Changes in lactose content of milk as indicators of longevity and udder health in Holstein cows

Milk lactose percentage (LP) is inversely related with somatic cell count and intramammary inflammation in dairy cows. Moreover, LP decreases with parity, likely as a result of the accumulated effect of udder infections and mammary gland epithelial stress. Thus, changes of LP across parities could be evaluated as potential indicators of longevity (LON) and udder health (UDH). In the present study we defined 6 descriptors of LP changes across the first 3 lactations of 69,586 Holsteins and we estimated their genetic correlations with LON and UDH. At least 5 test-day LP were guaranteed in each lactation, thus ≥15 test-day LP were available for each cow. β12 was the slope of LP in the first 2 lactations, β23 was the slope of LP from lactation 2 to 3, and β123 was the overall slope of LP. The 3 lactation means were also calculated for each cow in order to derive LP deltas, i.e. between lactations 1 and 2 (Δ12), 2 and 3 (Δ23), and 1 and 3 (Δ13). Genetic parameters and breeding values were estimated through a linear animal model for the 6 descriptors and the Calo’s method was used to assess genetic correlations between traits. Results showed that cows with greater changes of LP (i.e. stronger and negative slopes, and greater and positive deltas) were characterised by impaired LON and worse UDH than cows with smaller changes of LP. The h2 of descriptors ranged from 0.11 to 0.19 for the slopes and from 0.06 to 0.20 for the deltas. Genetic correlations with LON were 0.19 (β12), 0.08 (β23), 0.52 (β123), -0.19 (Δ12), -0.02 (Δ23), and -0.11 (Δ13), while those with UDH were 0.15 (β12), 0.05 (β23), 0.33 (β123), -0.19 (Δ12), -0.15 (Δ23), and -0.06 (Δ13). Selection responses for the descriptors of LP were also calculated. For comparison, both slopes and deltas of milk yield were investigated. Overall, genetic correlations of milk yield descriptors with LON and UDH were stronger than those of LP descriptors with the same traits, whereas h2 were smaller. Despite this, milk yield descriptors were slightly more efficient than LP descriptors in terms of selection response for both LON and UDH