Variability of genetic correlations of milk yield and fertility traits in Simmental cows in different regions of Serbia

The Simmental breed of cattle is mostly reared in the central part of the Republic of Serbia, where it makes up about 80% of all breeds. In areas of more intensive cattle production, populations of cattle with pronounced milk yield are reared. In more extensive, as well as hilly and mountainous areas, somewhat less productive animals are raised. The main goal of this study was to examine the variability of genetic correlations of milk and fertility traits on the farms of individual agricultural producers using modern methods, depending on the breeding area, i.e. the region in which they are bred and reared. This study included 2589 controlled Simmental heifers, with lactations concluded during one year. All first calving heifers were housed and reared on agricultural family farms in the area of Central Serbia. The paper examines genetic correlations between the following traits of milk yield and fertility: duration of lactation, milk yield in standard lactation, milk fat content in standard lactation, milk fat yield in standard lactation, yield of 4% FCM in standard lactation, age at first calving, duration of service period. The results of the study of genetic correlations were obtained using mixed LSMLMW models (Harvey 1990). The examined genetic correlations of milk yield and fertility traits in Simmental cows showed pronounced variability depending on the breeding area where the cows are reared.