Factors affecting and prediction of dystocia in Charolais heifers.

Calving difficulty at first and second parturition, using data recorded on 476 purebred Charolais heifers from a ranch in Southeastern Arizona, were analyzed by analysis of variance, step-wise multiple regression and discriminant analysis. Dystocia score (1 = no assistance through 4 = extreme difficulty) was the dependent variable with dam and calf effects independent variables. In this herd, 31.1 and 15.0% of the heifers experienced calving difficulty at first and second parturition, respectively. Birth weight of calf was the most important factor influencing dystocia, accounting for 71 and 61% of the total variation explained by the analysis of variance model when calf effects as well as dam effects were included at first and second parturition. Mean birth weight was 39.0 kg for primiparous heifers and 44.3 kg for second-half cows. A significant increase in dystocia (at both first and second parturition) occurred among male calves with birth weights of 45.5 kg or greater. At first parturition, no significant increase in dystocia occurred among female calves until birth weight exceeded 50 kg. Birth weight was not a significant factor influencing dystocia for female calves at second parturition. Yearling weight of cow and dystocia score of cow's dam were the only significant dam variables, and only at first parturition. Pelvic height measurements did not significantly affect dystocia scores at either parturition. A reasonably accurate prediction of dystocia based on the variables included in this study would be impossible, even if some means were devised to reliably predict birth weight of the calf.