Dystocia management must begin with proper heifer development. Fetopelvic disproportion is a major contributing cause of 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 moderate its effects should focus on replacement heifer development, sire selection using EBVs for calving ease, and early dystocia intervention.
Replacement Heifer Development:
Dystocia rates in beef heifers may not be controlled by nutritional restriction during late pregnancy. On the contrary, the loss of 0.5 kg/day during the last trimester of pregnancy in beef heifers is associated with weak labor, increased dystocia rate, reduced calf growth rate, prolonged postpartum anestrus, reduced pregnancy rate, and increased morbidity and mortality. It is recommended that heifers be fed to allow 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 selection of replacement heifers, even though pelvic area alone explains only a small proportion of the variability in dystocia. Pelvic area measurements before the breeding season or at the time of pregnancy examination have been used to estimate the pelvic area before calving. Those heifers with a small pelvic area before the breeding season may then be culled or selectively mated 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 based on pelvic area; however, such "pelvimetry" measurements may only detect the outlier animals in this multifactorial condition.
A combination of culling heifers with small pelvic areas and using bulls that sire calves with small birth weights may reduce dystocia significantly. Using only the sires’ birth weight to control calf birth weight and dystocia is not effective. A large number of nongenetic influences affect birth weight, such as age of the dam, environment, and birth type. The ability to identify sires appropriate for use on replacement heifers has advanced significantly.
The use of estimated breeding values (EBVs) or expected progeny differences (EPDs) for birth weight is more effective than using only sire birth weight in selecting for acceptable birth weights. EPDs are reported in the units of the trait they reflect (eg, pounds for birth weight). Along with each EPD is reported an accuracy ranging from 0 to 1. Higher accuracies indicate a higher level of confidence that the stated EPD truly reflects the bull’s effect. EPDs most effectively help compare bulls rather than identify 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 lb 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 EPD for use on heifers while maintaining at least moderate weaning and yearling weight EPD. This is best achieved by the use of AI sires with high accuracy EPD. EPD can be calculated on yearling bulls with no progeny, but the accuracy is low. Until recently, EPDs were useful only in comparisons within breeds; however, methods for across-breed EPDs have now been developed. This is of particular use in selecting bulls to control dystocia in crossbreeding programs. Two recent innovations in the use of EBVs/EPDs for management of dystocia are the calving ease EPD and the maternal calving ease EPD. Calving ease EPD is related to birth weight EPD 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 than female calves. Abnormal presentations of the calf accounted for 22% of dystocias and 4% of all births in one study. Most dystocias are seen in primiparous 2-yr-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 effects 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:
Despite the best efforts to avoid dystocia, some cases will be seen. Early intervention minimizes the effects of dystocia on calves; however, heifers especially may require significant time to dilate to point of delivery. Heifers should be monitored regularly and provided with assistance promptly if stage II labor is prolonged (eg, 1 hr). Producers need to identify the level of dystocia and growth that is economically acceptable and select a bull to match. They must be well trained to intervene appropriately in dystocia and recognize when to call the veterinarian. A general rule is that if a heifer has not made significant progress in delivering her calf within 30 min, it is time to get help.