Inheritance of somatic cell count and its genetic relationship with milk yield in different parities

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During the introduction of a cell count program, monthly somatic cell counts were collected between March 1985 and June 1986 for 3376 Friesian cows in 86 herds in The Netherlands. There were 977 cows in first parity, representing 42 sires; 718 cows in second parity, representing 28 sires; 1555 cows in third or greater parities, representing 30 sires. The objectives of this study were to estimate, in different parities, the heritabilities and repeatabilities of somatic cell count and the genetic relationships between somatic cell count and milk yield. Heritabilities for test-day log somatic cell counts (LSCC) and log total somatic cell counts (LTSCC) were low in first parity (0.03 and 0.06), higher in second parity (0.20 and 0.19) and lower in later parities (0.11 and 0.15). Repeatabilities for LSCC and LTSCC were 0.39 and 0.41 in Parity 1, 0.44 and 0.44 in Parity 2 and 0.46 and 0.47 in later parities. Heritabilities for lactation averages of LSCC (ALSCC) and LTSCC (ALTSCC) showed a similar trend over parities: low in first parity (0.07 and 0.08), higher in second parity (0.39 and 0.11) and lower in later parities (0.24 and 0.22). It is hypothesized that variation in somatic cell counts in first parity is due mainly to normal variation, whereas variations in second and later parities are largely due to incidental factors, such as mastitis infections. This might explain the increase in heritability and the change in genetic correlation from first to second parity. Consequently, selection against mastitis can be based on cell count observations in second lactation.