Impact of early lactation somatic cell count in heifers on somatic cell counts over the first lactation

The objective of this study was to estimate the impact of somatic cell count in early lactation (SCCel) from Belgian dairy heifers on test-day somatic cell count (SCC) in first lactation. Geometric mean SCCel [5 to 14 d in milk (DIM)] of the 14,766 available samples was 104,000 cells/mL, and decreased from 178,000 at 5 DIM to 74,000 cells/mL at 14 DIM. Proportion of SCCel >200,000 cells/mL was 27.5. Heifers calving in the period April-June had highest SCCel. In total, 117,496 monthly SCC were measured. A multilevel regression analysis revealed that an increase of the natural log-transformed SCCel (LnSCCel) by one unit on average resulted in an increase of test-day natural log-transformed SCC (LnSCC) by 0.22 unit. The impact of LnSCCel on LnSCC depended on when LnSCCel was measured; an elevated LnSCCel at 14 DIM was more consequential than an equally elevated LnSCCel at 5 DIM. The probability of having a test-day SCC >200,000 cells/mL during the first lactation, also increased with an increasing LnSCCel. The negative effect of an elevated LnSCCel was still present, although to a lesser extent, in heifers with a second test-day SCC