Genetic analysis of calf and heifer losses in Danish Holstein.

Mortality in dairy cattle is not only relevant with regard to economic losses but also to animal health and welfare. Thus, the aim of this investigation was to explore the genetic background of postnatal mortality in calves and replacement heifers in different age groups until first calving in Danish Holsteins. Records of Danish Holstein heifer calves born in the years 1998 to 2007 were extracted from the Danish Cattle database (Danish Cattle, Skejby, Denmark). The following periods (P) were defined for analyses: P1=d 1 to 30, P2=d 31 to 180, P3=d 181 to 365, P4=d 366 until the day before first calving or a maximum age of 1,200 d if no calving was reported, and the full period P5=d 1 until the day before first calving or a maximum age of 1,200 d if no calving was reported. Records of animals slaughtered or exported within a defined period were set to missing for this and following periods, whereas their records were kept for preceding periods. After further data editing, more than 840,000 calves and heifers born in the years 1998 to 2007 were investigated. Mortality rates were 3.23, 2.66, 0.97, 1.92, and 9.36% for the defined periods P1 to P5, respectively. For the estimation of genetic parameters, linear and threshold sire models were applied. Effects accounted for were the random effects herd × year × season and sire as well as the fixed effects year × month, number of dam's parity (parities >5 were set to 5), calf size, and calving ease. In total, the pedigree consisted of 4,643 sires and 20,821 animals. Heritabilities for the linear model were low, ranging from 0.006 (P3) to 0.042 (P5). Heritabilities estimated by threshold models showed a wider range, from not significantly different from zero for periods with low frequencies to 0.082 for P1. The mortality rate until first calving was higher than the stillbirth rate. Genetic and phenotypic variation seemed to be sufficiently high to genetically improve the trait calf and heifer mortality. Hence, a routine genetic evaluation would be valuable for monitoring and for selecting fitter animals in the Danish Holstein cattle population.