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

Abstract

Introduction: 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.
Materials & Methods: 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.
Results: 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.
Conclusion: 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.