Genetic analysis of lactation consistency using daily milk weights in U.S. Holsteins.

The ability of a dairy cow to perform similarly across time is clearly important and should be included in breeding programmes if feasible and heritable, since this would be linked to resilience. Consistency, defined as the quality of performing as expected each day of lactation, could be highly associated with resilience, where resilience is the animal’s ability to maintain health and performance in the presence of environmental challenges such as pathogens, heat waves and nutritional changes.

To investigate this possibility, the authors cited access 51 415 022 milk weights from data provided by 255 191 multi-lactation Holstein cows milked three times a day in conventional parlour systems across the USA. Milk yield consistency was measured from +5 to +305 days relative to calving and computed as the log-transformed variance of daily deviations between observed and expected milk weights. It was accepted that a higher consistency phenotype would imply greater variation, whereas larger deviations from expectations would indicate inconsistent performance. Expected values were obtained using three nonparametric and parametric regression models. The univariate statistical model included age at first calving and herd-year-season as fixed effects and cow as the random effect.

The heritability estimates (standard errors) of consistency phenotypes calculated over the entire lactation ranged between 0.227 (0.011) and 0.237 (0.011), suggesting that consistency is moderately heritable. The estimated genetic correlations between consistency traits and between lactations were high (>0.95), indicating consistency is repeatable across lactations and robust to the model used to compute expected daily milk yield. The correlations between predicted transmitting abilities (PTAs) for consistency and milk yield were 0.49, indicating that high-producing cows exhibit more day-to-day variation in performance. Correlations with productive life and livability were -0.33 and -0.39, respectively. Correlations between PTAs for consistency and those of post-calving health traits were also negative, ranging from -0.34 to -0.06, indicating that more consistent cows tended to have fewer health problems and greater longevity.

Overall, the results suggest that milk yield consistency can be used to select cows that maintain expected performance throughout their lactation, and which likely reflects greater resilience to management and environmental disturbances.