Factors affecting dystocia and early calf mortality in Angus cows and heifers.

Calving performance records (965,417) from purebred American Angus herds throughout the United States were used to study dystocia and early calf mortality during the period from 1972 to 1985. A sample of 53 (n = 83,467) herds was used to establish reasonable limits on the expected frequency of dystocia and mortality within and among herds that have good reproductive management programs and to verify the frequencies of scores in all other herds (n = 4,130) reporting calving performance information. The data were analyzed by logistic regression models. Dystocia and perinatal mortality, to a lesser extent, were found to be more frequent in heifers than in cows. The odds of unassisted births vs births with major difficulty were 11.58 times greater in cows than in heifers. In heifers, the odds of an unassisted birth increased with age of dam and decreased with birth weight of calf. At a baseline age of 22 mo the odds of an unassisted birth for calves weighing 30 and 40 kg were .13 and .02 times lower than the odds for calves weighing 20 kg. Alternately, at 29 mo, the odds of an unassisted birth for heifers producing a 20-kg calf were 4.53 times greater than at 22 mo. Survival of calves to 24 h in heifers was primarily affected by birth weight. Heifers producing calves at intermediate weights of 29 kg had higher odds of producing live calves at all ages. Considering all heifers calving at 22 to 29 mo relative to 20-kg calves, the odds for survival to 24 h were 3.83 times greater for 29-kg calves and lower by a factor of .52 for 40-kg calves.