Producers have a variety of informational tools available to monitor both the mastitis in their herds and the quality of milk being shipped to processors. Somatic cell counts (SCC) are a measure of mastitis in a dairy herd. The SCC will increase in a quarter as a result of an infection. The increase represents white blood cells entering the quarter to fight the infection. The bulk tank somatic cell count (BTSCC) reflects the total number of infected mammary quarters in the herd that are actually being milked into the tank. The bacterial count is another primary measure of milk quality. Bacteria are present in milk as a result of milking infected mammary quarters, contamination from the environment during milking, dirty milking equipment and growth during milk storage.

No one test simultaneously determines both SCC and bacterial content. Analysis of bulk tank milk samples yields information on the herd, while analysis of milk samples from individual cows/quarters yields specific information about that cow/quarter. Whole-herd information can also be obtained by testing individual milk samples from all cows in the herd.

Bulk tank milk testing
Tests run on bulk tank milk samples fall into three categories: regulatory tests; co-op/plant quality premium tests; and informational tests.

Regulatory tests establish that the milk being shipped by producers meets the legal minimum quality standards established by the Food and Drug Administration (FDA) and adopted by state health departments. These regulations are in the Pasteurized Milk Ordinance (PMO). Tests are conducted for SCC, bacterial numbers, the presence of antibiotics and the presence of added water. The primary bacterial count used is the standard plate count (SPC), sometimes referred to as the plate loop count.

Co-op/plant quality premium tests include determination of SCC and bacterial numbers, generally by both the SPC and preliminary incubation (PI) methods. Minimum standards to receive bonus payment for quality milk will vary among milk procurement agencies. There are generally other requirements that must be met, such as absence of antibiotics.

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Informational tests include the regulatory and co-op/plant quality premium tests. In addition, many veterinarians and other laboratories do bulk tank milk cultures (BTMC). This test estimates the total number of bacteria in the bulk tank milk and provides information as to the specific types of bacteria present. It is a good place to start when troubleshooting a herd mastitis problem. The SPC and PI tests do not reveal the specific types of bacteria present.

Testing milk from individual cows
The primary purpose for testing milk from individual cows/quarters is to evaluate mastitis. None of the tests applied to milk from individual cows are used by regulatory agencies or co-op/plant quality premium programs. Individual cow tests are simply informational and the next logical step for troubleshooting herd mastitis problems.

The most widely used program for determining the SCC of milk from individual cows is the Dairy Herd Improvement (DHI)-SCC program. The DHI-SCC program determines somatic cells in composite milk at monthly intervals. These data can be used to determine which cows are likely to be infected, but they cannot be used to determine which of the four mammary quarters are infected or the type of pathogen causing the infection. The California Mastitis Test (CMT) is the most widely used cow-side test for estimating the SCC of individual quarters.

Somatic cell counts measure inflammation and are an indirect measure of the presence or absence of infection. The true infection status of a cow/quarter can only be determined by microbiological analysis (culturing) of aseptically taken milk samples. Milk samples to be tested can be either composite samples (an equal volume of milk from each quarter is drawn into the same tube) or individual quarter samples.

This type of testing determines the presence or absence of pathogens and the type of pathogen. Results from multiple samples taken over time are more reliable indicators of infection status than results from a single sample.

Bulk tank milk tests
Regulatory tests are conducted in laboratories operated by state departments of health or equivalent agencies or in laboratories approved by these agencies. Somatic cell counts are determined almost exclusively by electronic counting methods using a series of milk samples with varying cell concentrations. Cell numbers in the standards are determined by the direct microscopic method, and the cell count is referred to as the direct microscopic somatic cell count (DMSCC).

The DMSCC method is considered the standard method to which all other cell counting methods are compared. Even with calibrated machines, there is approximately 15 percent error in any single number generated. A 15 percent error means that a cell count of 100,000 cells/ml could really be anywhere between 85,000 and 115,000 cells/ml.

The standard plate count method accurately determines the total number of bacteria present in a milk sample, but it does not determine the type of bacteria present. To run the SPC, a precise volume of milk is added to a given quantity of media and the plate incubated for 24 hours. The number of colonies is accurately counted and expressed as bacteria/ml of milk. Other names often used to describe the SPC are “loop count” and “bacteria count.”

Most co-ops/plants determine the number of bacteria in milk using both the SPC and PI methods. The value of the PI count is that it gives a better indication of the shelf life of fluid milk. To determine the PI count, the raw milk sample previously used to determine the SPC is incubated at 55°F for 18 hours and then the bacterial count is determined using the SPC method.

Veterinarians and others are frequently interested in not only the number of bacteria present in bulk tank milk, but also the various types of bacteria. A test that is growing in use is bulk tank milk culturing (BTMC). In contrast to the SPC and PI methods, BTMC is not standardized, not quantitative, and exact methods vary among laboratories.

Individual cow tests
Individual cow SCC are most frequently determined in DHI laboratories, but independent laboratories do offer this service. The DHI-SCC program generally reports the values as the Linear Score, but some report the value as thousands of cells (i.e. 100,000 = 100).

The scale for linear score is from 0 to 9, where a linear score of 4 is equal to 200,000 cells/ml and linear score 6 is equal to 800,000 cells/ml. The linear score is preferred, as it can be related directly to production losses. Each increase of 1 in linear score corresponds to a doubling of SCC and a milk loss of 400 lbs/lactation (1.5 lbs/day) on second-lactation or older animals and 200 lbs/lactation (.75 lbs/day) on first-lactation animals.

High SCC quarters can be determined at cow-side using the California Mastitis Test (CMT). The CMT test is an estimate of the SCC.

The primary purpose of culturing milk from individual cows is to determine the infection status of the cow or quarter and to determine the specific pathogens infecting the herd. Milk samples for culture must be taken with great care. Teats must be clean and dry and the teat end thoroughly scrubbed with alcohol pads prior to collection.

Either composite milk from all four quarters or individual quarter samples can be tested. The most accurate method is to culture individual quarter samples. Composite milk samples are often used in an attempt to reduce the cost of testing. However, composite milk samples have limited microbiological value and they do not reveal which quarters are infected.

Using the information to manage mastitis and milk quality
The production of quality milk begins with the understanding that mastitis must be controlled and sanitary conditions must exist throughout the cow’s environment. Bulk tank milk tests for SCC, SPC and PI provide producers valuable information about the current status of mastitis and sanitation in their herds. Co-op/plant incentive programs have greatly increased producers’ awareness of these quality tests.

Somatic cell counts are reduced by controlling mastitis and, more specifically, subclinical mastitis. The BTSCC is a function of the percentage of quarters infected in the dairy herd. The contagious mastitis pathogens are frequently causes of subclinical mastitis. On occasion, high-SCC herds are found to have a significant problem with the environmental streptococci. PD

—Excerpts from North Dakota State University Extension website

J W Schroeder