Genetic parameter estimates in the South African Jersey breed

Data obtained from the INTERGIS for Jersey breeders participating in the Na tional Dairy Cattle Performance and Progeny Testing Scheme were analysed. T he registered and grade records commenced in test year 1977 through 1992 (i ncomplete) and consisting of lactations between 240 and 300 days, were cons idered as normal completed records. Five production traits were evaluated, ie. milk, fat and protein yields and fat and protein percentages for first and second lactation records. Variance components and resulting heritabilit y estimates were obtained by DFREML procedures using 45 240 first and 27 41 4 second lactation records. The heritability estimates for first lactation milk, fat and protein yields and fat and protein percentages were 0.35, 0.3 5, 0.34, 0.57 and 0.58 respectively. Corresponding estimates for second lac tation traits were 0.29, 0.28, 0.28, 0.53 and 0.56. Repeatability estimates , calculated as interclass correlations when only two records per animal ar e available, were 0.73, 0.72, 0.74, 0.71 and 0.74, respectively, for the pr oduction traits considered. The heritability estimates were in agreement wi th literature results, indicating that selection for all five traits would be effective. It was suggested that an increase in environmental effects ma y be partly responsible for lower estimates of heritability of second lacta tion traits. The results indicate that selection of dairy bulls can be base d on first parity records.