Interpretation and Use of Calving Ease EPDs

by Andy D. Herring
Associate Professor, Department of Animal Science, Texas A&M University

Calving ease (the opposite of calving difficulty or dystocia) is dependent upon several factors. The most obvious factor affecting calving ease is calf birth weight. But, a small heifer can calve without difficulty just as a very large heifer can experience calving difficulty as not only the calf size (as measured through birth weight), but the ratio of the calf size to the size of the pelvic opening is the most critical factor. Calves of the same birth weight may not be shaped the same; broadness of shoulders, degree of muscle expression, length of the calf, etc. may be very different across calves even when birth weight is similar. Calf size, particularly for first-calf heifers, explains most calving problems.

Research conducted over the years has provided much useful information to understand calving difficulty. In the 1970s at the USDA Meat Animal Research Center, the Germplasm Evaluation (GPE) project was initiated to evaluate cattle breeds and sires bred to British females. Dystocia incidence across all sire breeds was reported to be 38.5 percent in two-year-old first-calf heifers, 13.3 percent in three-year-olds, and 7.4 percent in four- and five-year-old cows. Nix et al. (1998) studied 2,191 calving records from the Clemson University Beef Physiology Unit herd (1981-1993) and were analyzed to determine factors affecting malpresentation, mortality and dystocia. Only 20 (0.91 percent) of these calvings involved improper presentation of the calf. Among the 20 malpresentations, 14 were posterior, three were leg deviations, two were head deviations, and one was a breech birth. Overall, 94 percent of the births required no assistance; of the six percent of births that did require assistance, the vast majority was due to calf size. Calf size and female parity explained most of the dystocia.

Over the years, as breed associations recommended their members to collect birth weight records and eventually calculated birth weight EPDs, this provided breeders with tools to aid in reduction of dystocia. Dystocia is an added stress that increases postpartum anestrous, and this can be particularly harmful in first-calf heifers as they take longer to begin cycling after calving than older females even if no dystocia is experienced. Although birth weight EPD is useful in reducing incidences of dystocia, more precise measures of calving ease provide additional tools to breeders.

The Calving Ease Direct (CED) EPD is expressed as a difference in percentage of unassisted births in the birth of progeny from that animal (as when bulls are bred to first-calf heifers), with higher values indicating greater calving ease. Comparison of the CED EPDs between two animals predicts the average difference in calving ease percentage by which the two progeny groups will differ when they are being born. The Calving Ease Maternal (CEM) EPD is expressed as a difference in percentage of unassisted births from daughters of the animal in question, again with a higher value indicating greater calving ease. If two bulls are being compared for CEM EPDs and are bred to heifers, the CEM EPDs will represent differences in expected calving ease percentage among the two sires’ groups of first-calf daughters. It predicts the average ease with which daughters will calve as first-calf heifers when compared to daughters of other animals in the breed.

The table below provides some EPD values on calving ease direct, birth weight and calving ease maternal from the IBBA fall 2012 genetic evaluation. Among active sires, the range in CED EPD goes from -14.2 on the low end to 13.0 at the top, a difference of 27.2 percent calving ease between these extremes with an average CED EPD of +5.2. These values were taken directly from the fall 2012 evaluation on the IBBA web site (except for the fictitious bull OMG ANONYMOUS 4321). The range in CEM EPDs is from -4.0 to 12.4 with an average of +7.2. The range and average for birth weight EPD is also shown.

 

EPD comparison on four bulls in the Brangus Fall 2012 Multibreed cattle evaluation

Bull

Calving Ease

Direct

Birth

Weight

Calving Ease

Maternal

MC 661 JOHN WAYNE 535S9

11.9

-4.3

8.2

BRINKS BIG EASY 589F29

10.0

-5.7

5.5

OMG ANONYMOUS 4321

-9.0

6.5

-2.0

NMSU 94004

7.1

0.3

8.8

Average *

5.2

0.7

7.2

High*

13.0

8.3

12.4

Low*

-14.2

-7.9

-4.0

*Among active sires

The interpretation of calving ease EPDs is similar in concept to other trait EPDs, but the units are different. If the four bulls in the table were bred to a genetically similar set of heifers that were managed the same way in the same location, it is expected that calves of MC 661 JOHN WAYNE 535S9 would have 20.9 percent more calving ease (difference between 11.9 and -9.0) than calves sired by OMG ANONYMOUS 4321, calves sired by BRINKS BIG EASY 589F29 would have 19 percent more calving ease than calves sired by OMG ANONYMOUS 4321 (difference between 10.0 and -9.0), and calves sired by NMSU 94004 would have 16.1 percent more calving ease than calves sired by OMG ANONYMOUS 4321 (difference between 7.1 and -9.0). It can be seen that in general birth weight EPDs are related to CED EPDs, but this relationship is not exact. Among these bulls, although MC 661 JOHN WAYNE 535S9 is expected to sire calves with slightly more calving ease than BRINKS BIG EASY 589F29, BRINKS BIG EASY 589F29 is expected to sire calves slightly lighter in birth weight than MC 661 JOHN WAYNE 535S9.

The daughters of these four bulls (when all are bred to the genetically similar bulls and managed the same) would also be expected to express some differences in calving ease when they, in turn, are dams. Daughters of MC 661 JOHN WAYNE 535S9 would be expected to have calves with 10.2 percent more calving ease than daughters of OMG ANONYMOUS 4321 (8.2 minus -2.0); daughters of BRINKS BIG EASY 589F29 would have 7.5 percent more calving ease than daughters of OMG ANONYMOUS 4321 (5.5 minus -2.0), and daughters of NMSU 94004 would have 10.8 percent more calving ease than daughters of OMG ANONYMOUS 4321 (8.8 minus -2.0).

Use of calving ease EPDs can offer additional tools for Brangus breeders rather than only using birth weight EPD for control of dystocia, particularly in first-calf heifers. Incorporation of Calving Ease Direct EPD into breeding decisions is expected to give increased potential to reduce dystocia over birth weight EPD alone. No matter what location or expected market for calves produced, cattle breeders should always utilize balanced selection that considers reproduction and well as growth and size traits. Any trait or selection tool that provides increased potential for female fertility and calf survival should be economically advantageous.

This article was originally published in the 2012 November/December issue of the Brangus Journal.

Advertisements

Tell us what you think.

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s