Farmyard manures (FYM) have a dry matter content of more than 10% (weight/volume). By definition, slurries have a lower dry matter content of 6% (w/v) or less. Over the last 50 years, there has been a marked tendency towards slurry-based livestock systems because the waste can be pumped and so is more conveniently (and less labour intensively) handled (Strauch and Ballarini, 1994). In contrast, FYM often contains bedding materials such as straw or sawdust and the presence of such materials helps aerate the waste. The numbers of human pathogenic bacteria in livestock wastes will decline over time. But as a consequence of the different chemical compositions, pathogens decline at different rates during the batch storage of FYM and slurries. Compared with freshly deposited manure, the numbers of bacteria in batch stored material will nearly always be lower (Table 1). A number of factors influence how quickly microorganisms die off in livestock waste including, the waste temperature, the water content of the material, chemical properties of the waste such as its pH, the availability of nutrients that microorganisms can use to sustain themselves and external factors such as the intensity of sunshine (which contains sterilising UV light).
Table 1 A summary of the numbers of zoonotic pathogens measured in British livestock manures containing zoonotic agents in freshly deposited and non-batch stored manures. Data shown are calculated as both arithmetic (A) and geometric (G) means for positive isolations only. Highest levels observed for each pathogen and manure type are also shown (M). The number of positive isolations used to calculate each mean is shown (n)
Microbiological declines in manures directly deposited onto land
Avery and colleagues (2004) report information on the decline of Escherichia coli in livestock manures deposited directly onto land. Groups of cattle, sheep and pigs were penned outdoors on grass during November, and removed after 14 days. E. coli populations in the ground declined over time with the maximum observed survival time being roughly 6 months. E. coli originating from cattle and sheep had average decimal reduction times (D-values which are the amount of time it takes for a bacterial population to decrease by 90%) of 38 and 36 days, respectively. E. coli originating from pigs declined significantly faster (average D-value of 26 days). The workers concluded that E. coli from livestock faeces can commonly survive on grass for 6 months, affording opportunity for pathogenic biotypes to contaminate animals, plants or water during that time. This study is the basis of the minimum time interval of six months specified in the FSA manures guidance in cases where "livestock grazing is an essential part of the farming system". For most farms, the FSA advise a 12 month interval between crop harvest and last direct manure deposition.
Figure 1 Decline in Escherichia coli populations in faeces : soil mixtures taken from the ground after farming (a) cattle, (b) sheep and (c) pigs. Each point represents the average of 20 replicates. Error bars indicate standard deviations at each sampling time. (reproduced with permission from Avery et al., 2004)
Microbiological declines in manures from housing during storage
Hutchison and colleagues (2005) determined how E. coli O157, Salmonella, Campylobacter and Cryptosporidium decline in FYM heaps. Using funding provided by the UK Food Standards Agency, the workers artificially inoculated dairy cattle, beef cattle, pig, sheep and poultry FYM with each of these zoonotic agents and monitored their decline over time. The experiments were timed to mimic British agricultural practices such as stock turn out from winter housing in April. The project team observed initial rapid declines in the numbers of zoonotic agents that were introduced into the wastes (Figure 2). The longest that any of the pathogens survived was three months. The findings of the Hutchison work were recently verified by Berry et al 2013.
Figure 2 Typical decline observed when zoonotic agents were introduced into 5 m3 volume farmyard manure heaps. Data was generated from wastes produced by beef cattle. Bacteria shown are Salmonella (circle), Escherichia coli O157 (square), Listeria (triangle) and Campylobacter (diamond). The practically determined limit of detection for the test methods is shown by the dotted horizontal line (reproduced with permission from Hutchison et al., 2005).
Hutchison makes clear that the interval is not an absolute cut off and that it is valid only for the zoonotic agents and wastes used in the experiment. The project team that undertook the decline work believe that storing FYM for six months before application to land provides a reasonable balance between food safety and the logistics of extended manure storage.
Hutchison and colleagues have also assessed die off of E. coli O157 , Salmonella, Listeria, Campylobacter and Cryptosporidium in livestock slurries. As before, the zoonotic agents were grown in a laboratory and introduced into slurry from pigs or cattle as well as dirty water (a dilute slurry generated by dairy farms). Die off was monitored over time. The results showed that although all of the bacterial zoonotic agents were dead by 4 months, the Cryptosporidium oocysts were still viable in large numbers at that time (Figure 3).
Figure 3 Fate of Cryptosporidium parvum oocysts in dairy cattle slurry during Winter and stored in 35,000 litre batches. Oocysts (5 x 104) were added to polypropylene jars, which were filled with 100 ml volumes of dairy cattle slurry. Each jar was sealed at the neck with a semi-permeable membrane before submersion into a slurry tank. Data are arithmetic means from triplicate samples; error bars are the associated standard deviations. (reproduced with permission from Hutchison et al., 2004)
Avery, S. M., Moore, A. and Hutchison, M. L. 2004. Fate of Escherichia coli originating from livestock faeces deposited directly onto pasture. Lett. Appl. Microbiol. 38,355-359.
Berry, E., Millner, P. D. Wells, J. E., Kalchayanand, N. and Guerini M. N. (2013) Fate of naturally occurring Escherichia coli O157:H7 and other zoonotic pathogens during minimally managed bovine feedlot manure composting processes. Journal of Food Protection 73,1308-1321.
Strauch,D. and Ballarini,G. 1994. Hygienic aspects of the production and agricultural use of animal wastes. Journal of Veterinary Medicine Series B-Zentralblatt fur Veterinarmedizin Reihe B-Infectious Diseases and Veterinary Public Health 41, 176-228. (reference is too old to be available electronically).