Dystocia management must begin with proper heifer development. Fetopelvic disproportion is a major contributor to dystocia. Calf birth weight, the size of the pelvic area of the dam, and the interrelationships of these two factors are major determinants of dystocia. The weight of the calf is a function of genetic and environmental factors. Genetic factors include sex, length of gestation, breed, heterosis, inbreeding, and genotype. Nongenetic factors include age and parity of the dam, nutrition of the dam during various phases of gestation, and environmental temperature. Efforts to manage the dystocia rate and mitigate its effects should focus on replacement heifer development, sire selection based on estimated breeding values for calving ease, and early dystocia intervention.
Replacement Heifer Development in Dystocia Management of Cattle
Producers should not attempt to control dystocia rates in beef heifers by nutritional restriction during late pregnancy; the loss of 0.5 kg/day during the final trimester of pregnancy in beef heifers is associated with weak labor, increased dystocia rate, decreased calf growth rate, prolonged postpartum anestrus, decreased pregnancy rate, and increased morbidity and mortality. Instead it is recommended that heifers be fed quantities that enable modest rates of gain (0.5 kg/day) during late pregnancy. Protein malnutrition in late pregnancy has been associated with weak calf syndrome and may be a factor contributing to neonatal mortality.
Measurement of the pelvic area of the dam to predict dystocia is sometimes used as a criterion for the selection of replacement heifers, even though pelvic area alone explains only a small portion of the variability in dystocia. Pelvic area measurements before the breeding season or at the time of pregnancy examination can be used; heifers that have a small pelvic area before the breeding season may then be culled or selectively bred to easy-calving bulls, and those with a small pelvic area at the time of pregnancy examination may be aborted, culled, or identified for careful observation at calving.
Some evidence suggests that culling heifers with the narrowest pelvic width may be more effective than culling on the basis of pelvic area. However, such "pelvimetry" measurements may detect only the outlier animals in this multifactorial condition.
Sire Selection in Dystocia Management of Cattle
A combination of culling heifers with small pelvic areas and using bulls that sire calves with small birth weights may decrease dystocia substantially. Using only the sires’ birth weight to control calf birth weight and dystocia is not effective. Many nongenetic factors influence birth weight, including age of the dam, environment, and birth type. The ability to identify sires appropriate for use with replacement heifers has advanced markedly with the greater adoption of genomics and breeding values.
Selecting bulls on the basis of estimated breeding values (EBVs) or expected progeny differences (EPDs) to ensure acceptable birth weights is more effective than considering only sire birth weight. Genomics and genomic EBVs have greatly assisted the development of more reliable breeding-value predictions. EPDs are reported in the units of the trait that they reflect (eg, pounds for birth weight). Along with each EPD is reported an accuracy ranging from 0 to 1. Higher accuracies indicate greater confidence that the stated EPD truly reflects the bull’s effect.
EPDs are most effective for comparing bulls rather than identifying the specific effect a bull will have on a herd. For example, a bull with a birth-weight EPD of 4.0 would be expected to sire calves 6 pounds heavier, on average, than a bull with a birth-weight EPD of –2.0 when bred to the same group of heifers. An attempt should be made to identify bulls with good calving-ease EBV figures and low birth-weight EPDs for use on heifers, while maintaining at least moderate weaning and yearling weight EPDs. The most successful approach is to use AI sires with highly accurate EPDs. EPDs can be calculated on yearling bulls with no progeny, but the accuracy is low; however, genomics has greatly improved predictions.
Historically, EPDs were useful only in comparisons within breeds; however, methods for across-breed EPDs have now been developed. Interbreed comparisons are particularly useful in selecting bulls to control dystocia in crossbreeding programs. Two important innovations in the use of EBVs and EPDs for the management of dystocia are the calving-ease EPD and the maternal calving-ease EPD. Calving-ease EPDs are related to birth-weight EPDs but may predict calving ease more effectively. Maternal calving ease is a measure of the effect of the maternal grandsire and the ease with which a bull’s daughters will calve.
As the birth weight of the calf increases, the incidence of dystocia often also increases. Calving difficulty is higher for male calves than for female calves. Most dystocias occur in primiparous 2-year-old heifers, and the frequency decreases with increasing age and weight of the cow. Some studies have suggested that cows that previously experienced dystocia are more likely to do so again. Environmental factors may also have an effect on calf birth weight and dystocia. Cold weather may increase birth weights and subsequently increase the incidence of dystocia.
Appropriately Early Intervention in Dystocia Management of Cattle
Despite the best efforts to avoid dystocia, some cases do occur. Early intervention minimizes the effects of dystocia on calves; however, heifers in particular may require substantial time to dilate to the point of delivery. Heifers should be monitored regularly and provided with assistance promptly if stage 2 labor is prolonged (eg, more than 1 hour of effort). Producers must be well trained to intervene appropriately in dystocia and recognize when to call a veterinarian.