Genetic association between milk lactose, freezing point and somatic cell score in Italian Holstein cows

Milk data (n = 59,811) from 4,355 Holstein cows were used to estimate genetic parameters for lactose percentage (LP) and yield (LY), and other milk traits. Lactation curve of LP was very similar to that of milk yield and opposite to the lactation curve of somatic cell score (SCS). Heritability and repeatability of the investigated traits were assessed through univariate animal models, and genetic correlations with milk SCS and freezing point (FRP) were estimated using bivariate analyses. Fixed effects were herd-test-date, lactation stage, parity, calving season and the interaction between parity and stage of lactation. Random effects were animal, permanent environment and residual. Lactose percentage showed the highest estimates of heritability (0.44 ± 0.03) and repeatability (0.59 ± 0.01), whereas SCS and FRP showed the lowest estimates of heritability (0.12 ± 0.02 and 0.11 ± 0.01, respectively) and repeatability (0.26 ± 0.01 for both traits). Lactose percentage was moderately correlated with SCS both phenotypically (-0.34) and genetically (-0.25 ± 0.08), suggesting that there are opportunities for considering LP as a potential trait to indirectly select for resistance to mastitis along with SCS. Freezing point was negatively genetically correlated with LP (-0.46 ± 0.05). Further studies are required to evaluate correlated responses for all traits, before LP, LY and FRP can be considered to be part of the current selection index or breeding objective of Italian Holstein dairy cattle.