How much calving ease is enough? By Kelli Retallick, Genetic Service Director, Angus Genetics Inc.
Calving ease is a desired trait. Most would argue it is not only desired but necessary, as calving difficulty can lower calf survivability and extend post-partum intervals for cows, which then lowers breed-back rates. Expected Progeny Differences (EPDs) can be utilized to help manage this risk. Often, producers ask, “Can we have too much calving ease?” To answer, it is important to reflect on the basis of the calving ease argument, the build of the EPD and what to expect when using these tools. Knowing this, cow/calf producers – both seedstock and commercial alike – can make informed decisions about their own herds.
Using the Tool Calving Ease Direct, or CED, is the most effective tool when deciding which bulls to mate to first-calf heifers. Expressed as a probability percentage, CED aims to predict the percentage of unassisted births a bull will produce when mated to heifers. Let’s compare two bulls. Bull A has a CED EPD of +2, and bull B has a CED EPD of +7. When mating these two bulls to similar groups of heifers, phenotypically and genetically, one would expect, on average, bull B to produce 5% more unassisted births than bull A. While no one can indefinitely state the perfect cut‑off to be used across the industry, producers can rely on the information available to them to make the best decisions. It gets even simpler when producers can rely on past records to benchmark the amount of calving difficulty experienced to understand where selection pressure should be placed.
Behind the EPD Calving ease scores collected by breeders are utilized to predict CED. These scores range from 1-5, where 1 would indicate a birth with no assistance. For Angus cattle, only scores reported on first-calf heifers are used in the prediction of the CED EPD. Mature female scores, while they can be reported, are not used in the national cattle evaluation as not enough variation, or differences among reported scores, exists to add value to CED predictions. Birth weight is used as a correlated trait in the calving ease evaluation. The correlation, or strength of relationship, between calving ease and birth weight is -0.65 which is a moderately strong, negative relationship. A negative correlation suggests as one trait goes up the other goes down. Therefore, in most cases, as calving ease increases, birth weight tends to trend downward. When focusing on decreasing calving difficulty in first-calf heifers, it is most effective to focus on CED EPDs as this is the economically relevant trait. On the other hand, if the focus is to strictly increase or decrease birth weight, the BW EPD is the tool of choice and most effective to influence changes on actual birth weights of calves.
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