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 lac tation were obtained from all cows on a cell-counting program in Princ e Edward Island for a period of 4 years. The prevalence of elevated co unts (over 200 000 cells ml-1) declined sharply in heifers (primiparou s 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 o ccurred at approximately 12 days post-calving. Similar patterns were o bserved 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 pr evalence of elevated counts, and was substantial if the early lactatio n 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 co ws in the first 11 days be discarded before estimates of the early lac tation prevalence of elevated SCC are determined.