Estimation of Genetic Parameters and Trend for Somatic Cell Score Trait in Iranian Holsteins Using a Random Regression Test Day Model

Abstract

In this study, a total of 101,147 monthly test day records of somatic cell count collected from 13,977 first lactation Holstein cows (in 183 herds) calved between 2002 and 2006 were used. For the genetic analysis, a random regression test day model was utilized. Somatic cell score (SCS) was calculated based on natural logarithm of somatic cell count. In the model, fixed effect of contemporary group of province-herd-year-production season, as well as age at recording (as linear and quadratic covariables), Holstein gene percentage (as linear and quadratic covariables) were fitted. Orthogonal Legendre polynomial function with third order was applied to take into account of the stage of the lactation and genetic and permanent environmental variations of SCS. Genetic and environmental (co)variance components were estimated using restricted maximum likelihood method. In general, heritability of somatic cell score was found to be lower than 10% over the course of the lactation period. The lowest and highest heritability estimates were found for the second (0.03) and tenth (0.068) months of the lactation. Genetic and permanent correlations among the months of lactation decreased as the interval between them increased. Genetic trend was estimated based on linear regression of average breeding value on calving year. No statistical significant genetic trend was found for somatic cell score.