Genetic parameters for calving difficulty using complex genetic models in five beef breeds in Australia

Data on Angus (ANG), Charolais (CHA), Hereford (HER), Limousin (LIM) and Simmental (SIM) cattle were used to estimate genetic parameters for calving difficulty (CD), birthweight (BWT) and gestation length (GL) using threshold-linear models and to examine the effect of inclusion of random effect of sire x herd interaction (SxH) in the models. For models without SxH, estimated heritabilities for direct genetic effect of CD were 0.24 (+/- 0.02), 0.22 (+/- 0.04), 0.31 (+/- 0.02), 0.22 (+/- 0.04) and 0.17 (+/- 0.01) for ANG, CHA, HER, LIM and SIM, respectively, whereas maternal heritabilities ranged from 0.13 to 0.20. Estimated heritabilities for direct genetic effect of BWT were 0.38 (+/- 0.01), 0.37 (+/- 0.03), 0.46 (+/- 0.01), 0.35 (+/- 0.02) and 0.36 (+/- 0.01) for ANG, CHR, HER, LIM and SIM, respectively, whereas maternal heritabilities ranged from 0.08 to 0.11. Estimated heritabilities for direct genetic effect of GL were 0.59 (+/- 0.02), 0.42 (+/- 0.04), 0.50 (+/- 0.03), 0.45 (+/- 0.04) and 0.42 (+/- 0.03) for ANG, CHR, HER, LIM and SIM, respectively, whereas maternal heritabilities ranged from 0.03 to 0.09. Genetic correlations between direct genetic effects of CD with BWT were highly positive and with GL were moderately positive for all five breeds. Estimated genetic correlations between direct genetic effects and maternal genetic effects (r(dm)) ranged across the five breeds from -0.40 (+/- 0.05) to -0.16 (+/- 0.02), -0.41 (+/- 0.03) to -0.27 (+/- 0.08) and -0.47 (+/- 0.10) to -0.06 (+/- 0.12) for BWT, GL and CD, respectively. Fitting SxH interaction as additional random effect significantly increased the log-likelihood for analyses of BWT, GL and CD of all breeds, except for GL of CHA. The estimated heritabilities were less than or equal to the estimates obtained with models omitting SxH. The r(dm) increased (i.e. became less negative) for BWT, GL and CD of all five breeds. However, the increase for GL was not substantially high in comparison to the increase observed for BWT and CD. Genetic parameters obtained for BWT, GL and CD, by fitting SxH as an additional random effect, are more appropriate to use in the genetic evaluation of calving ease in BREEDPLAN.