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Measuring E. coli in irrigation water

University of California (UC) researchers are field testing a more accurate and potentially cost-saving water sampling protocol to measure for generic (indicator) E. coli in irrigation water in Imperial County, Calif.

The initial findings of the new testing protocol procedure are encouraging, according to Mark Trent, UC Cooperative Extension (UCCE) vegetable crops advisor, Imperial County, Holtville, Calif.

If the pilot test is successful, the new procedure could more accurately measure E. coli levels in irrigation water, possibly reduce the number of water tests required under the California leafy greens metrics, and even save growers’ money.

Collaborators on the project include: Trevor Suslow, Extension research specialist, and Adrian Sbodio, research specialist at UC Davis; Scott Lesch; Judy Li, Karen Xu and Daniel Jeske, Statistical Collaboratory at UC Riverside; and Hank Giclas and Sonya Salas of the Western Growers Association.

“Growers are currently required under the California leafy greens agreement to conduct extensive and laborious water testing to determine if irrigation water applied to winter vegetables in Imperial County has bacterial levels within an approved range,” Trent said. “Our research is designed to improve that process.”

Trent discussed the research at the 20th annual Fall Desert Crops Workshop held in Brawley, Calif., sponsored by Western Farm Press.

“We think the sampling protocol we developed will be effective,” Trent said. “We are currently validating the protocol during the winter months to determine how it works.”

Current water test procedures check for generic E. coli, an indicator for possible human pathogens. The tests determine if pathogens “could” be present in water, but not necessarily if they are actually present. The new UCCE protocol could provide a more definitive answer.

The water sample field tests are taken from the (mostly) concrete-lined Ash irrigation canal, which feeds off the All American Canal in Imperial County. Water samples are gathered from 26 sites from just north of the California-Mexico border to the Holtville area.

The researchers are using two testing methods. The first is simply gathering water in a bottle and checking the sample using a Quanti-tray analysis to test for indicator E. coli. The second is the new method; concentrating the samples by running canal water through a special filter to detect very low numbers of possible human pathogens.

“We run about 6 liters of water through the filter using a peristaltic pump at selected test sites,” Trent said. “The filter captures any microorganisms in the water. The water flows through the plastic tubing in the pump; not touching the pump itself to prevent cross contamination from sample to sample.”

Trent sends the filter to Suslow’s lab at UC Davis where a polymerase chain reaction (PCR) test is conducted to determine if pathogens are present, and if so further testing is required to determine how many.

“So far the indicator E. coli numbers generated by the Quanti-tray test have been fairly low for indicator E. coli,” Trent said. “The low levels fall within the acceptable levels in the leafy greens metrics.”

The irrigation water-E. coli research is critical to Imperial County agriculture. The county has the largest open water canal system in the United States, Trent says.

The issue is for vegetable, livestock, and other operations to coexist while keeping irrigation water clean to help ensure the production of safe food and retaining consumer confidence in the food supply.

Agriculture is a $1.6 billion industry in Imperial County (2008 figures), according to David Ritter of the county’s Agricultural Commissioner’s office. Imperial County agriculture ranks ninth in California in production value with about 500,000 farmable acres.

Vegetable production is the No. 1 farming industry in Imperial County with a gross value of about $550 million, followed by field crops ($480 million), and livestock ($400 million).

Trent gathers some water samples where CAFOs are located in close proximity to vegetable fields. Generic E. coli levels in water samples taken near CAFOs and composting operations are higher, Trent says, but again the levels fall within metric-approved limits.

The irrigation water study is funded by the California Department of Food and Agriculture.

Trent and his colleagues are also tracking sheep grazing practices in Imperial County. A common practice is for bands of 1,500-2,200 sheep to graze off 35-acre alfalfa fields. The sheep are then herded down public roads to “sheep off” another field.

Sheep manure dropped on the roadway is a concern as vegetable harvest equipment traveling down the road may come in contact with the manure before entering veggie fields.

The leafy greens metrics include plant back restrictions on fields grazed by sheep, Trent says. Some vegetable marketers require more stringent guidelines. Some buyers will not buy vegetables from fields where sheep were grazed.

“Science needs to determine if the manure left on the road combined with the movement of vegetable equipment is a high risk,” Trent says.

The sheeping studies are funded by the Leafy Greens Research Board, the Center for Produce Safety at UC Davis, and UCCE Imperial County.

“We are studying the prevalence of fecal shedding of E. coli 0157:H7 and Salmonella spp. and determining if rotational grazing between crop systems alters the prevalence of the pathogens,” Trent said. “We are very concerned about the movement of harvester equipment on roads where sheep are moved and the possible contamination of nearby vegetable fields.”

However, after taking weekly manure samples from four different bands of sheep for the past eight weeks Trent has found no human pathogens in the sheep manure. Testing will continue throughout the sheeping season.

Trent gathers manure from sheeped off fields using tongue depressors and rubber gloves.

“I want to determine if pathogens left on the road can survive in the hot desert environment,” Trent said. “I also want to find out if there is a pathogen level difference between sheep buzzing down an alfalfa field versus Sudangrass or bermudagrass.”

About 100 growers, pest control advisers, and other industry representatives participated in the Fall Desert Crops Workshop.

Commercial sponsors of the event included: BASF and Bayer CropScience (Platinum sponsors); Syngenta and Tessenderlo Kerley, Inc. (Gold); Dow AgroSciences, FMC Corp., and Valent (Silver); and Certis USA (Bronze).

email: [email protected]

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