FACTORS AFFECTING DYSTOCIA IN BRAHMAN-CROSS HEIFERS IN SUBTROPICAL SOUTHEASTERN UNITED-STATES

This study was conducted to determine relative relationships among fac tors affecting dystocia in Brahman-cross heifers. Body and pelvic meas urements were obtained in mid-June (when heifers were approximately 17 mo old), 45 d after a 60-d breeding season. Heifers studied were 207 Red Brangus, 209 Simbrah, and 250 Braford bred to Black Angus bulls; c alving began on December 1. Heifers grazed stockpiled bahia and hemart hria grass or ryegrass supplemented with 0.9 kg of 32% protein cottons eed meal and 1.8 kg of mill-run black strap molasses daily during calv ing. Calvings were scored (1 = no difficulty to 4 = major difficulty). A random sample of birth weights were obtained on 131 and 210 calves in 1992 and 1993. Data were analyzed within year by SAS procedures. Br eed differences in dam size and pelvic measurements were highly signif icant. Residual correlations between body weight and pelvic area were .20 and .35 (both P <.01) for 1991 and 1992. Dystocia incidence was 6. 9% in 1992 and 10.5% in 1993, with higher incidence in males than in f emales (1992, 10.6 vs 3.1%, P = .08; 1993, 15.6 vs 4.2%, P <.01, male vs female, respectively). Male birth weight exceeded (P <.01) that of females (26.7 vs 24.3 kg, 1992; 28.1 vs 26.1 kg, 1993). Correlations a mong dam size and calf birth weight were not significant. Birth weight was significantly correlated with dystocia score (.19 and .49, 1992 a nd 1993). Path analyses of influences on dystocia found birth weight, dam heart girth, and body weight significant in 1992, but only birth w eight significant in 1993. Regression of calving score on birth weight was curvilinear and significant in both years, with the birth weight inflection point at 22 kg. We conclude that dam body size data obtaine d following the breeding season were of minor value in determining dys tocia, but birth weight was consistently important.