Selecting for birthing ease not always clear-cut exercise

Most cow-calf operations finished calving season some time ago. In many herds, the bulls are out and next year’s calf crop is already being conceived.

Calving and the early post-partum period is a critical time in the life of a calf. Problems at this time can significantly affect the calf’s mortality risk.

A study published in 1987 estimated that 69 percent of pre-weaning mortality occurs within the first 96 hours of birth and about two-thirds of those losses are attributable to dystocia, or calving difficulties.

Other studies have shown that calves that experience dystocia are almost 13 times more likely to die within 12 hours of birth. Calving difficulties obviously hurt the bottom line for producers.

As a result, both purebred and commercial cow-calf producers have done a remarkable job of lowering the incidence of dystocia. One of the main ways this has been accomplished is by selecting for bulls with lower birth weights.

We seem to perform far fewer C-sections and pull fewer calves in our veterinary clinics these days than we did in the past.

More accurate estimates of dystocia levels have been published by Dr. Jennifer Pearson, Dr. Claire Windeyer and colleagues from the University of Calgary and University of Saskatchewan veterinary colleges in a recent paper based on herds enrolled in the western Canadian cow-calf surveillance network. In that study, 110 cow-calf producers reported assisting 4.9 percent of births during the 2016 calving season. When broken down by age, 13.5 percent of heifers were assisted at calving and 3.2 percent of cows were assisted. Stillbirth mortality was estimated at 2.1 percent (3.3 percent of heifers’ calves, and 1.9 percent of cows’ calves). Pre-weaning mortality was 4.5 percent overall.

These numbers show that despite major efforts at improving dystocia and stillbirth rates, we still have the potential to improve.

A recent paper published in the Journal of Animal Science written by researchers from Colorado State University looked at data on calving ease, birth weight, weaning weight and yearling weights from the American Simmental Association database. They used pedigree and performance data on more than 11.6 million animals in this genetic study. After analysis, they suggested that selection for low birth weights on its own may not be the best approach. Their analysis showed that selection for calving ease rather than low birth weight was more effective at reducing dystocia and if low birth weights were used as the primary measure, then weaning weights and yearling weights also tended to decrease.

The authors of the paper suggested that a more effective selection criteria would include a weighted score for birth weight and include calving ease and take into account weaning and yearling weights.

The authors’ premise is that using calving ease as a selection tool instead of just birth weights is more effective at lowering dystocia and does not compromise weaning weights. As a result, it would improve productivity.

I know from my experience in research that trying to measure calving ease can be difficult. It is a subjective measure and we often divide it into three categories: unassisted, easy pulls and hard pulls. C- sections are sometimes included as a fourth category.

However, what one producer classifies as a hard pull may be different than how another producer rates it.

Cow-calf producers also have different thresholds on when they intervene with a calving.

A producer with an intensively managed herd that calves in February may tend to bring a heifer into the barn and assist with calving far sooner than a producer with a more extensively managed herd calving on pasture in June. As well, in intensively managed herds, sometimes heifers are assisted because the herd manager wants to go to bed or go to town, and the assistance is not always absolutely necessary in every case.

This new paper in the Journal of Animal Science gives some new food for thought on the importance of classifying calving ease. However, I think we are still going to have challenges in implementing the suggestions because of the subjective nature of the calving ease measurement category.

Birth weights are more objective if they are accurately measured and recorded but they may tend to select for shorter gestation times and perhaps have a somewhat negative impact on weaning weights.

Do you record calving ease as part of your calving records? Seventy-four percent of producers in Pearson’s study of western Canadian herds said they recorded calving ease as part of their records. Only 44 percent of producers in the same study reported that they recorded birth weights. However, it should be noted that most of these herds were not purebred operations.

Perhaps in our western Canadian herds, calving ease is already being taken into account as part of the selection process.

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