LAUSR

Data collected from Australian Holstein cows that calved between 1995 and 2016 were used for estimating genetic parameters and genetic evaluation of gestation length (GL). Genetic parameters were estimated using a sire maternal grandsire model considering GL in heifers as correlated, but a different trait to adult cows and animal repeatability model. The key objective of this study was to assess if selective mating of bulls with short GL estimated breeding values (EBV) can help to modify calving patterns in predominantly pasture-based production systems and thereby contribute to the reduction of calving induction. The mean GL in heifers and adult cows was 280 and 281 d, respectively. The heritability of direct GL was 0.28 in heifers, which is slightly lower than in adult cows (0.36). The maternal heritability of GL was close to 0.04 in both heifer and adult cows. The genetic correlation between direct effects in heifers and adult cows was lower (0.88) than between maternal effects (0.94). A genetic evaluation model that considered heifer and adult cow data as the same trait in a repeatability animal model showed adequate variability in EBV for both direct and maternal GL. The genetic trend in direct GL EBV declined from 2005 in bulls and from 2006 in cows. Selective mating of bulls with short direct GL EBV showed that the GL and calving interval of their mates can be modified by up to 3.5 d for GL and 2.0 d for calving interval. Relative to parent average, the use of genotype data to calculate genomic EBV increased the reliability of EBV by up to 11% in validation bulls when daughter trait deviation of bulls with trait deviation of cows (11%) and deregressed breeding values (8%) were used as response variables. On average, for animals with only genotype data, the GL EBV can be predicted with about 50 to 60% reliability (expected) depending on the response variable (deregressed breeding values or daughter trait deviation and trait deviation) and the size of reference set. Overall, the results of this study show that calving patterns can be made tighter by selectively mating short GL EBV bulls to cows that do not become pregnant early in the mating season. Additionally, better reproductive management and the use of bulls with high female fertility EBV are still crucial for the success of a pasture-based dairy production system.