Foodborne illness outbreaks caused by Escherichia coli O157:H7 are on the rise, and tainted beef is not always the culprit. It is fresh produce that holds the dubious honor of being the most common source of such outbreaks in the United States. The two most common vehicles transporting E. coli to crops are livestock manure and runoff from manure piles or lagoons that taints irrigation water. Most organic growers rely heavily on livestock manure to improve soil fertility and structure. Does that mean organic crops run a higher risk of contamination than their conventional counterparts, which may not rely so heavily on manure-based amendments? A few studies have attempted to answer that question. Here are two, in particular.

Photo courtesy of Faey Szeuw/

During the 2002 growing season, researchers at the University of Minnesota, led by Avik Mukherjee, assessed bacteria loads on both conventional and organic fruits and vegetables. Three types of growers in Minnesota were included in the study: conventional (eight), certified organic (eight) and noncertified growers that reportedly used organic methods (24). Twenty-three types of produce were included in the study. By the end of the season, 129 conventional, 117 certified and 359 noncertified fruits and vegetables had been collected and tested. Samples were taken directly from fields; they were not washed, nor were soil particles rubbed or wiped away. They were simply dropped into labeled bags and boxes, placed in coolers and sent to the lab.

Once the lab results were analyzed, the researchers concluded bacteria levels on conventional vegetables and certified organic vegetables were not significantly different. Take lettuce, for example. E. coli was detected on zero of the 10 organic lettuce samples, and one of the six conventional lettuce samples (16.7 percent). This lack of a significant difference held true for all fruits and vegetables tested. All E. coli strains detected were of generic, harmless types.

The waters were muddied a bit by noncertified growers, however. Of the 39 noncertified lettuce samples analyzed, 12 (30.8 percent) were contaminated. It appears some were a little too relaxed about manure handling procedures. One grower in particular spread raw manure during the growing season. No surprise that 92 percent of that farm’s vegetables tested positive for E. coli.

The researchers decided to conduct a more comprehensive study in 2003 and 2004. This time, farms in both Minnesota and Wisconsin were asked to participate. A total of 57 participated in 2003 (19 conventional, 14 organic and 24 noncertified) and 46 participated in 2004 (14 conventional, eight organic and 24 noncertified). By the end of the growing season in 2004, 2,029 samples representing 16 types of produce had been collected from the field and tested.

Overall, E. coli was detected on 68 noncertified samples (8 percent), 34 organic samples (7 percent) and 13 conventional samples (2 percent). Some have used those numbers to support the argument that organic produce is more likely to harbor E. coli, but it’s important to take a close look at the data before coming to this conclusion. Many more samples of leafy greens and lettuces, which have a higher tendency to harbor the bacteria, were taken from organic (162) and noncertified (250) farms than conventional farms (41). So, the total number of contaminated samples was higher for organic and noncertified farms, but the highest proportions of contaminated samples were actually found on conventional farms. In 2003, two of the 30 organic leafy green samples were contaminated (6.7 percent), as compared to 17 of the 71 noncertified samples (23.9 percent) and three of the 12 conventional samples (25 percent). In 2004, five of the 22 organic lettuce samples were contaminated (22.7 percent), as compared to four of the 43 noncertified samples (9.3 percent) and three of the 12 conventional samples (25 percent).

Again, only generic, harmless E. coli strains were detected. None were the highly pathogenic O157:H7 strain.

So, what do all of these numbers tell us? They tell us fruits and vegetables growing in organically managed fields are not more likely to harbor E. coli bacteria, and they indicate the federal government’s organic certification program is working. They raise the question of whether handling and packaging procedures can influence bacteria loads postharvest. And, it hints the debate should be shifted from “Who is better? Conventional or Organic?” to “What are the best management practices for livestock manure?” It doesn’t matter if you’re conventional, organic or noncertified, if you use manure as a soil amendment you should be taking care to manage it in such a way as to minimize the risk of crop contamination.

Some think NOP standards on the use of livestock manure are a little too tight around the collar. Perhaps those are the folks spreading raw manure mid-growing season. Regardless, the standards are based on science, designed to reduce risk and should be understood by all growers applying manures to their fields.

The standards don’t prevent the application of raw manure on a field. But if that field will be used to grow a crop for human consumption, the manure must be applied no less than 120 days before harvest if the portion to be eaten comes into direct contact with soil, or no less than 90 days before harvest if the portion to be eaten does not come into direct contact with soil. Composted manure may also be used as a soil amendment, so long as the pile had an initial carbon to nitrogen ratio of between 25:1 and 40:1, and adequate temperatures were maintained. Adequate temperatures range between 131 and 170 degrees Fahrenheit for three days in an in-vessel or static aerated pile system, or for 15 days in a windrow composting system during which the materials are turned at least five times. Unlike raw manure, there are no restrictions as far as when properly composted manure can be applied during the growing season (NOP Regulations Section 205.203).

Since 1982, fresh produce has been at the heart of over three dozen E. coli outbreaks in the United States. None of those outbreaks have been linked to organic produce, but let us conventional and organic growers work towards soil management and handling practices that avoid future outbreaks.

The author is a biologist who lives and farms in Vermont’s Northeast Kingdom. Comment or question? Visit and join in the discussions.