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 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 and 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. 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. However, too small of a calf at birth is problematic as well. During severe cold stress, low body weight calves are more susceptible to hypothermia and subsequent death or disease issues. If you are using artificial insemination, select bulls with high accuracy Calving Ease EPDs to further minimize risk of dystocia events.
We’ll start our discussion on the use of Maternal Calving Ease (MCE) EPD (or Calving Ease Maternal (CEM) in some breeds) and its use in selection of bulls to produce replacement heifers. Maternal Calving Ease EPD describes the difference in the expected rate of dystocia among sire groups of daughters. For instance, if Bull A has a MCE EPD of +10 and Bull B has a MCE EPD of -2, we’d expect Bull A’s daughters to have 12% more unassisted calvings (i.e. fewer dystocia events) compared to daughters of Bull B when these daughters are mated to service sires of similar genetic merit for Calving Ease and birth weight. Remember, MCE is calving ease viewed as the ability of a sires’ daughters to calve unassisted. Typically, MCE has a negative genetic association with Calving Ease (direct) and a positive genetic relationship with growth and mature size. So it’s important that producers don’t just select for higher levels of Calving Ease in their herd as that will have a tendency to decrease the maternal calving ease genetic potential in the cowherd.
Once a producer has used MCE in the selection of sires to produce replacement heifers, one should transition the selection focus to identification of high Calving Ease (CE) EPD (Calving Ease Direct or CED in some breeds) sires to be mated to virgin heifers to produce their first calf. In this scenario, selection for high CE EPD helps increase the percentage of calves born without assistance to first calf heifers. In this case if Bull C has a CE EPD of +12 and Bull D has a CE EPD of +2, we’d expect Bull C’s calves to have 10% more unassisted births.
Recommendations for MCE EPD minimums for sires to be used to produce replacement heifers and CED EPD minimums for heifer service sires are in Table 1. Producers may adjust this recommendation up based on individual needs that reflect herd based experience in dystocia rates in first calf heifers.
Combining the use of Calving Ease and Maternal Calving Ease EPDs in your selection system will help assure a successful calving season and decreased dystocia in your first calf heifers.
Table 1. Recommended Calving Ease and Maternal Calving Ease EPD Recommendations for Service Sires (December 2016)