GENETIC PARAMETERS FOR DAYS TO CALVING, CALVING DATA AND CALVING SUCCESS OF EXPERIMENTAL HERDS OF NELORE CATTLE

Data from three experimental herds of Nelore cattle were analyzed to estimate genetic parameters for three fertility traits of cows: days to calving (DIAP), calving date (DATAP) and calving success (SP). The 7,951 records of 1,575 cows were obtained from 1978 to 2000 and up to 12 records by animal were available. DIAP was computed as the interval between calving date and the date of the beginning of the breeding season, DATAP was computed designing 1 to the first calving of the year and then successively, and non-calved cows received a predicted record according to higher observed record within breeding group. SP was considered as binomial trait (one for calving and zero for failure to calve). The three traits were analysed assuming a repeated model in single-trait analysis. The model included additive genetic effects, permanent environment and error as random, year of the beginning of the breeding season, herd, age, previous reproductive status and cow weight at the beginning of the breeding season as fixed effects. The variance components were estimated using REML methodology for DIAP and DATAP and quasi-REML in threshold model for SP. Estimates of heritabilities were 0.15 +/- 0.02; 0.16 +/- 0.02 and 0.19 +/- 0.03 for DIAP, DATAP and SP, respectively. The rank correlations between the expected breeding values for the DIAP and SP were -0.89 for sires and -0.90 for all animals. The high and desirable correlation between the expected breeding values for DIAP and SP, together with their similar heritabilities, suggests a quite similarity between both traits, and the decision of what trait to choose for the reproductive evaluation of the cows will depend on available information in the record systems.