Submitted to: Professional Animal Scientist
Publication Type: Peer reviewed journal
Publication Acceptance Date: 11/1/1991
Publication Date: N/A
Citation: Interpretive Summary: Breeding heifers to calve at 2 years of age can increase lifetime beef production; however, heifers at first calving are prone to calving difficulty (dystocia). In the absence of dystocia, heifers calving first as 2-yr-olds have a tendency to calve earlier in subsequent years, wean heavier calfs, and produce a higher percent calf crop than heifers calving first as 3-yr-olds. The problem is that heifers at first calving are thre to four times more likely to suffer dystocia than at second and later calvings. Fetopelvic incompatibility is likely the main reason for calving difficulty in heifers. The objectives of this research were to: 1) examine the relationships of calf birth weight and body size to dystocia levels, along with gestation, sex of calf, sire birth weight and dam weight, and then determine the effects of these various factors on incidence of dystocia in 2-yr-old commercial beef heifers, and 2) use discriminant analysis procedures to develop classification functions, which will aid management decision processes directed toward alleviating dystocia problems in heifers. Discriminant analysis indicated that birth weight of calves and age of heifer adequately classified heifers into dystocia groups. Birth weight of the calf was always needed to give acceptable classification accuracy. Heifer management systems that include breeding for lighter birth weights will dramatically reduce incidence and severity of dystocia in the 20 to 70% of animals which annually have calving difficulty.
Technical Abstract: Data were obtained on 1178 first-calf heifers from 11 commercial ranches and three experimental herds throughout Oregon. The objectives of the study were to examine the relationship among factors associated with dystocia and possibly to develop predictive formulas. Dystocia occurred in 34% of the heifers observed and ranged from 11 to 69% among ranches. Variables correlated (P<.05) with dystocia and the corresponding r values were heifer birth weight (.14), calf birth weight (.35), calf sex heifer age at calving (-.23), and the heifer's pelvic area/calf birth weight ratio (-.17). Pelvic area was not different (P>.05) between heifers experiencing dystocia and those calving without assistance. Factors associated (P<.05) with calf birth weight and the corresponding r values were gestation length (.17), sire birth weight (.25), calf sex (-.22), heifer birth weight (.37), heifer prebreeding weight (.38), and pelvic area (.15). Discriminant analysis indicated that birth weight of calves and age of heifer would adequately classify heifers into dystocia groups. Birth weight of the calf was always needed to give acceptable classification accuracy. Results would indicate that heifer management systems that included breeding for lighter birth weights will dramatically reduce incidence and severity of dystocia.