Additive genetic and permanent environmental correlation between different parts of lactation in moderate and cold regions

The objective of this study was to investigate genotype by environment interaction as well as genetic parameters for somatic cell score (SCS) in first lactation Holstein cows in Iran. Data were collected by the National Animal Breeding Centre of Iran during 2003 to 2018. Data consisted of 1 031 885 SCS test-day records of 145 817 first lactation Holstein cows. Records were classified into the cold and moderate climate on data of synoptic stations. Variance components and genetic parameters were estimated using a random regression model and restricted maximum likelihood method. The analyses were performed using the AI-REML algorithm of the DMU package. The mean SCS was 1.859 (1.598) and 1.823 (1.522) for the cold and moderate climate, respectively. Genetic variance of SCS was lower than the corresponding permanent environmental variance. The permanent environmental variance in the moderate climate was higher than that of the cold climate during lactation. The highest values of heritability were observed in the early stage of lactation. However, the estimates of heritability during the lactation curve were low. The estimates of heritability for the entire 305-day lactation were higher in the cold climate than those in the moderate climate. Genetic correlation between the cold and moderate climate ranged from 0.25 to 0.81. The results indicated the existence of genotype × environment interaction and hence the need for different breeding program for SCS in the studied climates.