AN EVALUATION OF THE VALIDITY OF INDIVIDUAL COW SOMATIC-CELL COUNTS FROM COWS IN EARLY LACTATION

Individual cow somatic cell counts (SCC) from the first 4 weeks of lactation were obtained from all cows on a cell-counting program in Prince Edward Island for a period of 4 years. The prevalence of elevated counts (over 200 000 cells ml-1) declined sharply in heifers (primiparous animals) from Day 5 to Day 10 of lactation, and then gradually from Day 10 to Day 28. For multiparous cows, the end of the steep decline occurred at approximately 12 days post-calving. Similar patterns were observed in the arithmetic and geometric mean SCC. The bias associated with including the post-calving SCC (i.e. SCC from the first 9-11 days) in estimates of the prevalence of elevated counts in early lactation was determined. The bias was most serious in herds with a low true prevalence of elevated counts, and was substantial if the early lactation period was defined to be 40 days or less. It is recommended that SCC from heifers in the first 9 days of lactation and from multiparous cows in the first 11 days be discarded before estimates of the early lactation prevalence of elevated SCC are determined.