Genetic parameters and calving ability index for direct and maternal calving difficulty and stillbirth in Iranian Holstein cows

A total of 72,124 calving difficulty and stillbirth records of Iranian Holstein cows in 15 herds that were collected, from 1981 to 2005, and used to estimate genetic parameters and also constructing a calving ability index. A maternal effect model was used for genetic parameter estimation. In order to construct calving ability index, direct and maternal calving difficulty and stillbirth were included in aggregate genotype. Estimated heritability for maternal calving difficulty and direct calving difficulty and stillbirth were 0.012, 0.041 and 0.066, respectively. Genetic correlation of stillbirth with maternal and direct calving difficulty was 0.22 and 0.10, respectively. The genetic correlation between direct and maternal calving difficulty was −0.43. Expected genetic gains for direct and maternal calving difficulty and stillbirth using first calving ability index, that included all traits in aggregate genotype were −0.335,−0.028 and −0.0054, in liability scale and −7,−0.5 and −0.4 percent in observed scale, respectively. Expected genetic gains for direct and maternal calving difficulty and stillbirth in a second calving ability index that included only direct calving difficulty were −0.375,0.13 and −0.0027 in liability scale and −7.2,+4 and −0.3 percent in observed scale, respectively. Selection based on direct calving difficulty cannot reduce calving difficulty in population; therefore in order to reduce calving difficulty selection should be based on both direct and maternal calving difficulties.