While monthly somatic cell count data are vitally important for mastitis control, remember that SCC values from DHIA are a composite of all four quarters. Because of dilution, these composite samples will always result in under-diagnosis of subclinical infections, cautions Pamela Ruegg, extension milk quality specialist at the University of Wisconsin.

For example, consider that a cow producing 40 pounds of milk  per milking — evenly distributed between four quarters — has one subclinically infected quarter. If the SCC from the three uninfected quarters is 100,000 cells per milliliter, the composite SCC value will not reach 200,000 cells per ml until SCC from the infected quarter exceeds 700,000 cells per ml.

To identify infected quarters, use a simple cow-side test for further screening of cows suspected of having subclinical mastitis, suggests Ruegg.