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 2010, the SCC in DHI herds averaged 228,000. This compares to 233,000 in 2009; 262,000 in 2008; 276,000 in 2007; 288,000 in 2006; and 296,000 in 2005.
Thirty-two states and Puerto Rico had lower average SCC than the previous year; 14 states had higher averages. A few Mexican herds tested through the US system were included for the first time.
Variation among states remains large, ranging from 170,000 (Idaho) to 421,000 (Arkansas). State average SCC was lower than the national average for mountain and western states, and often higher for southeastern states. Differences between adjacent states were substantial, which suggests that herd size and mastitis control practices, including genetic selection, are impacting state differences as well.
The current federal SCC regulatory limit in the US is 750,000, except in California where it is 600,000. 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 2010 were 18.0 percent, 10.0 percent, 5.7 percent and 2.7 percent, respectively. The 2.7 percent of 2010 DHI herd test days that were higher than the present legal limit for bulk tank SCC 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 2010, the average test-day SCC in herds with fewer than 50 cows was 286,000 compared to 251,000 in herds with 100 - 149 cows; 217,000 in herds with 500 - 999 cows; and 184,000 in herds with over 3,000 cows.
The percentage of test days with SCC greater than 750,000 in herds with fewer than 50 cows was 5.1 percent. This compares to 1.3 percent for herds with 50 - 99 cows; 0.7 percent for herds with 100 - 149 cows; and only 0.4 percent or less in herds with 150 or more 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.
Average test-day somatic cell count from US Dairy Herd Improvement herds during 2010 by state
Source: National Mastitis Council