THE EFFICACY OF PREDICTING DYSTOCIA IN YEARLING BEEF HEIFERS .1. USING RATIOS OF PELVIC AREA TO BIRTH-WEIGHT OR PELVIC AREA TO HEIFER WEIGHT

Three methods for predicting difficult births were tested on 4,140 yea rling heifers measured before breeding and(or) at pregnancy check appr oximately 6 mo later. These heifers were from 115 beef herds in Albert a and British Columbia. The overall incidence of dystocia for normal p resentations was 26.5%, which included 17.2% easy assists, 7.7% hard p ulls, and 1.6% Caesarean sections. In Method 1, heifers were predicted as difficult (hard pull and Caesarean section) or easy (unassisted an d easy pull) calvers by dividing their pelvic area (PA) by previously calculated PA to calf birth weight (PA/BWT) ratios. The ratio used dep ended on heifer weight and age. Method 1 predicted 63.7% to be difficu lt calvers. Of these only 10.4% were actually difficult calvers. The a ccuracy of this method was 40.0% and was not a useful on-farm method f or predicting difficult births in first-calf, 2-yr-old heifers. In Met hod 2, 3,278 heifers measured before breeding and 1,125 heifers measur ed at pregnancy check were predicted as difficult or easy calvers by d ividing their PA by 4.19 at prebreeding or 5.51 at pregnancy check. Th ese values were PA/calf BWT ratios previously determined to be thresho ld levels. The accuracy of Method 2 was 78.5% and culling by this meth od would have reduced difficult birth rate by 9.6%. However, of the 73 8 heifers (16.8%) predicted to be difficult calvers, 86.0% actually ca lved easily. Heifers predicted to be easy calvers by Method 2 were hea vier (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) t han heifers predicted to be difficult calvers. In Method 3, 3,269 heif ers measured before breeding and 1,087 heifers measured at pregnancy c heck were predicted as difficult or easy calvers by dividing their PA by their BW. Heifers having ratios that were among the lowest 16% of t he herd were predicted to be difficult calvers, and the rest were pred icted to be easy calvers. The accuracy of Method 3 was 79.4% and culli ng by this method would have reduced difficult birth rate by 9.5%. How ever, of the 677 (15.5%) heifers predicted to be difficult calvers, 85 .7% actually calved easily. Heifers predicted to be easy calvers by Me thod 3 were lighter (P < .001), had a larger PA (P < .001), had more P A per kilogram of BW (P < .001), and had lighter (1.2 kg) calves at bi rth (P = .04) than heifers predicted to be difficult calvers. In concl usion, previously published pelvic area to birth weight ratios could n ot be used across a wide variety of herds due to poor accuracy and a h igher than acceptable level of culling. In addition, the PA/BWT and PA /BW ratio methods as developed in this study were not useful diagnosti c tools for predicting difficult births. These methods predicted a hig h proportion of difficult calvers that turned out to be easy calvers a nd were unable to detect > 25% of the difficult births.