Heifers infected with coagulase-negative staphylococci in early lactation have fewer cases of clinical mastitis and higher milk production in their first lactation than noninfected heifers.

Intramammary infections (IMI) in recently calved dairy heifers are more common than was formerly believed but their relevance for future performance has been studied only rarely. In the present study, the association between the IMI status of fresh heifers and their subsequent udder health, milk production, and culling in first lactation was explored. Quarter milk samples were collected between 1 and 4 d in milk (DIM) and between 5 and 8 DIM from 191 dairy heifers in 20 dairy herds for bacteriological culturing and somatic cell count (SCC) analysis. Monthly milk recording data including composite milk SCC and test-day milk yield (MY) were obtained for the first 285 DIM or until culling. Farmer-recorded clinical mastitis cases were available. Data were analyzed using mixed models and survival analysis. Approximately 80% of the fresh heifers (79.8%) had at least one culture-positive quarter. Coagulase-negative staphylococci (CNS) were the most frequently isolated pathogens (72%), followed by esculin-positive streptococci (4.6%) and Staphylococcus aureus (3.5%). Overall geometric mean SCC at quarter level decreased between the first and second samplings from 348,000 to 116,000 cells/mL. Heifers infected with CNS had an intermediate average test-day SCC (84,000 cells/mL) during the first 285 DIM compared with noninfected heifers (53,000 cells/mL) and heifers infected with major pathogens (195,000 cells/mL). Heifers infected with major pathogens had a much lower average daily MY (18.3 kg) during first lactation compared with noninfected animals (21.3 kg). That CNS-infected heifers out-produced their noninfected counterparts could be at least partially explained by their significantly lower incidence of clinical mastitis (incidence risk 3.6 vs. 21.0%) during first lactation compared with noninfected heifers. We conclude that although CNS cause the majority of IMI in heifers around calving, they should not be a reason for serious concern