Comparison in the same environment of calving problems in heifers from herds with a history of a low and high incidence of dystocias.

The incidence of dystocias and the calving performance of primiparous Hereford heifers from 2 herds, one with a low (A) and the other a high (B) dystocia history, were compared when all animals were run on the same property in southern Queensland. The dystocia incidence was 21.4% for Herd A heifers and 40.5% for Herd B heifers (P approximately equal to 0.10). While Herd A heifers had significantly (P < 0.05) heavier calves than Herd B heifers (29.1 v 27.7 kg), the ratio of calf birth weight to dam pelvic brim area was 0.11 kg/cm2 for both groups. Differences in the size of the maternal pelvis, calf oversize and the type of presentation recorded for assisted births did not satisfactorily explain the difference in the incidence of dystocias. The circumstantial evidence indicated that inherent factors may affect dystocia.