Move 160 m away from the farm

Recalls of fruits and vegetables following outbreaks of foodborne illnesses caused by microbes like E. coli and Salmonella seem to be becoming the norm, but new manure application research may help growers reduce risk.

The South Australia Health Department has been notified of 233 cases of salmonellosis since the beginning of last December, with 43 people hospitalised. Darwin and the Northern Territory are also experiencing a spike in cases.

And, once again, raw vegetables, in this instance bean sprouts, are implicated.

Vegetables and fruits really suffer the double whammy — they are often exposed to manure used to fertilise the crops and many are eaten raw so the usual kill step provided by the high temperature during cooking isn’t effected.

Across the world a lot of work is going on to minimise the risks of contaminating produce during crop-raising.

Produce can be tainted with bacteria when farmers apply animal manure to fields near fresh produce. Tiny particles, including bacteria, may become airborne and drift to nearby fields, contaminating the crop.

Until now, no-one knew just how likely it was for microbes to travel from manure application sites to downwind produce. Field research out of Clarkson University in upstate New York is providing an answer.

Shane Rogers, an associate professor of civil and environmental engineering, led a research team that looked into the issue. They measured how far common bacteria — including Salmonella and E. coli — are likely to travel downwind from manure application sites. They hoped to better understand how fresh produce might be contaminated by nearby animal agriculture practices.

The team used field data to understand how these bacteria travel from manure application sites to produce. The research lasted three years. They took samples at several distances from manure application sites and measured the presence of illness-causing bacteria.

The researchers used computer models to expand their understanding. “It is not possible to obtain measurements for every possible set of circumstances that may exist,” Rogers said. “The models allow us to predict produce contamination over a larger range of probable conditions than our raw measurements would provide.” These include the type of manure, the terrain of the farm and weather conditions at the time the manure is applied.

The team also evaluated the risk of illness. This gave the team a better understanding of how likely someone is to get sick from produce when a certain amount of bacteria is present.

Combining all that data, the team found that produce fields should be set back from areas of manure application by at least 160 m. That distance should help lower the risk of foodborne illness to acceptable levels (1 in 10,000).

Rogers emphasised that the advice is for a minimum setback. “[160 metres is] the minimum distance that produce growers should maintain between manure application activities and produce growing areas,” Rogers said. Additional distance and delay between manure application and harvest would provide further protection.

The study appears in Journal of Environmental Quality.