Abstract
Factors associated with pelvic area were studied in 943 yearlings and 2-year-old cows and factors associated with dystocia were studied in 599 2-year-old primiparous cows. There were three goups of animals: yearlings and 2-year-olds representing 14 breed groups, and 2-year-old Hereford and Angus fed three levels of energy the last trimester of gestation. Variables studied in relationship to pelvic size were breed, precalving energy level, age, cow weight, weight change, condition score, muscling score and pelvic slope score. Factors studied in relation to dystocia were breed, precalving energy level, pelvic height, pelvic width, cow weight, calf sex, calf birth weight, calf shoulder width, hip width, chest depth, wither height and body length. Cow weight was the largest source of variation associated with pelvic area, but breed, adjusted for cow weight, had a significant (P<.01) effect on pelvic area. Precalving energy levels (4.9, 6.2 or 7.7 kg TDN/head/day) influenced (P<.01) calf birth weights, but had no influence on dystocia or calf measurements. Traits known or measured before parturition accounted for 26% of the variability in percentage of dystocia in the 2-year-olds representing the 14 breed groups and 5% in the cows fed three levels of energy before calving. The amount of accountable variability associated with dystocia was increased to 39 and 25% in the two groups of cows when calf sex, birth weight and the five calf shape measurements were included in the analysis. Results of this study indicate that the relationship between dystocia and the physical measurements and subjective scores of cows included in this study are too low for these variables to be used as accurate predictors of dystocia. Larger 2-year-old cows have larger pelvic openings, but they also have calves with heavier birth weights. Although differences in dystocia rates among breeds with similar birth weights suggest calf anatomy as a possible effect on dystocia, calf shape measurements taken in this study, independent of birth weight, were not related to dystocia. Copyright © 1974. American Society of Animal Science . Copyright 1974 by American Society of Animal Science