and family lines, calves are either large or small mainly because of gestation length. The calves that are smaller than average generally are born a few days sooner than average and those that are larger are often carried longer. If you select for cattle with short gestation lengths, you usually have calves that are smaller at birth. This is one way that breeders in some of the hard calving have improved calving ease of their cattle—by selecting for shorter gestation length.

The selection process, trying to ensure reasonable calf size and shape, is ongoing.

Bull buyers (and anyone buying female replacements) need to pay attention to EPD's for calving ease. Some seedstock producers are now recognizing the value and need for guaranteeing ease of calving again. Stockmen need to work closely with their seedstock producers to make sure the bulls they select are compatible with their cow herd for ease of calving.

There are some advantages to using minor breeds that have not been “improved” for increased beef production. If you can't be there at calving time, you may want to select cattle that calve swiftly and easily. A small live calf at birth is always worth more than a dead big one!


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