The research has been carried out with cows of the Bulgarian black-and-white cattle in 10 farms from various regions in the country. The cows calved in the period 1995 – 2006. A total of 1460 cows with 2852 lactations from І-st to ІІІ-rd and respectively 27228 controls have been included. The cows are daughters of 125 sires. Heritability for age at first calving is 0.37 and its values for test day milk yield and % of fat are 0.10 and 0.08, respectively. Both phenotypic and genetic correlations between the age at first calving and test day productivity have values around 0. The genetic correlation between the age at first calving and test day milk yield is very low and has positive value r = 0.07 whereas with % of fat it is negative r = –0.09. The age at first g g calving of cows has to be included in the models for evaluation of the breeding value of dairy cattle as a factor for obtaining more precise scores.
Zh. Gergovska, L. Yordanova