EN
The effect of inbreeding on the age of the first calving and on the length of the first and second intercalving periods in red-white cows was determined by means of linear and parabolic regression. To eliminate the effect of sires on the evaluation of inbreeding depression, the linear and parabolic regression coefficients were calculated within the groups of sires. All the calculations were made for the herds of low, medium and high production levels as well as for all three levels togetheri In the population studied, considerable frequency of inbreeding resulted: 9,5% of all the heifers and 15,7% of the older cows were offsprings of related parents. The average inbreeding coefficient for 14743 cows was 0,8% and 6,9% in the inbred group. The inbred cows calved 0,4 month later and their first and second intercalving periods were 0,24 and 0,16 longer than in the outbred group. The inbreeding depression was observed mainly in the low productivity of herds; this tendency proved to be curvilinear. The survival rate of the inbred group was - until the end of the second and third lactation - considerably higher than in the non-inbred group.