PELVIC GROWTH, CALF BIRTH-WEIGHT AND DYSTOCIA IN HOLSTEIN X HEREFORD HEIFERS

The pelvic area was measured in 129 Holstein x Hereford heifers at 10, 16 and 22 months of The heifers were fed an all forage diet. Pelvic g rowth was not linear over time, changing from an increase of 0.27 +/- 0.2 cm2/day during the first 6 months of the study to 0.13 +/- 0.13 cm /day during the last 6 months (P < 0.01). The relationship of pelvic a rea to body weight, height at hooks, and distance from hooks to pins d id not change with age, and a moderate correlation between the pelvic area and these other measures (R=0.20 to 0.80) was noted. The pelvic a rea was measured within 24 hours after calving in 76 of the heifers. T he rate of increase of pelvic area/day increased significantly (P < 0. 01) in the month prior to calving from 0.14 +/- 0.13 cm2 to 1.15 +/- 0 .88 cm2. As a result, the pelvic area at calving had a moderate correl ation (R=0.29 to 0.52) to the pelvic area prior to calving. Logistic r egression and discriminant analysis techniques were used to model the influence of the pelvic area and calf birth weight on the incidence of dystocia. Ratio of the pelvic area at calving to calf birth weight si gnificantly (P < 0.01) influenced the incidence of dystocia. Logistic regression techniques were not superior to discriminant analysis; both correctly predicted 73% of the cases. Pelvic area measurement at any time other than calving was not associated with dystocia (P > 0.05). P elvic area and calf birth weight are important determinants of dystoci a in heifers. The high degree of variation noted in pelvic growth, in particular during the month prior to calving, resulted in low correlat ion between pelvic area at calving and the precalving measurement. The refore, we were not able to predict dystocia by measuring the pelvic a rea prior to calving.