Relationships of pelvic structure, body measurements, pelvic area and calving difficulty.

Data on 186 Hereford heifers from five South Dakota ranches were collected to evaluate the relationships of pelvic structure and body measurements with calving difficulty (CD) and pelvic area. Body measurements obtained prebreeding and precalving included two internal pelvic and seven external rump measurements, three pelvic angles and two slope of rump measurements. A calving difficulty score (CDS) of 1 (unassisted) to 8 (Caesarean) was assigned at birth, and calf birth weight was recorded. Data were analyzed using both regression and discriminant analyses. Stepwise regression analysis including all 49 variables showed that calf birth weight was the most important variable influencing CDS (R2 = .33), with precalving pelvic area ranking second (cumulative R2 = .45). Prediction equations that included all variables or only prebreeding variables accounted for 63% and 25% of the variation in CDS, respectively. Pelvic angles and slope of rump variables had low correlations with CDS and pelvic area. Ratios of prebreeding and precalving pelvic areas to calf birth weight significantly decreased as CDS increased. A prebreeding ratio of 4.7 cm2/kg may be beneficial in selection of replacement heifers. In discriminant analyses, the most informative variable in differentiating among levels of CD was calf birth weight. All models significantly discriminated between two CD categories (assisted and unassisted). Models using prebreeding variables only performed as well as those based on precalving variables.(ABSTRACT TRUNCATED AT 250 WORDS)