Herd and state means for somatic cell count from dairy herd improvement.

To determine the impact of reducing the current legal limit of 750,000 cells/ml for somatic cell count (SCC) in US market milk, data were examined from 539,577 herd test days for Dairy Herd Improvement herds on test during 1996 and 1997. Somatic cell scores for individual cows were converted to SCC. The SCC for each cow was weighted by milk yield and used to compute herd mean on test day. The mean for each state was derived by weighting herd test-day SCC by herd test-day milk yield. State means were lowest in the West and highest in the Southeast. The percentage of herd test days with an SCC of >750,000 cells/ml ranged from 0 to 14% across states; the mean was 4%. Only 1% of the herd tests were >750,000 cells/ml on 2 consecutive test days. Mean SCC in the United States was 307,100 cells/ml for 1996 and 313,500 cells/ml for 1997. Mean SCC was lower during October through January (280,000 to 300,000 cells/ml) than during July and August (340,000 cells/ml). Herd size and SCC were negatively related; larger herds had lower SCC. Because records of some cows treated with antibiotics were included in the data, herd SCC means likely were higher than corresponding bulk tank SCC. Most herds had test-day SCC that were substantially below legal bulk tank limits and could have met lower limits (e.g., 500,000 cells/ml).