Factors Affecting Milk Somatic Cell Counts and Its Parameter Estimation in Holstein of Khorasan Razavi Province by Random Regression

Introduction Mastitis is recognized as one of the most costly diseases of the dairy industry. Somatic cell count (SCC) is a widely used indicator trait for mastitis, which in many countries is used as an indirect selection criterion for improving mastitis resistance. This is based on the positive genetic correlation between clinical mastitis (CM) and SCC. Recent estimates of the genetic correlation between mastitis and SCC range between 0.53 and 0.77 and the heritability is estimated between 0.05 and 0.31. Individual test day SCC are influenced by systematic environmental effects, such as stage of lactation, lactation number, season of calving, and season of recording. The aim of this study was to investigate factors affecting milk somatic cell score (SCC) and estimation of its heritability and correlation with the other productive traits.
Materials and Methods A total of 185688 test day records of production and pedigree from 54 herds in Khorasan Razavi State were used. To normalize SCC, log transformation of natural records were considered from cows, which had at least one calving up to 10th. Environmental effects such as herdyear of record (247 levels), stage of lactation (15 levels), lactation number (10 levels), month of recording (12 levels), year of calving (9 levels) and season of calving (4 levels), and the 1st and 2nd regression coefficient of milk production were included in the model. Heritability of SCS was estimated by animal model test-day random regression and correlations between traits were analyzed by bivariate animal model of Wombat. Fixed effects in model included herdyear of record, Holstein gene percentage, and stage of lactation as covariate, as well as additive genetic effect and common permanent environmental effects as random effects with 5-degree of Legendre.
Results and Discussion Analysis of variance revealed that all factors in models were significant (P