Pf. Arthur et al., Factors influencing dystocia and prediction of dystocia in Angus heifers selected for yearling growth rate, AUST J AGR, 51(1), 1999, pp. 147-153
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.