For the first time in Australia, the applicability of carcass-grinding and composting was evaluated for mass disposal in an emergency animal disease (EAD) outbreak. NSW DPI has successfully completed a composting field trial on a large commercial piggery in Southern NSW, providing important preliminary data on process performance and insight into how grinding and composting could be implemented effectively in an EAD outbreak.
The feasibility of mass disposal options for an EAD outbreak in piggeries is disease and site-specific. On-farm disposal is likely to play a prominent role in cases where restrictions are placed on the movement of infected materials including carcasses (e.g., for African swine fever (ASF) or foot-and-mouth disease (FMD) outbreaks).
Composting is an approved mass disposal method under AUSVETPLAN (Animal Health Australia, 2021), but its practical application in an Australian EAD is hampered by a number of issues, including (Price et al., 2020):
Yet, mass disposal of large animals (e.g., pigs, sheep and cattle) is likely to be more challenging than has often been assumed in the past. Aside from issues to do with movement controls placed on carcasses in an EAD, gaining access to sufficient landfill space is not guaranteed, and there are concerns around the sustainability of on-farm trench burial as a mass disposal method.
For this reason, both industry and government agencies are committed to investigating alternative carcass disposal options, particularly now that both ASF and FMD are pressing concerns. The trial reported in this paper makes a significant contribution to the growing body of work currently underway on this subject.
Three separate composting windrows were set up in early March 2022. The 'control' treatment (Treatment 1) involved the traditional layered method of carcass composting in which pig carcasses were placed on a bed of straw followed by a layer of spent bedding and a second layer of carcass material (Figure 1). The whole pile was then capped with straw. The second windrow (Treatment 2) was set-up just like the first, but the contents of the whole windrow were fed through an industrial horizontal grinder after 14 days of composting (Figure 2). After grinding, this windrow was reformed to continue composting. The third treatment (Treatment 3) involved the pre-grinding of carcasses before windrow formation. The carcasses were ground up and then combined with spent bedding using a front-end loader. This mix was then placed on a bed of straw with more straw placed on top. Windrow temperatures were monitored with wireless probes and each windrow was subjected to six turns with a front-end loader.
At the time of this composting trial, the piggery was affected by an outbreak of Japanese encephalitis (JE). This resulted in a mix of mortalities comprising mainly of sows, some boars, neonatals, aborted foetuses and entrails.
Immediately upon completion of windrow set-up, 4 motion-sensitive infra-red cameras (Swift Enduro, wide angle outdoor cameras) were installed at each corner of the site to monitor any activity by scavenging animals during the trial.
Samples of raw material and compost were taken at various times throughout the trial for microbiological testing including faecal coliforms, E. coli, Listeria monocytogenes, Salmonella sp., Campylobacter spp., Yersinia enterolitica, Streptococcus suis, Clostridium perfringens, coagulase positive Staphylococci and Adenovirus (PCR).
Composting for 14 days resulted in a mean 99.97% reduction in faecal coliform counts, 99.98% reduction in E. coli and complete elimination of Adenovirus. In addition, Salmonella was undetected after 132 days composting. After 202 days composting, E. coli counts were below the established limit of <100 cfu/g but faecal coliform counts consistently exceeded the accepted threshold of 1,000 cfu/g. Furthermore, elimination of other pathogens (e.g., Listeria monocytogenes and Leptospira sp.) and Helminths was proven to be less reliable even though standard time-temperature requirements for pathogen reduction had been met. Given the weight of global experience supporting the efficacy of mass mortality composting in eliminating pathogens (Costa et al., 2019; Guan et al., 2010; Pepin et al., 2021; Wilkinson, 2007), the cause of pathogens persisting in this trial is probably due to a lack of process efficiency. Meeting this KPI quickly is highly desirable from a biosecurity perspective, and the project has identified how the composting processes used in this trial can be better optimised.
This composting trial was successfully conducted despite many challenges faced due to the nature of the mortalities (being affected by Japanese encephalitis), minimal equipment (e.g., only one loader and the weighbridge could not be used as originally planned), resources not being available 'on call' and project budget limitations. Contrary to conventional 'wisdom' (Bonhotal et al., 2022; Oshins et al., 2022), we found that composting can be conducted at low C:N ratios with the resultant end-product having similar composition to straw-based piggery spent bedding.
All treatments successfully reached thermophilic conditions (i.e., >45°C) within a few days of set-up (Figure 3, Table 1). Temperatures remained high in all treatments until about day 100, after which each windrow began to cool off. This does not necessarily indicate that the composting process had been completed, however, since the windrows were no longer actively managed after the last turn on day 84. The main objective was not to produce a manure compost, but to dispose of carcasses in a biosecure manner.
|Temp probe position||Days >55°C||Max temp recorded (°C)||Mean temp for trial (°C)|
|Treat 1||Treat 2||Treat 3||Treat 1||Treat 2||Treat 3||Treat 1||Treat 2||Treat 3|
There are sufficient grounds showing that grinding combined with composting may have a significant role to play in a mass mortality disposal operation involving pigs. This trial has provided some useful insight into the applicability of the different processes used, as well as what equipment and other resources are needed for process optimisation and validation.
When comparing the various treatments, the following conclusions can be drawn: