Dystocia (calving difficulty) contributes heavily to losses in the production and profitability of beef cow-calf herds. Some of the factors that result in economic losses associated with dystocia include calf deaths, increased veterinary costs, decreased rebreeding rates, extended breeding and calving seasons, increased labor costs and injury and death to cows.
The 2007 National Animal Health Monitoring Systems (NAHMS) survey indicated dystocia was responsible for approximately 26 percent of all calf losses and approximately 17 percent beef breeding cattle losses.
In the beef industry, first-calf heifers account for the majority of calving difficulties and associated calf losses. Nebraska researchers studying the effects of age of the dam on the incidence of dystocia found 54 percent of 2-year-old heifers and 16 percent of 3-year-old females experienced calving difficulties.
Four-year-old females had calving difficulty rates of 7 percent, and cows 5 years and older had calving difficulty rates of 5 percent.
Even though there are a number of factors that contribute, the higher rates of dystocia in young beef females is generally due to the fact they have not yet reached their structural maturity, and a disproportional relationship between maternal and fetal size exists in the younger beef females.
It is generally accepted that the largest contributor to a cow-calf producer’s income/profitability are the proceeds from calf sales. Calf death loss from dystocia (or any other means) represents a significant loss for the cow-calf enterprise. Results from a 2002 study estimated the annual cost of dystocia in beef cow-calf herds to be $185 million.
As producers consider bulls to produce replacement heifers or bulls to use on first-calf heifers, they should make selection decisions that lead to a reduced incidence of dystocia. A couple of tools producers can use in this selection process are maternal calving ease (MCE) expected progeny differences (EPDs) and calving ease (CE) EPDs.
Maternal calving ease (MCE) EPDs are expressed as a difference in percentage of unassisted births and predict the difference in ease with which a sire’s daughters will calve as first-calf heifers. Larger values indicate greater calving ease (larger percentage of unassisted births) in a sire’s daughters.
An example of how MCE EPDs are used and interpreted follows: In a comparison of two bulls being considered for use to produce replacement heifers, it is noted that Bull No. 1 has a MCE EPD of +2.5 and Bull No. 2 has a MCE EPD of +8.5. This suggests that 6 percent more of Bull No. 2’s daughters would calve unassisted as first-calf heifers than Bull No. 1’s daughters.
Calving ease (CE) EPDs are expressed as a difference in percentage of unassisted births and predict the difference in ease with which a sire’s calves will be born when he is mated to first-calf heifers. Larger values indicate greater calving ease (larger percentage of unassisted births) in first-calf heifers.
An example of how CE EPDs are used and interpreted follows: In a comparison of two bulls considered to be used on first-calf heifers, it is noted that Bull No. 3 has a CE EPD of +14.0 and Bull No. 4 has a CE EPD of +4.0. This suggests that Bull No. 3, when bred to first-calf heifers, would sire 10 percent more calves that are born unassisted than Bull No. 4.
With the disproportion between maternal and fetal size (fetus too large or maternal pelvis too small) being the most frequent cause of dystocia, producers have used birthweight EPDs as a selection tool to reduce the incidence of dystocia in their beef cattle herds.
However, beef producers can be more effective and efficient in reducing dystocia in their herds by selecting for calving ease. In relation to dystocia, calving ease is referred to as the economically relevant trait, and birthweight is referred to as an indicator trait.
This implies that calving ease is where the economics or costs of dystocia are associated. Selection for calving ease allows us to focus selection efforts on the trait that most readily affects the bottom line.
As has been implied previously, birthweight has often been used as an indicator or selection criterion for calving ease. However, birthweight does not tell the whole calving ease story. When calving ease EPDs are calculated, birthweights and calving ease scores are collected and incorporated.
Studies have shown that birthweight only accounts for approximately 50 to 55 percent of the genetic variation in calving ease. Selecting for reduced birthweights alone will not result in the same level of improvement in calving ease (i.e., decreased dystocia) as selecting directly for calving ease.
Producers should also keep in mind that birthweight is strongly and positively associated with other weight and growth traits. This relationship suggests that as producers select to change birthweight, other weight and growth traits follow in the same direction.
Reductions in birthweight are usually associated with decreased growth performance in the form of decreased weaning and yearling weights.
CE vs. MCE
As trait relationships are considered, it is also important to note the relationship between maternal calving ease and calving ease. Several studies have shown maternal calving ease is negatively associated with calving ease. This relationship suggests that as producers select to improve calving ease in their herds, there is a tendency to reduce the maternal calving ease potential in their herds as well.
To address this concern and keep calving ease potential at optimal levels in the whole herd, producers should select bulls that will be mated to cows to produce replacement heifers and then turn their attention toward the bulls that will be used on first-calf heifers.
Today, beef producers have excellent tools to estimate the genetic worth of animals for a number of economically important traits, including those related to calving ease.
To fully address dystocia in beef cattle herds, producers must consider bulls that will be used to produce replacement heifers (MCE EPDs) and bulls that will be used on first-calf heifers (CE EPDs).
As producers make selection decisions for bulls related to calving ease, they should also make sure the selection objectives for calving ease are in balance with the selection objectives of other economically important traits.
PHOTO: Birthweight in calves is strongly and positively associated with other weight and growth traits. Photo by Julie Brown.
J. Benton Glaze Jr.
- Extension Beef Cattle Specialist
- Department of Animal and Veterinary Science University of Idaho
- Email J. Benton Glaze Jr.