Large herds generally have lower somatic cell counts due to better adherence to milking protocols.
Impact depends on final details
At the end of this month, the National Conference on Interstate Milk Shipments will meet in Baltimore to lower the U.S. standard for somatic cell counts (SCC).
The current standard of 750,000 cells/ml will likely be lowered in a stepwise fashion to 400,000 cells/ml by Jan. 1, 2014. If that standard were implemented today, 8% to 14% of U.S. herds would not be in compliance.
"The Southeast is definitely running high, and some of the mid-Plains states—Arkansas, Kansas, Missouri, Nebraska, Oklahoma—are marginal," says Duane Norman, a supervisory research geneticist with USDA’s Animal Improvement Programs Laboratory (AIPL).
The devil is in the details. It’s long been known that smaller herds and herds in the South will struggle to meet the 400,000 cells/ml level. But exactly which herds and how many will only be determined by the final rule.
AIPL recently analyzed the test-day records of nearly 15,000 DHIA herds with roughly 4 million cows. The SCC data from these herds includes cows being treated for mastitis and withheld from commercial sales. As a result, the average tends to be slightly higher than in SCC data collected through the Federal Milk Marketing Orders, which represents only salable milk. In 2009, for example, the DHIA average was 6,000 cells/ml higher than the Federal Order average.
Nevertheless, the huge DHIA data set allows researchers to calculate more closely how many herds and how much milk would exceed federal standards as SCC levels are reduced.
The AIPL analysis shows that 2.7% of DHIA herds (0.7% of milk) would not be in compliance if the limit is reduced to 600,000 cells/ml and three of five consecutive months must be below this level.
That number jumps to 6.2% of herds (2% of milk) when the limit is reduced to 500,000 cells/ml and to 14.1% of herds (5.8% of milk) when it is reduced to 400,000 cells/ml.
Ironically, if the European Union (EU) methodology and rules are used, only 7.8% of herds (3.1% of milk) would not be in compliance at 400,000 cells/ml. That’s due to two factors, Norman says.
First, the EU calculates averages geometrically, not arithmetically. The rolling geometric mean is calculated by converting the SCC values of the three most recent months to base 10 logarithms, adding the three values, dividing the sum by three and then converting the resulting value back to an arithmetical number by finding the antilog.
Mastitis researchers say that using the geometric mean is the correct method, both biologically and statistically, for calculating cell count averages. It has the added benefit of producing a lower average. "Use of the rolling geometric mean really helps decrease the number of noncompliance herds," Norman says.
Second, the EU does not suspend producers until they exceed 400,000 cells/ml. Norman says producers are warned once their three-month geometric mean reaches 400,000 cells/ml and have another three months to bring the count below 400,000. The U.S., however, suspends producers if they exceed the standard in three out of the most current five months.
"The EU is more conservative on suspensions," Norman says. "The U.S. Federal Orders system is tougher on producers, no question."
Proposals are being submitted by the National Milk Producers Federation (NMPF) and NMC to lower the U.S. standard to 400,000 cells/ml.
The NMPF proposal retains the rule that licenses are suspended if three of the five most recent tests exceed the limit. It calculates averages by arithmetical means: somatic cells are summed and then simply divided by the number of tests. And it ratchets down the level over three years: 600,000 cells/ml on Jan. 1, 2012; 500,000 cells/ml on Jan. 1, 2013; and 400,000 cells/ml on Jan. 1, 2014.
The NMC proposal requires that a geometric mean be used to calculate a herd’s SCC. It leaves the current 750,000 cells/ml limit in place through Dec. 31, 2012, but phases in the rolling geometric mean in 2012. On Jan. 1, 2013, the limit would be lowered to 550,000 cells/ml and on Jan. 1, 2014, to 400,000 cells/ml.
2010 DHIA Somatic Cell Counts
DHIA herds had an average somatic cell count (SCC) of 228,000 cells/ml in 2010, which was 5,000 cells/ml less than in 2009, reports USDA’s Animal Improvement Programs Laboratory.
The data includes 198,218 herd test days and involves some 4 million cows. The data reflects all cows tested, including treated cows. (Milk from treated cows, however, is withheld from market.) That’s why Federal Milk Marketing Order data on SCCs is typically lower than DHIA data.
This year’s numbers follow a trend of declining SCC averages. The average level has declined every year since 2005 and has increased only once since 2001. In 2010, 32 states and Puerto Rico had lower SCC averages than in 2009. Fourteen states saw their averages increase.
Eighteen percent of herd test days exceeded the 400,000 cells/ml limit at least sometime during the year. Ten percent exceeded 500,000, and 5.7% exceeded 600,000.
- April 2011