The association between herd- and cow- level factors and somatic cell count of Irish dairy cows

Somatic cell count (SCC) is an indicator of both udder health and milk quality and is measured at an animal level through national milk recording schemes. The objective of this study was to assess the animal and herd factors contributing to elevated SCC (i.e. poorer milk quality). Test day records (n = 2,658,928) from 519,456 cow lactations obtained between 2007 and 2011 were included in the analyses. Herd factors tested included the geographical region of the herd and production system operated (spring calving or mixed calving system). Animal factors tested included breed, parity and age nested within parity. Four definitions of normalised SCC (i.e. SCS) were considered: 1) average test-day SCS within a 24 hour period (TD_SCS), 2) maximum SCS (peak_SCS), 3) minimum SCS (min_SCS), and 4) average SCS (avg_SCS) recorded across cow lactation; in addition, the proportion of test day records with an SCC count >200,000 (prop_200) or >250,000 (prop_250) within cow lactation were included. Following adjustment for fixed effects, average TD_SCS was 179,308 cells per mL while avg_SCS, and average min_SCS and peak_SCS were 119,481, 50,992 and 298,813 cells per mL, respectively. All animal and herd factors had a significant effect on SCC. Older animals, animals which were younger at calving than contemporaries and Holstein animals had higher SCC than younger alternative breed animals who calved at the median age. In addition, mixed calving production systems and herds in Connaught had higher SCC than spring calving herds in the other regions of Ireland.