Bull Selection for Heifers: Calving Ease and Birth Weight EPDs
Livestock Update, March 2006
Dr. Scott P. Greiner Extension Animal Scientist, Beef, VA Tech
The economics of cow-calf production emphasize the importance of replacement heifers giving birth to their first calf by the time they are two years old. Since this female is bred shortly after reaching puberty, her first calf will be born before she is fully developed and mature. Therefore, this first-calf heifer warrants special management and breeding considerations to minimize the frequency of calving difficulty (dystocia).
Dystocia must be minimized because it is expensive. Direct consequences of calving difficulty in heifers include calf death loss and injury, and in extreme cases the loss of females. Increased calving intervals due to delayed rebreeding and reduced weaning weights also contribute to economic losses of calving difficulty. Calving difficulty has been shown to be a problem primarily in two-year-old first-calf heifers. Dystocia occurs in much lower frequency in older females.
Research has demonstrated that there are several factors contributing to calving difficulty in heifers. The single factor most highly related to calving difficulty in heifers is calf birth weight. As calf birth weight increases, the percentage of heifers requiring calving assistance also increases. However, in addition to birth weight a number of other factors contribute to differences in calving ease or difficulty including gestation length, calf gender, female pelvic area, calf shape, season, calf presentation, breed effects, and others.
Beef producers have at their disposal excellent tools in the form of Calving Ease and Birth Weight EPDs to assist in the management of calving difficulty. While these two EPDs are highly related and are under the influence of many of the same genes, they describe different traits associated with management of calving difficulty. Consider the following example for two bulls:
|Calving Ease EPD||Birth Weight EPD|
Calving Ease EPDs predict the ease with which a sire's calves will be born when he is bred to first-calf heifers. Calving Ease EPDs are expressed as a percentage of unassisted births. Using the example above, if we mated both bulls to 100 first calf heifers we would expect 6 more heifers to calve unassisted due to the difference in CE EPD of 6% between the two bulls. Higher CE EPDs reflect more calving ease. The Birth Weight EPDs for the two bulls suggest that Bull A's calves would weigh 2.5 pounds less on average at birth compared to calves sired by Bull B.
There is a strong genetic correlation between calving ease and birth weight, suggesting that sires with favorable CE EPDs also tend to have lower BW EPDs. However, this relationship is not perfect. Calving Ease EPDs reflect multiple factors that contribute to calving ease genetics, including birth weight. Birth weight EPDs on the other hand only predict differences in genetics for birth weight. While both EPDs provide opportunity for managing calving difficulty in heifers, Calving Ease EPDs directly estimate the trait of economic consequence- calving ease, as reflected in the number of heifers that are expected to calve without assistance. Therefore, Calving Ease EPDs should be utilized as a primary tool in selecting heifer bulls, in conjunction with other EPDs of economic importance.