Comparison of models for estimating genetic parameters and predicting breeding values for birth weight and calving ease in Czech Charolais cattle

The most appropriate model for genetic parameters estimation for calving ease and birth weight in beef cattle was selected. A total of 27 402 field records were available from the Czech Charolais breed. For estimation of genetic parameters for calving ease and body weight, three bivariate models were tested: a linear-linear animal model (L-LM) with calving ease classified into four categories (1 - easy; 2-4 - most difficult), a linear-linear animal model (SC-LM) in which calving ease scores were transformed into Snell scores (Snell 1964) and expressed as percentage of assisted calving (ranging 0-100%), and a bivariate threshold-linear animal model (T-LM) with calving ease classified into four categories (1 - easy, 2-4 - most difficult). All tested models included fixed effects for contemporary group (herd x year x season), age of dam, sex and breed of a calf. Random effects included direct and maternal genetic effects, maternal permanent environmental effect, and residual error. Direct heritability estimates for calving ease and birth weight were, with the use of L-LM, SC-LM, and T-LM, from 0.096 +/- 0.013 to 0.226 +/- 0.024 and from 0.210 +/- 0.024 to 0.225 +/- 0.026, respectively. Maternal heritability estimates for calving ease and birth weight were, with the use of L-LM, SC-LM, and T-LM, from 0.060 +/- 0.031 to 0.104 +/- 0.125 and from 0.074 +/- 0.041 to 0.075 +/- 0.040, respectively. Genetic correlations of direct calving ease with direct birth weight ranged from 0.46 +/- 0.06 to 0.50 +/- 0.06 for all tested models; whereas maternal genetic correlations between these two traits ranged from 0.24 +/- 0.17 to 0.25 +/- 0.53. Correlations between direct and maternal genetic effects within-trait were negative and substantial for all tested models (ranging from -0.574 +/- 0.125 to -0.680 +/- 0.141 for calving ease and from -0.553 +/- 0.122 to -0.558 +/- 0.118 for birth weight, respectively), illustrating the importance of including this parameter in calving ease evaluations. Results indicate that any of the tested models could be used to reliably estimate genetic parameters for calving ease for beef cattle in the Czech Republic. However, because of advantages in computation time and practical considerations, genetic analysis using SC-LM (transformed data) is recommended.