Obviously, using low birth weight, calving-ease bulls on heifers helps avoid dystocia. Heifers with difficult deliveries take more days to cycle and are less likely to rebreed. However, that is not the end of the story… birth weight can also affect reproductive efficiency in cows. First, heavier birth weights often result in longer gestation lengths thus decreasing the days a cow has to rebreed. For example, a calf born at 277 days versus 285 days will result in eight extra days for the cow to cycle and re-breed while maintaining a 365 day calving interval. In the arid southwest, where cows may not see green grass before or even during the breeding season, an extra eight days to cycle can be the difference between being open or bred, especially in young, higher-risk cows.
Second, during the last trimester of pregnancy, a heavier calf means higher nutritional demands. Compare a 70 pound calf with 35 pounds of placenta (approximately half the fetus weight) to a 100 pound calf with 50 pounds of placenta. The cow with the heavier calf has to support 45 extra pounds. Either she eats more feed (which on dormant forage is nearly impossible) or she loses more body condition. Forty-five pounds of body condition is about half of a body condition score. We see this in cows that have twins. They are slightly thinner when they calve than the other cows and have more problems rebreeding even if they only raise one calf. In short, a slightly thinner cow with less time to re-breed increases the risk of her calving later next year or ending up open.
“Last year we calved out 125 heifers with almost all of the heifers bred to your bulls. We only pulled a handful of calves. We have had good years calving before but never anything like that!”
Richard Vaughan, Cibola Ranch, Fort Sumner, NM
Reading Birth Weight Numbers
We get many questions about reading the various figures relating to birth weight. Calving Ease Direct (CED) is the likelihood that a bull or female’s offspring will be born unassisted. A higher number is more desirable, indicating greater calving ease in first-calf heifers. Birth weight EPD (BW) is expressed in pounds and indicates the predicted size of a bull or female’s offspring at birth. Lower BW EPD is generally preferred, especially on heifers. Usually a lower BW is associated with a higher CED, however sometimes they are not perfectly correlated and therefore both should be taken into account when evaluating calving-ease bulls.
Although research has shown using EPDs is a more accurate selection tool than weights and ratios, actual birth weight is another tool to use when selecting calving ease bulls. When looking at weights you must keep in mind
that older cows will have heavier calves at birth. The table from the American Angus Association illustrates the effect of cow age on birth weight.
Adjusted birth weights allow weights to be compared regardless of dam’s age. However, all weights are adjusted to reflect a 5-10 year old cow. For example a 65 pound bull calf out of a two-year old will have an adjusted BW of 72 pounds (65+6.7=71.7). The reverse can be done when looking at the actual BW of a bull out of a mature cow. A 75 pound calf out of a 6 year-old cow is 68 pounds when adjusted to a two-year old basis.
Ratios simply compare the animals within contemporary groups with 100 being average. Calves are grouped by sex, age of dam and area where they were raised to form contemporary groups. A lower BW ratio simply means a lighter weight compared to contemporaries. A lower ratio (less than 100) means the calf was lighter than the average. While this is good, almost all Manzano Angus bulls out of two-year old cows are calving-ease bulls but some will ratio over 100 for BW. This just means they are above the average for the group.
Calving Ease Maternal
Calving ease maternal (CEM) is expressed as a difference in percentage of unassisted births with a higher value indicating greater calving ease in first-calf daughters. It predicts the average ease with which a sire’s daughters will calve as first-calf heifers when compared to daughters of other sires.
Calving ease maternal (CEM) is a value that doesn’t receive as much attention as calving ease direct (CED). While CED has an immediate effect on the amount of dystocia experienced in first calf heifers, CEM will have a long term effect through the heifers a bull produces. Like CED, a high CEM value indicates greater calving ease in a bull’s daughters. Although CEM is not highly heritable (.12 vs .42 for birth weight) it is important in developing a
herd with low rates of dystocia. Therefore, cattlemen should pay some attention to this value. At Manzano Angus we strive to produce low maintenance, trouble free cattle. To that end, we are working to improve our CEM values.