Skip to Content.
Sympa Menu

market-farming - Re: [Market-farming] manures/GAP training

market-farming AT lists.ibiblio.org

Subject: Market Farming

List archive

Chronological Thread  
  • From: Road's End Farm <organic87 AT frontiernet.net>
  • To: Market Farming <market-farming AT lists.ibiblio.org>
  • Subject: Re: [Market-farming] manures/GAP training
  • Date: Wed, 3 Mar 2010 20:43:56 -0500


Yeehah, Liz has found me the paper!

They did in fact check for survival of the specific strain. The strain used was a laboratory-sourced strain modified apparently by subtracting two genes and adding at least two others:

"E. coli O157:H7, strain B6914, without the stx1 and stx2 genes, was used for all experiments. [ . . .] . In addition, E. coli O157:H7 strain B6914 contains genes for green fluorescent protein (GFP) and ampicillin resistance on a stable plasmid"

Whether any of thise modifications might affect survival in the field is briefly addressed near the end of the study, but as near as I can tell only some of the modifications are addressed, and they are assuming that results would not vary between field situations and manure/manure slurry situations; so I am not convinced that it's certain that this organism would behave the same way in the field.

Some of the modifications caused the specific strain to produce fluorescence for easy identification, so questions of possible later re-contamination of the field are moot, as it was this specific fluorescing strain that was identified.

The study was done at the Horticultural Farm of the University of Georgia, Tifton, Georgia. Carrots and onions were produced "according to production guidelines by the University of Georgia Cooperative Extension Service" (I would assume this was conventional, not organic, production, but they don't address that question); no chemical weed control was used; irrigation was used; seeding techniques are discussed. 

I don't find any mention of soil types, or of what techniques were used to prepare the soil. I also don't find any reference to previous use of the field, to levels of organic matter in the field, or to any tests for bacterial or fungal organisms in the field other than the one they introduced themselves.

References to weather during that season include the statment that "light irrigation was required frequently during warm, dry periods" and that the winter had been unusually cold for the area, so growth was slow and yield was below normal (The crops were seeded in late October and onions were harvestable on day 140 and carrots on day 126: apparently these crops in Georgia are grown over the winter. This I think is highly relevant because activity of other soil life was probably relatively low.)

Survival rates in the field ranged from 154 days to 196 days depending on treatment and crop, and on the crop for 168 days on carrots and 74 days on onions from the date of application. (The chart showing survival on fields does look like it goes up to 210 days, but the text gives a maximum of 196, and I think the 210 is simply the end point of the chart -- though the authors do mention 210 days as a possible interval for similar situations.) If I'm reading the results correctly, the E coli tested for survived longer in the fields than on the crop itself, even while the crop was in the fields. All the charts show a continuing drop in E coli during the test period.

The researches note that "the levels of E. coli O157:H7 used in this study are far greater than what would likely be found on an agricultural field".

The authors conclude that the persistence of the strain of E coli in soil is dependent on the type of vegetable grown in the soil, with inactivation more rapid with onions than with carrots; and suggest that the higher concentrations of antimicrobial phenolic compounds in onions might explain this.

They further conclude, not that a 210 day interval should immediately be imposed on all crops grown in the entire country, but that the 120 day interval "may not be adequate for Georgia's climate"; and that "the impacts of on-farm practices . . . have not been sufficiently explored". As is common, the researchers probably have a better sense of the limits of their study than do at least some of those referencing the study. 

This does, except for the failure to consider the soil life, look to me like a reasonable start on the huge amount of work to be done in this area, done in one particular situation and understood by the researchers as such; and it does indicate that, in some situations, at least some E coli strains are likely to survive longer than 120 days. 

The failure to consider the other soil life present, and how current year and previous management techniques may affect such soil life, strikes me as a huge omission, especially as I didn't see anything in the study acknowledging that it is an omission. It's possible that the field did not have otherwise healthy soil life, and that if it had, the results would be different; but there's no way to judge that from this information.

I would also note that survival over the winter may well be different than survival over the summer; and that the inoculation with larger quantities of E coli than are likely to occur in actual agricultural practice may lead to bigger differences in practice than they assume, especially combined with the failure to consider variations in other soil life: a large dose of the introduced E coli may potentially overwhelm defense mechanisms that can deal with a smaller quantity.

I'd like to see the study repeated in a variety of soil types, under different types of management, and with better attention paid to what's living in the various soils to start with. I'd also, if possible, like to see at least some replications done with the actual pathogen, though those would of course have to be done in carefully controlled circumstances so that the surrounding area wasn't accidentally contaminated. And the information that it makes a difference what crop is grown is really useful, but there are, obviously, a lot of other crops in the world besides carrots and onions (and might there also be variety differences?) For that matter, the level of antimicrobials in the crop might well vary depending on the life in the soil they're grown in; and if onions are in the same field as carrots, does that provide any protection to the carrots?

In the meantime, for those growing carrots, especially over the winter, it's probably a good idea to be extra careful what's applied to the field (it seems to me that the survival on the onions themselves was comfortably under the 120 day limit -- in fact at 74 days it was under the 90 day point.) But I'm not inclined to immediately take this as evidence that the occasional presence of mammals and birds in the field means that I shouldn't be growing crops at all; and I'm certainly not going to take it as evidence that the only possible management technique for dealing with the potential problem is to try to exclude all possible contact of non-human animals with the field.


-- Rivka; Finger Lakes NY, Zone 5 mostly
Fresh-market organic produce, small scale






Archive powered by MHonArc 2.6.24.

Top of Page