Genetic (co)variances for calving difficulty score in composite and parental populations of beef cattle: I. Calving difficulty score, birth weight, weaning weight, and postweaning gain

Heritability of a-yr-old heifer calving difficulty score was estimated in n ine purebred and three composite populations with a total of 5,986 calving difficulty scores from 520 sires and 388 maternal grandsires. Estimates wer e 0.43 for direct (calf) genetic effects and 0.23 for maternal (heifer) gen etic effects. The correlation between direct and maternal effects was -0.26 . Direct effects were strongly positively correlated with birth weight and moderately correlated with 200-d weight and postweaning gain. Smaller negat ive correlations of maternal calving difficulty with direct effects of birt h weight, weaning weight, and postweaning gain were estimated. Calving diff iculty was scored from 1 to 7. Predicted heritabilities using seven optimal scores were similar to those using four scores, The predicted heritability using only two categories was reduced 23%. Phenotypic and direct genetic v ariance increased with increasing average population calving difficulty sco re. The estimated direct and maternal heritabilities for 2-yr-old calving d ifficulty score were larger than many literature estimates. These estimates suggested substantial variance for direct and maternal genetic effects. Th e direct effects of 2-yr-old calving difficulty score seemed to be much mor e closely tied to birth weight than were maternal effects.