Estimation of genetic and environmental parameters of milk production traits in Slovak purebred sheep using test-day model

Abstract

Covariance matrices and genetic parameters for milk production traits in sheep were estimated for the Improved Valachian and Tsigai breeds. These multipurpose breeds are of local importance, and are bred for milk, meat and wool. The traits under study were daily milk yield and fat and protein content. Genetic analyses were based on test-day records gathered over the period from 1995 to 2001. Each breed was analyzed separately. Only purebred animals with at least four test days per lactation were considered. The data set for the Improved Valachian consisted of 117,959 test-day records of 28,753 lactations belonging to 18,867 ewes. The corresponding pedigree comprised information on 32,804 animals. For the Tsigai breed, the data set consisted of 52,191 test-day records of 12,382 lactations belonging to 8,132 ewes and the pedigree comprised 14,576 animals. The three-trait animal model was used for analyses. To account for different lactation stage, a fixed regression on individual test days was employed. In addition to the function describing, effects of days in milk, effects of lactation, litter size, number of milkings per day, age at lambing, and flock-test day as well as additive genetic and permanent environmental effect of the animal were included in the model. Of these, flock-test day, additive genetic and permanent environmental effects were treated as random. Restricted maximum likelihood (REML) algorithm was used to estimate genetic parameters and covariance matrices. Heritabilities were 0.10 and 0.19 for daily milk yield, 0.06 and 0.12 for fat content, 0.07 and 0.17 for protein content for the Improved Valachian and Tsigai breeds, respectively. Genetic correlations between milk yield and fat content for the two breeds were −0.29 and −0.23 and those between milk yield and protein content were −0.30 and −0.27. Genetic correlations between fat and protein content were 0.58 and 0.57. Higher values (either positive or negative) were found for the breed with the larger number of records. Variance ratios and correlations were also estimated for nongenetic effects for both breeds. Variance ratios for the flock-test-day effects were rather high, ranging from 0.34 to 0.48. Higher values for these effects were found in the Improved Valachian sheep, reflecting more variability in management conditions in this breed. The genetic parameters estimated in this study should be used in routine genetic evaluation of milk production traits of local sheep breeds to replace present selection criteria based on phenotype values of dam milk yield.