There are several factors that affect birth weight in calves, including breed (some breeds tend to have larger calves and some are known for smaller calves at birth) genetics of sire and dam (size of calf at birth is heritable), length of gestation (also heritable), age and size of the dam (heifers tend to have smaller calves than mature cows, and large cows tend too have larger calves than small cows), sex of the calf, environmental factors, and nutrition and health of the dam. Bull calves in the same breed tend to be larger than heifer calves, partly due to the fact that in cattle males are larger than females and partly because male calves tend to be carried a few days longer than heifer calves. If a cow goes past her due date she more often than not will have a bull calf, whereas if she calves a few days early the calf is often a heifer.
Gestation length is heritable, however. Some breeds and some family lines within breeds tend to have gestation lengths slightly shorter or longer than the “average” 283 days. Low birthweight cattle often have a shorter-than-average gestation and high birth weight cattle tend to have a longer than average gestation. The fetus is growing fastest in the final stages of gestation, so several more days of gestation create a larger calf. One study showed that each extra day of gestation amounts to at least a one-pound increase in the size of the calf.
Nutrition of the dam can also be a factor in calf birth weight. If the cow is underfed and thin during late gestation, her calf may be slightly smaller than he would have been otherwise (and may also be weak and immune-compromised if the cow was seriously shortchanged on important nutrients). Some stockmen underfeed their heifers in late gestation in an attempt to reduce calving problems, but this is not a good idea. A heifer that is inadequately fed, especially if she is short on protein and trace minerals, may not have optimum colostrum, putting her calf at risk for disease. If she is drastically underfed she may not be strong enough to give birth without assistance.
Overfeeding can be just as detrimental as underfeeding. If a cow or heifer is too fat at calving time there may be too much fat in the pelvic area, resulting in difficult birth, and fat females tend to have more incidence of retained placenta. Overfeeding, especially of protein, during the last 90 days of gestation, can increase birth weight of calves by a few pounds, which can sometimes result in calving problems.
WEATHER AND CALVING SEASON
Birth weight (in similar types of cattle) tends to be lower in hot seasons and higher in cold seasons. During hot weather the cow’s body is trying to dissipate heat and more of the blood supply is routed toward the body surface for cooling. In cold weather, by contrast, the animal is trying to conserve heat and more of the blood supply is concentrated toward the internal organs, including the uterus, which can lead to faster growth of the fetus. This is one reason winter born calves may present a slightly higher incidence of dystocia (difficult calving).
A study in the mid 1990s reported in the Journal of Animal Science looked at the major causes of dystocia in two-year old heifers, and included weather data. During the three-year study, looking at calves that were born in late winter, temperatures were warmer during the second year, and even warmer the third year. Along with the temperature increase came a lower average birth weight — the warmer the winter, the fewer the calving problems.
In the first winter, which was coldest, the average daily temperature was about minus 6 and calf birth weights averaged 82 pounds. During the third winter of the study, daily temperatures averaged about 0 and birth weights averaged 72 pounds. With the lighter calves and warmer temperatures there was a 28 per cent drop in calving difficulty.
The researchers surmised that blood flow to the uterus is increased during cold weather when cattle are trying to conserve heat by concentrating more of the blood in the body core. This would make more nutrients available to the fetus, resulting in more growth. It was also noted that the heifers ate more during cold weather — as cattle must do in order to generate enough body heat to keep warm — but not enough more to explain the entire difference in calf birth weights. The gestation lengths were not different, so the best explanation they could find for the major difference was the alteration in blood flow.
Calves that are genetically large at birth also tend to be large at weaning and as yearlings. Small calves tend to have lower weaning and yearling weights. This is one reason many breeds have inadvertently developed more calving problems in the past half century, in selecting for heavier weaning and yearling weights. Some of the “big” cattle, even in the traditionally “easy calving” breeds, produce calves with 100 to 120 pound birth weights or even larger. This got to be such a problem by the 1980s that some seedstock breeders began selecting for lower birth weights, and most of the breeds eventually developed birth weight EPDs so stockmen could have a way to try to select bulls that sired smaller calves at birth, especially for use on heifers.
Birth weight is highly heritable and influenced by gestation length, which is also heritable, and one of the things stockmen should evaluate when selecting bulls or replacement heifers. The calf will inherit tendencies for birth weight from both his sire and dam. You may not resolve all your calving problems by purchasing bulls with low birth weight, if your heifers were large at birth themselves. The calves may inherit large size from the dams and still be too large at birth. A good rule of thumb is to never keep a heifer that was heavier than 90 pounds at birth, since her calves will also tend to be large.
It’s wise to avoid extremes. Even though a small calf will be born easily, a too-small calf is at a disadvantage in severely cold weather, because he chills more quickly than a calf with more body mass. The small calf may get too cold before he can get up and nurse. Another drawback is that a really small calf may not give you the performance you want, being too small at weaning time and never catching up. As a general rule, extremely small birth weight calves tend to perform less optimally in the feedlot; they may have less genetic potential to grow fast.
A really large calf, on the other hand, may be clumsy and slower to get up than a smaller calf, or may be compromised by a longer, slower birth. If he’s slow to get up in cold
weather, he may also chill before he can nurse. But he may not freeze to death before you find him, just because it takes longer for his extra body mass to chill clear to the core.
Regarding birth weight, it’s best to try to find a happy medium — the medium size calves at birth that still give good performance. Or, you can look for the exceptional individuals that are born small but grow very quickly, catching up to the larger birth weight calves by weaning time. There are a few bloodlines that have both traits: small birth weight and large weaning weight. Some stockmen keep trying to bend the curve a little with these exceptional individuals. There are cattle with a 70 to 80 pound birth weight that grow fast enough to give excellent performance at weaning and as yearlings.
Dr. Ron Skinner, a veterinarian and seedstock producer near Hall, Montana says you need to be cautious with the really small 60-to-65 pound calves. “If you keep these as breeding stock, you may start building a body type that is not optimal for calving. Some of these will have short bodies and not enough thickness. You can build yourself into a trap where they can’t give birth to a 90-pound calf. It may not happen for 15 or 20 years down the road, but if you get into that situation it can be a wreck and you are better off to disperse that herd and start over,” says Skinner.
It’s more common, however, for stockmen to get into trouble at the other extreme, since people tend to keep the biggest heifers as replacements. Those beautiful big heifers probably were large at birth, and as you keep selecting this type of animal you soon have cows that are a frame size or two larger than you started with, and bigger and bigger calves at birth.
“Do not keep heavy birth weight females,” says Skinner. “Even if a big heifer is the biggest one in the bunch, you may still have to assist her at birth. It’s just as important to watch birth weight on females as it is on the bulls you select.”
When selecting a bull, you need to make sure that not only his birth weight, but also that of his mother, were moderate rather than large, since this is a heritable trait and many of the traits a bull passes to offspring will be those from his mother as well as his sire. His daughters, especially, tend to take after their paternal grandmother in many important traits.
“When trying to bend the curve and have moderate birth weights and high performance, much of your success will depend on where you get your genetic seedstock. You need to know how your bull producer selects genetics and how careful he is on keeping good records. Some purebred breeders are merely multiplying registration papers rather than practicing selective breeding for optimizing important trait,” says Skinner.