Environmental Factors Influencing Test-Day Somatic Cell Counts in Holsteins

Abstract

Between Feb. and Dec., 1977, 133,493 test-day observations of somatic cell count were taken on 27,009 Holstein cows in 676 herds on the Quebec Dairy Herd Analysis Service. Data were transformed to a log (natural) scale, and analyses were separate within lactation age group (.ltoreq. 2, 3, 4, 5 and .gtoreq. 6 yr). Joint estimates of fixed effects of month of test, age of sample at time of laboratory analysis, stage of lactation, age of cow at calving (mo.) within lactation age (yr), and season of calving, and components of variance for herd, cows and error were obtained by maximum likelihood. Patterns of month of test, age of sample and stage of lactation were similar for all lactation ages. Somatic cell counts were lowest in May, rose through summer and autumn and peaked in Dec. As age of sample increased somatic cell count decreased. Somatic cell counts were highest shortly after freshening, declined rapidly to a nadir between 25 and 45 days in milk and rose throughout the remainder of lactation. Effects of age at calving within lactation age were generally small. Effects of season of calving were small and were significant only for 3 and .gtoreq. 6 yr olds. Within lactation, repeatability of somatic cell count ranged from 0.26 for .ltoreq. 2 yr olds to 0.40 for mature (.gtoreq. 6 yr) cows and averaged 0.35 over all ages.