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Assessment of the risks to food safety associated with spreading of animal manure and abattoir waste on agricultural land

Project Code: B17002

30/04/2002

WRc-NSF Ltd
Gale, P ; Stanfield, G

Traditionally, animal manure and certain wastes from abattoirs, such as “gut contents” and blood, have been used as a fertiliser. The use of animal wastes as a fertiliser may pose risks to food safety by direct contamination of crops and livestock or by more indirect routes such as the contamination of water sources.

Investigation of all the potential routes for transfer of pathogens from these wastes to food crops and livestock is impractical, because of the wide spectrum of microorganisms which could be present and the analytical difficulties of quantifying them in the various environments involved.

Microbiological risk assessment (MRA) can be of assistance in this type of situation. MRA can be envisaged in three stages referred to as load, pathway and receptor. The ”Load” is the number of each of the pathogens likely to be present. “Pathway” is the route that can bring about transfer to the “Receptor” which in this project are food crops and livestock.

In this project the load/pathway/receptor concept is being applied to the routes of disposal of farm manures and abattoir wastes with the aim of producing a model or series of models that can be used to assess the relative risks of different strategies of use.

The overall objective of the study agreed with the Food Standards Agency is: - To carry out a risk assessment to determine whether the application of animal manure and abattoir waste to agricultural land poses a significant risk of pathogens contaminating crops and livestock.

In this the final report of the contract, the conceptual model in the form of a series of event trees is described. The model has been used to predict the extra pathogen loadings on crops that could result from the application of manures and abattoir wastes to agricultural land. The pathogens used in the model are Salmonella, E.coli O157, Listeria, Campylobacter, Giardia and Cryptosporidium. The model has been developed to encompass the agricultural use of slurry and farmyard manure produced on farms and wastes from abattoirs such as blood and gut contents. Included in the disposal strategies were the latest recommendations from the Food Standards Agency draft guidelines for “Managing Farm Manures for Food Safety”. This document was available as a draft for consultation at the time of writing and as such the periods mentioned in this report may not reflect those in the final guidelines.

The study indicated that a disposal strategy which involved storage of manures (slurry and farm yard manure) on farms for prescribed minimum periods, followed by application to agricultural land, could form an effective barrier to the transfer of pathogens from manures to crops or grazing livestock. Time was an essential component of this strategy, since this can ensure adequate decay of pathogen numbers can take place.

Storage of farmyard manure (FYM) is particularly effective since the composting action of these manure heaps generates temperatures sufficiently high to inactivate enteric organisms. Storage of slurries, which do not generate heat, did not appear to be so effective for pathogen inactivation. .

Application of the stored manures to land leads to further pathogen inactivation through exposure to desiccation, solar radiation and predation by other micro-organisms.

The predictive model developed as part of this study allowed the effectiveness of various manure management and disposal strategies to be assessed in terms of pathogen reduction. During the study, the Food Standards Agency issued its guidelines (for comment) on Managing Farm Manures for Food Safety. The effectiveness of these guidelines for achieving pathogen inactivation were tested using the model and were found to be generally good for bacterial pathogens. Predictions for the hardier protozoan pathogens indicated not such a high margin of safety, but this may, in part, be due to reservations about the values used for the decay rate of these pathogens.

Establishing reliable data on pathogen decay is an essential requirement for microbial risk assessment and this decay needs to be able to be expressed mathematically for use in
predictive modelling. Given the wide range of conditions encountered in the agricultural environment, it would be unrealistic to expect a single decay rate to suffice for each pathogen. A range of decay rates would therefore be anticipated but this range needs to embrace the worst and best survival scenarios.

The study indicated that there are some shortcomings in the current state of knowledge about microbial decay and this casts some uncertainty on the results of the risk assessment.

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