The bulk tank milk culture has long been the standard method for screening dairy herds for the presence of contagious mastitis pathogens (Streptococcus agalactiae, Staphylococcus aureus or Mycoplasma).
Whenever these bacteria are present in a bulk tank milk sample, it is concluded that there are infected mammary glands in the herd. Because these mastitis pathogens are intermittently shed into the milk, single day bulk tank samples are not reliable indicators of the absence of contagious mastitis infections. However, research has shown when 3 or more consecutive daily bulk tank samples are taken and pooled for culturing, test results are dependable.
The bulk tank milk culture is also recognized as an excellent monitor of the effectiveness of pre-milking teat prep. Since most environmental mastitis pathogens found in bulk milk generally do not come from infected mammary glands but rather from the teat surface, it is reasoned that whatever environmental bacteria (coliforms and non-ag streps) not removed from teat surfaces during pre-milking teat prep will then end up in the bulk tank milk.
Thus, the level of environmental bacteria in bulk tank milk samples is a direct reflection of the effectiveness of pre-milking teat prep. As in the case for contagious infections, pooling multiple days of bulk tank milk samples will give a more representative assessment of pre-milking teat prep across days and milking shifts (see Quality Count$ Fact Sheet FC-MC-1).
Many research studies indicate that production of high quality milk with minimum mastitis and optimum milking speed requires consistent cleaning of teat surfaces (including teat ends) and correct machine attachment timing (see Quality Count$ Fact Sheet F-MR-4 - Principles of Ideal Pre- and Post-milking Cow Prep). A survey of milking protocols will yield numerous pre-milking teat prep methods all hoping to achieve clean dry teats.
A lot of the pre-milking teat prep protocol variation is due to housing and milking equipment differences as well as personal preference and inconsistent application of pre-milking prep procedures by milking personnel.
Recent introduction of robotic milking and such things as mechanical teat scrubbers has challenged the more conventional DIP - STRIP - DRY and APPLY teat prep protocols. But regardless of your teat prepping procedure method, the expected outcome must be the same – CLEAN, DRY TEATS.
The bottom line is pretty simple: If the numbers of coliforms and non-ag strep bacteria on your bulk tank culture results are high, you are NOT getting teat surfaces clean enough!
Why does it matter? At the farm, mastitis, both clinical and subclinical (high SCC), is the main concern because current market access depends on having low SCC. But that is only half the story. In addition to bacteria that cause mastitis, there are many other organisms in the cow's environment that also contaminate cow's teats.
These environmental organisms are usually neither a cow nor human health concern but can have a detrimental effect on milk shelf life and product quality. Some of these bacteria can be heat-resistant spore formers that, if they survive pasteurization, can end up in dried milk powder and milk protein products. Today approximately 15 to 17% of U.S. milk production is exported. The greatest portion is exported as dried milk powders and proteins. The fact that our importing partners are very concerned about spore producing contaminants should put this concern high on our list. Just ask Fonterra in New Zealand about the importance of having products free of spore contamination. Their recent product recall was a brand-marring scare that was too close for comfort. Fortunate for them the suspected Clostridium spores in their milk powder turned out to be garden variety food spoilage spores with no human health concerns.
TAKE HOME MESSAGE: There are normally many undesirable organisms in the cow's every day environment, which can easily contaminate cow's teats. Keeping cows clean and dry by attentive bedding, stall and alley management is needed. Research has repeatedly shown that the number of bacteria in bedding materials is positively correlated to the number of these bacteria on teats surfaces. Likewise, the greater numbers of bacteria on teat surfaces, the greater the risk of mastitis infections. In addition, whatever food spoilage bacteria are on teat surfaces at milking machine application will also end up in bulk tank milk.
Therefore, the last opportunity to assure that bulk tank milk is not contaminated with undesirable organisms is to thoroughly clean teats before every milking. That is why monitoring coliform and non-ag strep bacteria levels on bulk tank cultures is the best means to know if your pre-milking procedures are effective regardless of the method you are using. If these bacteria levels are low, we are doing an adequate job! If they are too high, we are not being effective enough and we must improve.
To access the Quality Count$ materials, go to our U of MN Dairy Team website: http://www.extension.umn.edu/dairy, click on "Milk quality and mastitis", then click on "New! Quality Count$ 2012".