The efficacy of predicting dystocia in yearling beef heifers. I. Using ratios of pelvic area to birth weight or pe lvic area to heifer weight

Three methods for predicting difficult births were tested on 4,140 yearling heifers measured before breeding and(or) at pregnancy check approximately 6 mo later. These heifers were from 115 beef herds in Alberta and British Columbia. The overall incidence of dystocia for normal presentations was 26.5%, which included 17.2% easy assists, 7.7% hard pulls, and 1.6% Caesarean sections. In Method 1, heifers were predicted as difficult (hard pull and Caesarean section) or easy (unassisted and easy pull) calvers by dividing their pelvic area (PA) by previously calculated PA to calf birth weight (PA/BWT) ratios. The ratio used depended on heifer weight and age. Method 1 predicted 63.7% to be difficult calvers. Of these only 10.4% were actually difficult calvers. The accuracy of this method was 40.0% and was not a useful on-farm method for predicting difficult births in first-calf, 2-yr-old heifers. In Method 2, 3,278 heifers measured before breeding and 1,125 heifers measured at pregnancy check were predicted as difficult or easy calvers by dividing their PA by 4.19 at prebreeding or 5.51 at pregnancy check. These values were PA/calf BWT ratios previously determined to be threshold levels. The accuracy of Method 2 was 78.5% and culling by this method would have reduced difficult birth rate by 9.6%. However, of the 738 heifers (16.8%) predicted to be difficult calvers, 86.0% actually calved easily. Heifers predicted to be easy calvers by Method 2 were heavier (P < .001), had a larger PA (P < .001), had more PA per kilogram of BW (P < .001), and had heavier (1.0 kg) calves at birth (P = .05) than heifers predicted to be difficult calvers. In Method 3, 3,269 heifers measured before breeding and 1,087 heifers measured at pregnancy check were predicted as difficult or easy calvers by dividing their PA by their BW. Heifers having ratios that were among the lowest 16% of the herd were predicted to be difficult calvers, and the rest were predicted to be easy calvers. The accuracy of Method 3 was 79.4% and culling by this method would have reduced difficult birth rate by 9.5%. However, of the 677 (15.5%) heifers predicted to be difficult calvers, 85.7% actually calved easily. Heifers predicted to be easy calvers by Method 3 were lighter (P < .001), had a larger PA (P < .001), had more PA per kilogram of BW (P < .001), and had lighter (1.2 kg) calves at birth (P = .04) than heifers predicted to be difficult calvers. In conclusion, previously published pelvic area to birth weight ratios could not be used across a wide variety of herds due to poor accuracy and a higher than acceptable level of culling. In addition, the PA/BWT and PA/BW ratio methods as developed in this study were not useful diagnostic tools for predicting difficult births. These methods predicted a high proportion of difficult calvers that turned out to be easy calvers and were unable to detect > 25% of the difficult births.