Each year, test-day data from all herds enrolled in Dairy Herd Improvement (DHI) somatic cell count (SCC) testing in the United States are examined to assess milk quality on a national basis.
During 2012, the SCC in DHI herds averaged 200,000 cells/ml. This compares to 217,000 in 2011; 228,000 in 2010; 233,000 in 2009; 262,000 in 2008; and 276,000 in 2007.
National average test-day herd SCC has declined every year since 2005, and every year except one since 2001.
Forty-two states had lower average SCC than reported the previous year; five states had higher averages. A few Mexican herds tested through the US system were included for the third time.
Variation among states remains large. State average SCC generally was lower than the national average for mountain and western states, and often higher for a few southeastern states. Differences between adjacent states were substantial, which suggests that factors such as mastitis control practices and genetic selection are impacting state differences.
The current federal SCC regulatory limit in the US is 750,000. In many other major dairy countries, the SCC limit is 400,000.
The overall percentage of herd test days that exceeded 400,000, 500,000, 600,000, and 750,000 during 2012 were 12.0%, 6.1%, 3.3% and 1.5%, respectively, which was lower for all levels than during 2011. The 1.5% of 2012 DHI herd test days that were higher than the present legal limit for bulk tank SCC in the US may overestimate the percentage of herds that shipped milk exceeding the legal limit because milk of cows treated for mastitis is excluded from the bulk tank even though included in DHI test data. The percentage of herd test-days that exceeded the legal limit also would have been higher than the percentage of herds that were rejected from the market because market exclusion only occurs after repeated violations.
As herd size increased, milk yield generally increased and SCC decreased. During 2012, the average test-day SCC in herds with fewer than 50 cows was 200,000 compared to 222,000 in herds with 100 – 149 cows; 195,000 in herds with 500 – 999 cows; and 168,000 in herds with over 3,000 cows.
The typical seasonal pattern was also evident. Average SCC increased from May through August and declned quickly from September through November. The lowest average SCC was in November and December.
Source: Source: National Mastitis Council