Date of this Version
A study was conducted over 3 yr to evaluate effects of sire birth weight EPD, calf birth weight and shape, and heifer pelvic area and weight, individually and in combination, on dystocia and subsequent rebreeding of 2-yr-old heifers. Heifers (n = 550), MARC II yearlings, were assigned for breeding to one of four Angus sires with birth weight EPD of -.95, -.82, +2.9, and +2.7 kg. At calving, heifers were assisted as needed. A gauge attached to the calf puller recorded applied traction pressure. Analysis of traction pressure detected only slightly larger amounts of variation (2 to 3%) affecting dystocia than the standard five-point scoring system. Dam weight did not affect calving difficulty score (CDS), except dam birth weights were heavier ( P < .05) for CDS 5 (Caesarean section) than CDS 1 (unassisted). Dams requiring Caesarean section had smaller pelvic areas ( P < .05), with no other differences among CDS. The CDS increased as calf birth weight and calf external measurements increased. Low EPD sires produced calves with smaller ( P < .05) birth weights and smaller calf head and foot circumferences and caused less dystocia than high EPD sires. The CDS did not affect subsequent pregnancy rates but did affect conception date of the second calf. Calves delivered by Caesarean section were lighter ( P < .05) at weaning than other calves but had similar slaughter weights. As mean winter temperature increased (6.1°C) from yr 1 to 3, calf birth weight decreased (4.6 kg) and calving difficulty decreased 23%. Results indicate sire birth weight EPD, calf birth weight and shape, dam pelvic area, and climate affected CDS, and CDS affected subsequent conception date.