When one begins the process of selection of bulls to produce replacement heifers or bulls to be service sires of replacement heifers a number of criteria come to mind. Certainly among these are breed composition and the contribution the bull may provide to direct and/or maternal heterosis, as well as a variety of growth, maternal and carcass traits. Perhaps among the most important is calving ease.
In the case of replacement heifers we need to think of calving ease as both a trait of a calf (how easy it is born or direct calving ease) as well as a trait of the cow (how easy the cow gives birth or maternal calving ease). There is a genetic component to both the direct and maternal aspects of the calving ease trait. As such, producers should be aware of when to use which measure to aid in the production of high quality replacement females with the expectation of long productive lives as well as to minimize dystocia in first calf heifers.
Before we discuss the two different Calving Ease EPDs, a brief discussion on why producers should use Calving Ease EPDs rather than birth weight EPDs to control dystocia rates in heifers and cows. For cow-calf producers, calving ease is the economically relevant trait associated with dystocia. Economically relevant traits (ERTs) are those that directly generate revenue or incur costs in beef production systems.
For a commercial cow-calf producer, dystocia (or lack of ‘calving ease’) is what generates costs in a cow herd through direct losses of calves and their dams, increased labor costs, and certainly lower reproductive rates among cows that have experienced dystocia. Birth weight is an indicator trait. In this case, birth weight provides some information on calving ease. Birth weight alone doesn’t directly generate revenue or incur costs independent of calving ease.
It’s important to recognize that there is an optimal range of birth weights in beef cattle. Certainly, too heavy of a calf is a problem during delivery of the calf hence our selection, at least historically, for lower birth weights. However, too small of a calf at birth is problematic as well. This is especially true for winter/spring calving herds. During severe cold stress, low body weight calves are more susceptible to hypothermia and subsequent death or disease issues. Indeed, very low birth weight calves in northern latitudes can have dramatically reduced survivability when born in winter months.
Birth weight only accounts for 55 to 60 percent of the genetic variation in calving ease. So, selection for reduced birth weight alone won’t improve calving ease as much as selecting directly on calving ease. And since birth weight is strongly correlated with other growth traits, reduction in birth weight is usually associated with decreased growth performance at weaning and yearling. When selecting a service sire for use on virgin heifers, it is recommended to focus on selection of bulls with Calving Ease EPDs in the top 20% of the breed being considered or better. If you are using artificial insemination, select bulls with high accuracy Calving Ease EPDs to further minimize risk of dystocia events.