EFFECTS OF SIRE, DAM TRAITS, CALF TRAITS, AND ENVIRONMENT ON DYSTOCIAAND SUBSEQUENT REPRODUCTION OF 2-YEAR-OLD HEIFERS

A study was conducted dyer 3 yr to evaluate effects of sire birth weig ht EPD, calf birth weight and shape, and heifer pelvic area and weight , individually and in combination, on dystocia and subsequent rebreedi ng of 2-yr-old heifers. Heifers (n = 550), MARC II yearlings, were ass igned for breeding td one of four Angus sires with birth weight EPD of -.95, -.82, +2.9, and +2.7 kg. At calving, heifers were assisted as n eeded. A gauge attached to the calf puller recorded applied traction p ressure. Analysis of traction pressure detected only slightly larger a mounts of variation (2 to 3%) affecting dystocia than the standard fiv e-point scoring system. Dam weight did ndt affect calving difficulty s core (CDS), except dam birth weights were heavier (P <.05) for CDS 5 ( Caesarean section) than CDS 1 (unassisted). Dams requiring Caesarean s ection had smaller pelvic areas (P <.05), with no other differences am ong CDS. The CDS increased as calf birth weight and calf external meas urements increased. Low EPD sires produced calves with smaller (P <.05 ) birth weights and smaller calf head and foot circumferences and caus ed less dystocia than high EPD sires. The CDS did not affect subsequen t pregnancy rates but did affect conception date of the second calf. C alves delivered by Caesarean section were lighter (P <.05) at weaning than other calves but had similar slaughter weights. As mean winter te mperature increased (6.1 degrees C) from yr 1 to 3, calf birth weight decreased (4.6 kg) and calving difficulty decreased 23%. Results indic ate sire birth weight EPD, calf birth weight and shape, dam pelvic are a, and climate affected CDS, and CDS affected subsequent conception da te.