Decreasing dystocia may provide a reproductive benefit to the cow/calf enterprise. In a study involving almost 2,000 cows, Laster and co-workers (1973) described the reproductive performance of cows that experienced dystocia versus those that did not. When the 45- to 60-day artificial insemination period was evaluated, 74 percent of the cows that calved with no difficulty were detected in estrus while 59 percent of the cows that experienced dystocia were detected in estrus. Additionally, there was a 16 percent advantage in conception rate to those cows that experienced no dystocia. Not surprisingly, these researchers found a strong relationship between calving difficulty and calf birth weight. Therefore, decreasing birth weights may be a consideration in reducing the postpartum interval, especially in an operation where dystocia or length of postpartum interval has been a problem. By decreasing the postpartum interval, cows have more chances to be bred and should achieve higher conception rates.

     The postpartum interval may also be reduced by shorter duration of labor. In a study by Doornbos and co-workers (1984) involving 127 cows, about half of the females were allowed to calve normally unless assistance was needed. The calves of the other cows were taken beginning at Stage II of labor. Stage II is described by these researchers as that point in parturition at which the cow begins an abdominal press. Thereafter, postpartum reproductive performance was evaluated. Ten percent more of the cows that experienced short-duration labor had been observed in estrus by the beginning of the breeding season, and about 14 percent more were pregnant at palpation. If lighter birth weight calves result in shorter duration of labor in cows, then again reproductive efficiency might be enhanced.

     If lower birth weight in calves correlates with better future reproductive performance of their dams, then certainly birth weight should receive emphasis in sire selection. Many producers may hesitate to reduce birth weights, and it may not be necessary for many operations. Although selection for any single trait over a period of time can produce unwanted results, if a herd shows a moderate or high number of assisted births, then birth weight may need to be considered in selecting a sire.

     Regardless of the breed of the selected sire, birth weight EPDs provide a means of lowering birth weight through individual sire selection. Arnold and co-workers (1990), evaluated the efficacy of birth weight EPDs in purebred Angus cattle. Low and high birth weight EPD sires with accuracies of at least 0.8 were identified for each of the lines. Sires in each of the lines had to have yearling weight EPDs of at least 44 pounds. Progeny from each of the sire lines were evaluated for birth, weaning and yearling performance. Based on the sire EPDs for birth, weaning and yearling weight, an expected response difference was calculated and compared with the actual progeny performance difference. The expected response difference between the lines for birth weight using EPDs was 6.4 pounds while the observed birth weight difference was 8.1 pounds. The expected response difference between the lines for yearling weight using EPDs was 14 pounds, while the observed yearling weight difference was 15 pounds. This study supports the use of EPDs to select bulls that have low birth weight genetics but still excel for growth.