USDA: Farms’ Use of Milk Culturing Increasing

By Ron Johnson Dairy Editor
Friday, January 2, 2009 9:05 AM CST

In the battle against bacteria, more U.S. dairy farms are culturing bacteria from samples of their milk. The idea is to find out what kinds of bacteria might be causing mastitis.

According to the USDA’s National Animal Health Monitoring System (NAHMS) “Dairy 2007” study, more than half of the farms questioned said they culture milk samples. Some grow and identify the bacteria right on their farms, while others turn the task over to a veterinary lab or lab run by a university or state.

Doing such culturing provides a trio of benefits, notes the NMC, formerly known as the National Mastitis Council.

First, it pinpoints the most common types of bacteria a particular dairy farm faces. Second, this knowledge lets the dairy battle the bacteria with the correct antibiotics and management. And third, culturing for bacteria lets a potential cow purchaser know beforehand what kinds of problems he or she might be buying.

In addition, the survey found a link between the size of the dairy herd and its likelihood to use bacteria culturing. The larger the herd, the more likely it was to employ this tool.

Seventy-six percent of the “large” dairy farms that were surveyed reported testing bulk tank milk samples. Nineteen percent of the large farms said they use string sample cultures. For the purposes of the survey, the USDA defined “large” herds as those having 500 or more cows.

By contrast, 46 percent of “medium”-sized herds were found to use bulk tank culturing. Just 3 percent of the medium-sized operations reported using string. The USDA defined “medium”-sized herds as those with 100 to 499 cows.

When it came to farms with “small” herds, 25 percent reported using bulk tank culturing. The percentage plummeted to zero for small herds using string sample cultures. The USDA defined “small” herds as those with fewer than 99 cows or less.

In all cases, the report says herd size classifications were based on cow numbers as of Jan. 1, 2007. Only farms with at least 30 dairy cows were surveyed.

The survey found regional differences, too. Dairy operations in the West were more likely to use both kinds of bacteria culturing.

Sixty-one percent of the surveyed western dairy farms reported testing bulk tank milk. Eleven percent said they used string samples.

Dairy farms in five western states were included in the survey n Texas, New Mexico, California, Washington, and Idaho.

In the East, 31 percent of the farms used bulk tank milk sampling. For string sampling, the percentage was one percent.

Wisconsin, Iowa, Minnesota, Michigan, Missouri and Indiana are the Midwestern states that were covered by the survey. The other eastern states surveyed are Kentucky, Ohio, New York, Virginia, Vermont, and Kentucky.

When it came to culturing the milk of individual cows, both the West and East yielded similar results. Approximately 43 percent of the farms in each region reported using this practice.

There were two types of cows that most often had their milk individually cultured n those that had chronic, clinical mastitis and those whose clinical cases had not responded to treatment, the survey found.

Fifty-nine percent of the farms reported culturing the milk of cows that suffered from chronic, clinical mastitis. Fifty-four percent of the farms were found to culture the milk of individual cows that had not improved after a mastitis treatment.

As with the overall use of bacteria culturing, more of the larger farms said they tested the milk from individual cows.

Slightly more than 47 percent of the farms with 500 or more cows reported culturing the milk of their fresh cows. That compares to 14.9 percent of the medium-sized herds and eight percent of the small herds.

Sixty-five percent of the large herds cultured the milk from cows that had clinical mastitis. For medium-sized herds the percentage was 35.4, and for small herds it was 22.2.

Sixty-seven percent of the large herds cultured the milk from cows with recurring, or chronic, cases of mastitis. Sixty-five percent of the medium-sized herds also reported doing that, along with 55 percent of the small herds.

When it came to cows with clinical cases of mastitis that were not responding to treatment, 53.5 percent of the larger herds used individual culturing. That compares to 61.1 percent of the medium-sized herds and 50.1 percent of the small herds.

Farms with large herds did not rank highest in one category: culturing the milk of individual cows when there was a high somatic cell count (SCC). Here, 31.5 percent of the large herds employed this practice.

But 49.6 percent of the medium-sized herds used individual culturing for cows with high SCC. For small herds, the number was 37.9 percent.

Bacteria found

The most-commonly found disease-causing bacterium was environmental Streptococcus. In all 60.1 percent of the surveyed farms cultured this bacterium. It was found on 78.3 percent of the large farms, 67 percent of the medium-sized farms, and on 52.4 percent of the small farms.

Coming in second in terms of prevalence were E. coli, Klebsiella and other gram-negative bacteria. They were cultured from milk samples 53.3 percent of the farms. They were found on 78.9 percent of the large dairies, on 64.3 percent of the medium-sized dairies, and on 41.8 percent of the small dairies.

Staph aureus was cultured on 52.3 percent of the farms, with 64.4 percent of the large herds finding it. Slightly more than 51 percent of the medium-sized herds cultured Staph aureus, while 50.5 percent of the small farms found it.

Another bacterium, Strep ag, was cultured on 34.4 percent of the surveyed farms. It was cultured from the milk of 35.6 percent of the large farms, 42.2 percent of the medium-sized farms, and 29.4 percent of the small farms.

Coagulase-negative Staph bacteria were cultured on 33.5 percent of the farms. More than 63 percent of the large operations found it, compared to 37.6 percent of the medium-sized herds, and 25.3 percent of the small herds.

Mycoplasma were cultured on 5.7 percent of the surveyed farms. This organism was found in the milk of 21.4 percent of the large farms, 3.8 percent of the medium-sized farms, and four percent of the small farms.

The USDA says this survey represented 79.5 percent of U.S dairy farms and 82.5 percent of U.S dairy cows.

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