Variation in Somatic Cell Counts in Dairy Herd Improvement Milk Samples

Abstract

Information on variability of somatic cell numbers in Dairy Herd Improvement samples analyzed by the Filter-DNA method came from two field studies. Data consisted of 13,733 samples taken in 16 herds monthly for 2 yr in one survey and 6285 observations in 134 herds each sampled once in the other. Mean cell numbers in the studies were 692,000 and 625,000 cells per ml. Median cell number in the 16 herds was 390,000 cells per ml. Herd percentages of cows over 1 million, between 500,000 and 1 million, and below 500,000 cells per ml averaged 17.9, 20.5, and 61.6 with wide ranges. Data were transformed to a log scale and analyzed with two models, one involving herd, year, season, and age and another involving management practices. Mean differences among herds, years, seasons, age of cows, and stages of lactation were large in the 16-herd study. The model of management practices found no association of herd size and cell numbers. Herds with parlor milking systems that dipped teats and practiced selective dry cow treatment had the lowest mean cell numbers.