Managing Calving Difficulty

Livestock Update, March 2004

Scott Greiner, Extension Animal Scientist, Beef, VA Tech

For many beef herds, calving difficulty is the largest single factor resulting calf losses over time. In addition to the obvious economic consequences associated with calf mortality as a result of calving difficulty, dystocia (calving difficulty) also results in increased cow mortality, increased calving intervals (as a result of delayed return to estrus), lower conception rates, reduced weaning weights, and increased veterinary and medical costs. Losses attributed to calving difficulty have been estimated to cost the beef industry $750 million annually.

Calving difficulty has been shown to be a problem primarily in two-year-old first-calf heifers. The proportion of first-calf heifers requiring assistance at birth is commonly cited at 25%. Although dystocia is not uncommon in older females, it occurs at a much lower frequency. Calving difficulty in first-calf heifers has been shown to be three to four times higher than in three-year-olds. Incidence of calving difficulty is twice as high for three-year-olds as four-year-olds. Therefore, management strategies to control calving difficulty will be targeted primarily at first-calf heifers.

Dystocia is the result of a disproportion between calf size at birth (birth weight) and the dam's birth canal (pelvic area). Along with these two factors (birth weight and pelvic area), a number of additional factors may contribute to differences in calving difficulty, including: cow age, calf sex, gestation length, pre-calving nutrition and cow body condition, season, calf presentation, and maternal effects. Of these factors, research has clearly demonstrated that calf birth weight is the primary factor that contributes to differences in calving difficulty. As calf birth weight increases, the percentage of cows requiring calving assistance also increases.

Genetic selection is the primary tool for effective management of birth weight and calving difficulty. Birth weight is a highly heritable trait, and therefore responds favorable with selection. Several tools are available for this purpose, namely utilization of breed difference and direct selection through the use of EPDs for birth weight and calving ease.

Breeds differ considerably in their genetic merit for economically important traits, and birth weight is no exception. A recent study conducted in Montana evaluated differences in calving difficulty in commercial Angus heifers when mated to either low Birth Weight EPD Angus bulls (top 10% of breed) or high Calving Ease EPD Simmental bulls (top 10% of breed). The two-year study was conducted on four ranches, and 1038 calvings were recorded. Results showed the Simmental-sired calves were 4.7 pounds heavier at birth than the Angus-sired calves. Correspondingly, the percentage of heifers requiring assistance at calving was 41% for those bred to Simmental sires vs. 29% for heifers carrying Angus-sired calves. These results confirm the commonly reported 10-15% difference in calving difficulty associated with British vs. Continental sires when. Results from the current germplasm evaluation study at the Meat Animal Research Center in Nebraska reports differences in calving difficulty in British vs. Continental sired calves are not as large in magnitude for mature cows. The most recent data available on each breed's current genetics shows a slightly higher percentage of unassisted births for Angus and Red Angus (99.6 and 99.1%) compared to Hereford (95.6%) and the Continental breeds (Simmental 97.7%, Gelbvieh 97.8%, Limousin 97.6%, Charolais 92.8%). These results confirm that calving ease differences between breeds are not as great today as they were 20 years ago. However, the British breeds remain superior to Continental breeds on average for siring calves that are lighter at birth and born with less assistance. Consequently, British breed sires are utilized with more frequency on commercial heifers. . Identification of sires with genetics favorable for birth weight and calving ease within breed is most critical.

Expected progeny differences (EPDs) provide the most effective tools to manage calving difficulty. Three EPDs are directly related to calving difficulty: Birth Weight EPD, Calving Ease EPD, and Maternal Calving Ease EPD. Birth Weight EPDs: predict the expected average difference in birth weight between two sires when mated to the same set of cows. It is important to recognize that EPDs predict the expected difference in performance, not the actual performance. EPDs do not predict what the actual birth weight of the calves will be, nor the variation in the calf crop. Actual birth weight will be impacted by the genetic merit of the cows the bull is mated to as well as environmental effects. Several breed associations (Gelbvieh, Hereford, Limousin, Red Angus, Simmental) publish Calving Ease EPDs (also referred to as Calving Ease Direct). This EPD predicts the ease with which a bull's calves are born to first-calf heifers. Calving ease EPDs are reported as deviations in percentage of unassisted births. Calving ease EPDs consider differences between animals in calf birth weights and actual observed levels of calving difficulty. Since calving ease EPDs directly predict calving ease, they should be used (when available) as the primary tool for avoiding dystocia problems in the cowherd. As maternal implies, Maternal Calving Ease EPDs (also referred to as Calving Ease Daughters or Calving Ease Total Maternal) reflect the ability of a bull's daughters to calve as first-calf heifers. When comparing bulls, a larger maternal calving ease EPD indicates that a bull's daughters will calve with a higher percentage of unassisted births. Maternal CE EPDs may be used to develop a cow herd that is superior for calving ease.

Proper interpretation and application are essential for effective use of EPDs. Target EPD values for birth weight or calving ease need to be established when selecting heifer bulls. An individual herd can establish benchmarks based on previous experience, knowing the EPDs of bulls that have been used. The level of risk that one is willing to accept is an additional consideration. Progeny-proven, high accuracy sires are available for use via AI as a means of reducing risk. More risk is associated with young sires which have lower accuracy values associated with their EPDs, and therefore more possible change in their EPD value. Therefore, the target BW EPD established for a natural service sire may be lower than for a sire to be used AI. Genetic and a number of other factors that vary from herd to herd make a universal recommendation on target BW or CE EPDs difficult. As a benchmark, the Virginia Premium Assured Heifer Program has a maximum BW EPD of +2.3 (top 40% of breed) for Angus service sires.

In general, measures of birth weight and calving ease are negatively correlated with growth traits. In other words, selection for calving ease and growth tend to be antagonistic. However, utilization of EPDs allows for the identification of sires that offer a balance of calving ease and growth, along with maternal traits and carcass merit. This allows for simultaneous progress to be made in several traits, and specifically allows for the management of calving difficulty to not occur at the expense of growth and other economically important traits.

In addition to EPDs, another valuable management practice to reduce the frequency of calving difficulty is to cull heifers with small pelvic areas. A substantial amount of research has been done to quantify the relationship between pelvic area and birth weight as it relates to dystocia. Heifers with yearling pelvic areas less than 150 cm2 are at increased risk of calving difficulty and should be culled. This selection practice can be applied in conjunction with a pre-breeding exam.

In summary, several tools are available to manage calving difficulty in beef cattle. Proper sire selection to reduce birth weights and increase calving ease is of primary importance, along with eliminating potential problem heifers. These factors, along with optimum nutrition and management and good husbandry will ultimately lead to a larger calf crop.

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