Explaining trends in bulk milk somatic cell count from dairy herds in the Netherlands from 2018 to 2023

In the Netherlands, bulk milk somatic cell count (BMSCC) has decreased steadily from 2002 to 2018. From 2018 onward, the BMSCC has increased again, with a steep increase in 2022. This is an unfavorable development, and therefore, the aim of this study was to retrospectively explain the increase in BMSCC from 2018 onward. A multivariable (population averaged) regression model has been used to find associations between BMSCC and explanatory variables such as herd characteristics. The BMSCC showed a clear seasonal effect, being highest in summer. The increase in BMSCC over the whole study period can be explained by an increase in the percentage of herds using an automated milking system, an increase in the average age of milking cows, and an increase in herd size. These factors exemplify the sudden change in 2022, and are therefore less likely to explain the sudden steep increase in BMSCC, as was observed in 2022. The increase of average milk price from €30/100 kg to €60/100 kg in 2022 partly explained the steep increase in BMSCC. In addition, a change in the calibration reference of the test for BMSCC at the Dutch Milk Quality Laboratory (Qlip, Zutphen) to the European international test calibration reference from January 1, 2022, also partly explained the steep increase in BMSCC in 2022. Understanding the possible causes for the unfavorable trend in BMSCC can help farmers and veterinarians with measures to decrease the BMSCC of their own herd.