Estimation of genetic parameters of somatic cell score and longevity in Iranian Holstein cows

Profitability in dairy herds depend on animal survival in the herd, which is provided by reducing involuntary culling and increasing the voluntary culling in the herd. The aim of this study was to investigate the non-genetic factors affecting longevity and to estimate the genetic parameters of longevity and somatic cell score in Iranian Holstein cows. For this purpose, the data of 277715 cattle that were collected by Animal Breeding Center of Iran during 2001 to 2018, were used. Longevity was defined as length of life (LL), lifetime number of days in milk (LDIM), lifetime milk yield (LMY), lifetime fat yield (LFY), and lifetime protein yield (LPY), and the effect of environmental factors on traits and risk of culling were determined using Survival kit and cmprsk statistical packages. Variance components of longevity traits were estimated based on exponential distribution and censored data along with somatic cell score trait using two-trait analysis by Gibbs sampling method. The results showed that the effect of herd, year and season of calving and calving age of cows was significant on all longevity traits (P <0.001). Cows with a high degree of dystocia had a higher culling risk than other cows and the culling risk was higher for cows born in the spring than in other seasons (except to the LL). The range of heritability for different longevity traits varied from 0.076 to 0.186 and the heritability of the somatic cell score was estimated to be 0.31. The results showed that genetic selection for somatic cell score and some longevity traits such as LPY can be effective in improving these traits, although due to the importance of environmental factors on longevity traits, these factors should also be considered. Genetic and residual correlation between somatic cell score with longevity traits were negative and the amount of genetic correlation between traits were higher than their residual correlation. Therefore, genetic selection for somatic cell score and considering it as a selection criterion in breeding programs can indirectly improve the longevity traits in Iranian Holstein cows. Increasing and improvement of longevity is not only economically important but also improves animal welfare.