On-farm culturing can enable dairy farmers to make more-informed decisions about treating for mastitis. Mastitis is associated with the most frequent antibiotic use in dairy cows. The cost of treatment and discarded milk associated with clinical mastitis could exceed $350 per cow per year, according to one study.

Antibiotics are frequently used to treat clinical mastitis. But they’re often either ineffective or unnecessary. Producers who use unnecessary antibiotics will lose profit and risk developing antibiotic resistance on their farms.

Identifying species of bacteria responsible for causing mastitis can be beneficial in determining treatment options and reducing unnecessary antibiotic use. Management strategies can be changed to help prevent specific pathogen types in a herd. Staphylococcus aureus is an example of a pathogen transmitted through the parlor during milking. Wearing gloves, teat dipping and milking infected cows last are some ways to prevent and control the pathogen.

Identifying the pathogen can help producers change management strategies to prevent additional cows from being infected. An on-farm culture may help a producer decide not to treat a cow. Results from one study found that 10 percent to 40 percent of cultures from clinical mastitis showed no growth following culturing. Cultures that show no bacterial growth usually indicate that a cow doesn’t require treatment because its immune system already has cleared the bacterial infection.

Traditionally producers send milk samples to laboratories for culture results. But the time can be long between sample submission and learning test results. Implementation of an on-farm culture program can help producers make proactive decisions in a timelier manner. Submitting a sample for bacteriological culture also can be costly.

Pennsylvania State University has created a quad-plate culturing system. Each quadrant of the plate can selectively grow different species of bacteria. Quadrant I MacConkey’s Agar detects gram-negative bacteria such as coliforms and non-coliforms. Quadrant II is Edwards Modified Agar, which detects streptococci bacteria. Quadrant III is Baird Parker Agar, which detects staphylococci bacteria. Quadrant IIII is Blood Agar, capable of growing most types of bacteria. It's used to confirm results of other quadrants.

To start culturing producers will need a few supplies.

  • sterile test tubes to collect aseptic milk samples
  • sterile swabs to plate milk onto agar plates
  • agar plates to grow bacteria
  • an incubator maintained at a constant temperature to grow bacteria

Once a milk sample is aseptically taken it should be plated onto a quad plate. Plates should be incubated for 24 hours and observed for preliminary bacterial growth. An additional 24 hours of incubation may be needed to collect final results. Pennsylvania State University has created a free guide to help producers identify bacterial growth. Farm personnel handling on-farm culture responsibilities should consult with a veterinarian to make appropriate treatment decisions for the herd based on culture results.

On-farm culture can’t identify every mastitis-causing bacterium. For example Mycoplasma bovis requires special media and conditions to grow. That can only be done in laboratories.

On-farm culture is designed to help producers make more-proactive treatment decisions regarding mastitis. Producers will be able to identify cows with no bacterial growth. Those cows have self-cured the bacterial infection. Producers also will be able to identify gram-negative pathogens that are often self-limiting or unresponsive to treatment, or gram-positive pathogens that generally respond more effectively to antibiotic treatment. But some aren’t susceptible to antibiotics. The udder-health history of the cow should be examined. A producer should talk with a veterinarian to establish a treatment protocol.

The Pennsylvania State University-Extension dairy team recently released an online course titled “On-Farm Milk Culturing for Mastitis Control.” Email anl113@psu.edu for more information.

Amber Yutzy is a Pennsylvania State University-Extension specialist in dairy-herd health and reproductive management.