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This study investigated the potential value of traits measured in young bulls as genetic indicators for female reproductive performance in two tropical beef cattle breeds. Genetic parameters for the male traits percentage of normal sperm (PNS) and scrotal circumference (SC), and their genetic relationships with first (DTC1) second (DTC2) and all days to calving measures (maximum of six) (DTCall) were estimated in Brahman (BRAH) and Santa Gertrudis (SANTA) cattle from records submitted for BREEDPLAN evaluation. Estimated heritabilities for PNS and SC were 0.20±0.06 and 0.45±0.02 for BRAH and 0.17±0.05 and 0.43±0.02 for SANTA, respectively. Genetic correlations between PNS and DTC1, DTC2 and DTCall were -0.67±0.28, - 0.79±0.25 and -0.47±0.22 in BRAH and -0.18±0.21, -0.28±0.27 and -0.20±0.20 in SANTA, respectively. Genetic correlations of SC with DTC1, DTC2 and DTCall were -0.26±0.12, - 0.25±0.13 and -0.19±0.08 in BRAH and -0.02±0.09, -0.19±0.13 and 0.00±0.09 in SANTA. These results showed that PNS and SC measured at 18 to 24 months of age in young BRAH and SANTA bulls were moderately heritable and their genetic correlations with DTC were in the same direction in both breeds. PNS and SC had higher genetic correlations with early DTC measures compared to DTC measured in older cows, indicating that they may be more related to early reproduction than lifetime reproduction. In addition, PNS was more strongly related with all DTC measures than SC in both breeds, which suggests that PNS may be a better indicator trait than SC for improving female reproduction in tropical breeds in Northern Australia. |
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