Genetic analysis of somatic cell count and milk traits in Manchega ewes -: Mean lactation and test-day approaches

A total of 36 873 lactation records of Somatic Cell Counts (SCC) and 32 844 of milk yield, protein and dry matter contents of 28 694 Spanish Manchega ewes from 118 herds were used to estimate genetic parameters under different approaches. In a first step estimations were carried out under multivariate animal models including lactation mean somatic cell score (SCSL) and other milk traits (milk yield and protein and dry matter contents). These analysis were made within 1st, 2nd and 3rd lactations separately and also with multiple lactations under repeatability and multivariate animal models, considering SCSL as repeated measures of the same trait or as different traits among lactations. In a second step, a Test Day approach was analysed also within and among lactations. SCS test day were considered as different traits within lactations and as repeated measures of the same trait within and across lactations by fitting two permanent environmental effects. Heritability estimates of SCSL and SCS test day under models without repeatability were higher (0.12-0.24) than estimates obtained when permanent environmental effect was fitted (0.04). This fact also happened for the other milk traits. Genetic correlations between SCS test day in all lactations were high (0.78-0.99), showing similar genetic basis of this trait within lactations. Genetic correlations between SCSL in different lactations were also high (0.97-0.99), suggesting similar behavior of this trait across lactations. Genetic correlation between SCSL and milk yield was low and negative (-0.12 to -0.16), showing a slightly favourable genetic relationship between these traits, however was positive between SCSL and protein content (0.22). Positive genetic correlations between SCSL and milk yield found in cattle and other sheep breeds could be due to models, population's structure and selection process of breeds. Thus, in breeds with a long history of selection to improve milk yield, a fixation of alleles that diminish mastitis resistance could occur. (C) 2003 Elsevier B.V. All rights reserved.