Dairy antibiotics degrade in soil

In the first large study to track the fate of a wide range of antibiotics given to dairy cows, University of California, Davis, scientists found that the drugs routinely end up on the ground and in manure lagoons, but are mostly broken down before they reach groundwater.

Key Points

• This is the first large study to track antibiotics given to dairy cows.

• Antibiotics are mostly broken down before they reach groundwater.

• The study looked at large freestall operations in the San Joaquin Valley.

Data alleviate fear

The findings should help alleviate longstanding fears that dairy farms, and the fields fertilized with their waste, might lead to large-scale groundwater contamination.

“What we found is that antibiotics can frequently be found at the manure-affected surfaces of the dairy operation [such as corrals and manure flush lanes], but generally degrade in the top 12 inches of soil,” said Thomas Harter, an expert on the effects of agriculture on groundwater quality, and UC Davis chairman for Water Management and Policy.

Shallow groundwater affected

“A very small amount of certain antibiotics do travel into shallow groundwater. Our next task is to determine whether these particular antibiotics are further degraded before reaching domestic and public water wells,” he said.

Harter said the study findings should be particularly useful to people who get drinking water from wells (such as water companies and homeowners), dairy producers and policymakers. It provides the first comprehensive data set to assess and compare potential local impacts to groundwater from the wide variety of antibiotics in use on “freestall” dairy farms, where cows are free to enter and leave resting cubicles rather than being confined in stanchions or pens.

California is the nation’s largest producer of milk and cheese, with 1.8 million milking cows. More than 90% of them are housed in freestall operations.

California dairies typically administer antibiotics to calves and heifers — cows that have not had a calf — and to non-lactating adult cows, but not to “milking” cows.

Health officials are concerned that antibiotics could travel from cows’ urine and feces into the groundwater that supplies drinking water to people and livestock, potentially fostering antibiotic resistance in disease-causing bacteria.

Harter said that the health effects of antibiotics in drinking water at the low levels he detected are not known.

Examination of freestall operations

The study looked at two large freestall operations in the San Joaquin Valley, a region with highly vulnerable groundwater due to its shallow depth and sandy soils. The two dairies had a total of more than 2,700 milking cows and 2,500 heifers.

The study did not test surface water, such that of creeks. Dairies are not permitted to discharge waste-containing runoff to surface water. The study was published in the online version of Environmental Science & Technology.

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DRILLING FOR SAMPLES: Researchers Mike Mata of UC Davis, left, and Brian Bergamaschi of the U.S. Geological Survey are drilling core samples from the ground under a dairy freestall. Photo by Thomas Harter, UC Davis

This article published in the October, 2010 edition of CALIFORNIA FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.