Factors influencing dystocia and prediction of dystocia in Angus heifers selected for yearling growth rate

Data on 914 Angus heifers and their calves that had been selected for yearl ing growth rate (from birth to 1 year of age) were used to (i) evaluate the effect of dystocia on subsequent cow and calf performance, (ii) identify f actors influencing dystocia, and (iii) develop and validate equations for p redicting dystocia. Records on heifers born in the high, control, and low g rowth selection lines from 1975 to 1990 and their calves were used. Calving was classified as normal (no assistance), with a code of 0, or difficult ( requiring assistance), with a code of 1. The incidence of dystocia was 4.5% , 9.7%, and 6.2% in the high, control, and low lines, respectively, with th e difference between the high and control lines being significant (P < 0.05 ). Dystocia resulted in a significant reduction in calf survival to weaning (97% for normal v. 85% for difficult calvings). Of all the calving and pre -calving traits studied, the ratio of calf birth weight to heifer weight ac counted for the highest variation in dystocia in the control (19.6%), low ( 15.2%), and high (5.4%) lines. The total variation explained by models gene rated by stepwise regression and discriminant analyses ranged from 5.4% to 36.8%. Prediction equations developed by discriminant analysis using traits measured prior to calving had high accuracy of predicting normal calvings (72.6%-90.3%), when tested on independent data sets. However, the sensitivi ty (ability to identify those heifers having difficult birth) of the equati ons was low (0%-40%). These results imply that the approach to be used to r educe dystocia and minimise its effects is to develop an overall management strategy involving both planning of mate selection at joining and supervis ion at calving.