Raw spent litter can contain pathogens that may limit its end use, particularly on horticultural crops for direct human consumption, such as leafy vegetables. The level of risk generally depends on the period between applying the spent litter and harvesting the crop. Nicholas et al. (2007) listed a variety of factors that affect the survival of pathogens in the environment outside their host after land application, including:

1. the presence, quantity and viability of the organism
2. method of application (surface or incorporation)
3. solids content of amendment (solid or liquid)
4. soil conditions, such as permeability, infiltration, soil moisture, texture, pH
5. climate exposure (especially temperature and UV light)
6. the presence of competitive organisms in the soil (Epstein et al., 2002, cited in Nicholas et al., 2007).

From the variety of pathogens that may be present in spent litter and relevant to animal/human health, Runge et al. (2007) categorised the following pathogens into high and medium-risk categories:

– Campylobacter jejuni (high)
– Clostridium botulinum (high)
– Salmonella (high)
– Cryptosporidium (medium)
– Listeria (medium)

For most crops and pastures, Chinivasagam (2008), found that simple precautions will reduce health or food safety risks associated with chicken litter application, with stockpiling found to achieve effective pathogen destruction.

The risk of pathogen transfers to grazing or foraging livestock on pasture/forage that has received spent litter depends on treatment processes and the livestock withholding period after application (Runge et al. 2007). They suggested that using spent litter for forage production poses negligible risk to livestock due to the time period between application and harvest.

Spent litter should be managed to minimise risks to personnel handling the material during spreading, with the main risk being via ingestion of material contaminated by litter. Risks can be minimised by ensuring high standards of personal hygiene when handling litter and avoiding very dusty and windy conditions.

Pathogen risk with horticultural crops

The risks associated with pathogens contained in spent litter primarily relate to contamination of food products with spent litter that are subsequently ingested by humans. Hence, the greatest risk relates to leafy vegetables that are consumed fresh. Refer to the Guidelines for Fresh Produce Food Safety 2019 (FPSC A-NZ, 2019) for information regarding the application of organic manures. Some recommendations include:

– The period between application of products containing manure and crop harvest should be as long as possible.
– Untreated manure must not be used on growing sites within an exclusion period of 45 days from harvest for most vegetables. This period should be 90 days for vegetables like rockmelon and lettuce, where the outer layer comes in contact with the ground.
– Manure may be used within the exclusion limits if subjected to a treatment verified to achieve coli <100cfu/g and Salmonella Not Detected/25g.
– To prevent dust or runoff contaminating adjacent crops, ensure untreated manure is not applied in rainy or windy conditions and is incorporated into the soil after application.
– Manures are considered treated if they have been subjected to the times and temperatures proven to kill human pathogens. Evidence must be obtained when purchasing treated fertilisers and soil additives from a supplier, such as a certificate confirming that compost has been treated in accordance with Australian Standard AS 4454-2012 (Australian Standards, 2012). This standard specifies that:
– The materials must be kept aerated and outer layers turned into the centre
– Piles must be heated to ≥55 °C for three consecutive days
– For windrows, the materials must then be turned, and the heating process repeated four more times to ensure all materials are thoroughly treated.