cows in field

Puzzled By PI Spikes?

Production Resources | Animal Health

By Linda Tikofsky, DVM, Quality Milk Production Services
Cornell University

Added May 7, 2009. A variety of bacteria counts are available to today’s dairy producers and processors to provide information about the quality of milk on our dairies, the health of our cows or the adequacy of milk harvesting practices. For years our single measure for bacteria counts has been the Standard Plate Count (SPC). The SPC is used to determine the total bacteria count in raw
milk (expressed as the number of colony forming units in a milliliter of milk or cfu/ml). The maximum level for Grade A milk is 100,000 cfu/ml; counts under 10,000 cfu/ ml are achievable for every farm. There are many possible sources for bacterial contamination and increases in the SPC.

The most common ones are:

1. Mastitis: Cows with strep, and sometimes coliform infections have been shown to shed more than 10 million bacteria per ml of milk. One infected cow, especially in small herds, can have a tremendous impact on bacteria counts in bulk milk. Bacteria can be shed intermittently.

2. Teat and udder cleanliness: Manure and mud clinging to the surface of teats will contain millions of bacteria as well and if udder prep at milking time is inadequate, these bacteria have the potential to contaminate bulk milk.

3. Milking and milk storage equipment: Milk is the perfect food for humans and for bacteria, so residues on equipment surfaces supports the growth of bacteria that will contaminate future milkings.

Recently, some processors have begun to place more importance on additional measures of quality and what is most frustrating to producers is the Preliminary Incubation Count (PIC). PICs are used in conjunction with the SPC to further understand the causes of high bacteria count milk. A sample of milk is incubated at 55 0F for 18 hours and an SPC is performed on that sample of milk. No universal PI limit has been scientifically established but a common industry recommendation is to have a PIC that is not any PICs detect bacteria that grow well under cooler storage. Mastitis bacteria do not grow well under cool conditions; however, bacteria from manure contamination, poor cleaning and contaminated water will increase the PIC. Failure to cool milk below 400 F will also potentially increase PIC so issues with bulk tank cooling or warm environmental temperature will have an impact. High PI counts are most commonly caused by Gram negative bacteria, such as Pseudomonas.

Because the bacteria that cause high PICs are usually contaminan ts, it is not unusual for PICs to fluctuate widely. Opportunities for contamination of milk may vary day-to-day so it is not unusual for a farm to have low PI counts for several days and then suddenly have a count that is in the millions. On that particular day, there may have been contamination of the milk from a dirty udder, an issue with the milk cooling (forgetting to turn the bulk tank on), cleaning problems or even a swing in environmental temperatures. It’s easy to understand why PICs give EVERYONE headaches.

What are some practices you can implement on your farm to reduce the risk of having a high PIC? Since high PICs are most commonly affected by deficiencies in hygiene, cleaning and cooling consider these:

• Evaluate the cleanliness of cows and their udders. Hairy udders should be clipped or flamed so that mud and manure do not cake on the surface. Teats should be washed or predipped and dried before units are attached. Attaching units to wet teats increases the chance that droplets of contaminated
water will enter the milkline and contaminate the bulk tank. Avoid hosing down udders or spraying excessive amounts of water in the parlor.
• Regularly evaluate the temperature of your wash and rinse waters. One of the most common causes of high PI counts is not having hot enough water or not having a sufficient amount of hot water. An electronic cooking thermometer is an inexpensive, easy to use tool for your dairy. Wash water should enter the milking system at approximately 160-1650 F and return water should be no cooler than 115-1200 F.
• Be sure your cleaning chemicals are appropriate for your water’s hardness. Have your supplier test your water and find soaps and acids that are compatible.
• Have your milking system function evaluated. If your pump is not sized adequately for your system, there may be inadequate airflow for slug formation. Slugs are the necessary ‘elbow grease’ required to scrub the system clean. Dead ends in pipelines, dips and too many elbows impede drainage. Blind spots in the bulk tank can allow for residues and may need to be cleaned manually. Be sure rubber parts (inflations, gaskets, etc) are changed according to manufacturer recommendations.
• Water quality can also impact PIC. Well water may contain coliforms or water lines may become contaminated with Pseudomonas. If sanitized equipment is rinsed with water, milk may become contaminated. There are ultraviolet (black light) water treatment systems that kill bacteria found in well water; these systems are quite effective.
• Rapidly chilling milk will help prevent PI spikes so for farms that have consistent issues with counts, installing a precooling system can help.
• Finally, sampling and handling errors can cause PI spikes. Partial pickups will leave a residue on the sides of a bulk tank that will contaminate the next milking. Bulk tank drivers should be educated to always take samples from the top of the tank (not the outlet valve) and to be sure that samples are promptly chilled. Old samples will have increases in the PI count. Dipping household containers to retrieve milk for the evening meal may introduce bacteria.

There are proactive measures farmers can take to help diagnose causes of PI spikes on their farms. Have your hauler take an extra vial and label it with the date. Freeze this sample IMMEDIATELY. If you should have a bacteria count spike on your next report, you’ll have a little bit of ‘history’ in your freezer. This frozen sample can be sent to an independent laboratory for confirmatory testing and to help you troubleshoot issues. Additional information on sampling may be found at htm
or by calling the Canton QMPS laboratory at 315-3798-3930.

PIC spikes are frustrating for everyone. Although the organisms causing PI spikes are killed by pasteurization, consistently high counts indicate there may be some deficiencies in milk harvesting practices, cooling of milk or cleaning of equipment that require further investigation.