A method is described to obtain approximate reliabilities for breeding values calculated with a multiple-lactation random regression test day model. The method was based on a concept of an equivalent number of progeny for animals with records, which is further used to derive reliabilities for related animals. The procedure accounted for the size of contemporary groups. Approximate reliabilities were calculated for estimated breeding values of the average milk yield in the first three lactations, using the data of daily milk yield of Polish Black-and-White cows in randomly selected herds. Results of approximation were analysed by comparison with corresponding exact reliabilities, obtained from the inverse of the coefficients matrix of the mixed model equations. High correlation of 0.98 for bulls between reliabilities from both methods and very low computer requirements facilitate the implementation of the approximate method into a routine genetic evaluation procedure.
In the present paper the heritabilities and repeatabilities of difficult calvings in Black and White cows are estimated. The highest heritability (0,135) was found for the first calving and a slightly lower one for the second and third. The lowest (0,058) heritability was noted for the fourth and later perities. This result indicates the possibility of an effective selection of bulls for the direct effect on calving difficulty.
The purpose of the present paper was to estimate the frequencies of difficult calvings and calf mortality in Black and White and Red and White cattle. Calvings were recorded for 24,045 Black and White and 5,353 Red and White cows. Dystocia was more frequent in Red and White heifers (15.5%) than in Black and White heifers (9.2%). In cows the frequency of difficult calvings decreased by half. The highest calf mortality was found in Black and White heifers (11.4%) and Red and White heifers (9.6%). In older cows the mortality was much lower.
The study investigated the existence of heterogeneous variance in first-lactation daily milk yield of Polish Black-and-White cows across herds in different years. Bayesian Information Criterion was used to show that the model with unequal residual variances for different herd-years was more plausible than the model assuming equal variances. A method of adjusting phenotypic records was developed to account for unequal variability in herd-years. Factors used for the data adjustment considered variation of general residuals and residuals for specific herd-years. The size of herd-year was also taken into account. Varied power of corrections was used to analyze the effect of adjustment on estimated breeding values. The method was applied to daily milk records of 817 165 primiparous cows. The effectiveness of the data adjustment was evaluated by the analysis of differences between each bull’s breeding value and its parental index. Data correction reduced the average difference and variance of differences between breeding values and parental indices. Accounting for the size of herd-year classes in correction factors improved the efficiency of heterogeneous variance adjustment.