Composting has long been used as a suitable management tool for handling the solid wastes produced in layer hen facilities and is now becoming a more common practice for managing spent hens and mortalities.

Spent hens have traditionally been removed from farm and converted to human and animal food at processing facilities. However, the Australian egg industry is investigating other viable options to handle this byproduct due to the potential lack of availability and capacity of these processing facilities to accept and process spent hens. Additionally, transport costs and the loss in farm productivity while a shed is being destocked has made this traditional method less financially viable.

FSA Consulting, from Toowoomba in Queensland, Australia, was engaged by the Poultry Cooperative Research Centre (CRC) to undertake a project titled "Odor Measurement and Impact from Spent Hen Composting."

Project leader Eugene McGahan explained, “To quantify odor generation, emissions produced by composting spent hens on-farm, and the likely impact these have on community amenity, different odor modeling scenarios were run for two sites.” The modeling scenarios included composting emissions only; sheds emissions only; and shed and composting emissions combined.

Two objectives

This research had two primary objectives: quantify odor generation and emissions produced by composting spent hens on-farm and the likely impact these have on community amenity; and to determine if different cover materials, moisture levels or compost ages influence odor generation at the Pittsworth (QLD) and Tamworth (NSW) trial sites.

To address these objectives, a comprehensive review of the literature on odor emissions from composting (specifically animal composting) was conducted, along with the measurement of odor emission rates from the two trial sites.

This emission data profile was used to assess the likely increased odor impact of a layer farm if it changed from exporting spent hens off-farm to a composting process on-farm.

The two trial sites were chosen to provide varying demographics and meteorological conditions under which the composting was undertaken. The farms also used different methods of composting; however both were typical of practices used by the industry.

Two different composting substrate materials (layer manure and sawdust) were used to represent typical materials that are available and used by Australian egg producers that currently utilize composting as a method to manage hen mortalities and spent hens. Summer and winter compost trials were conducted at each site.

A total of 99 odor emission samples were collected across the two sites using a flux chamber. These were then analyzed at the Department of Agriculture, Fisheries and Forestry olfactometer in Toowoomba, using the AS/NZS 4323.3 standard. The odor samples collected included the two different substrate materials (sawdust and manure); both wet and dry windrow surfaces; and disturbed and undisturbed windrows.


Additionally, testing was conducted to investigate the decay in odor emission up to three hours post disturbance of a windrow. Some of the samples were also assessed for character to determine if aged compost produced a different type of odor to freshly placed windrows.

Wet vs. dry

Results indicated odor emissions for wet and dry compost windrows were typically higher in the initial few weeks and then dropped away with time. The difference between wet and dry manure substrate did not appear to be significant in the period close to placement. However, wet manure, when aged, appears to have higher emissions than dry manure. The sawdust substrate-based emissions rose from Day 7 to about Day 28 and then decreased. The data indicated that wet windrows tended to have elevated emissions compared with dry windrows, at least in the first few weeks. In contrast, the manure substrate emissions were highest at Day 7, dropped at Day 10, then rose slightly at Day 28 before dropping away to a background value.

The odor decay experiment, where emission rates were estimated immediately after turning, 1 hour after turning, and 3 hours after turning, showed that the emissions rose after turning but dropped off again rapidly.

Minimal impact on overall emissions

Experience with composting has shown that the finished product is often far less odorous than the initial product. This might be expected, as the initial material is high in protein and fats, which decompose creating a wide range of odorous gases. Decomposition converts the original complex chemistry into simpler, less odorous products. Different odor compounds are expected to be produced at different stages of composting. Odor descriptors were collected for 56 of the odor samples collected and whilst not conclusive, it was observed that the general character of the odor changed from “decaying, putrid, pungent, chicken manure” for a new windrow to  “silo smell, earthy, damp soil, veg patch” for a composted windrow.

One item not factored into the assessment was the change in character, from more offensive to less offensive, of the odor over time from the windrows. The impact assessment modeling assumed that the odor is additive to that from other sources.

In reality, compost odor is less offensive than shed odor (if covered and managed appropriately) and therefore the assumption that it is additive is only likely to be relevant for the first few weeks after placement. At other times, the additive assumption is likely to be conservative.

Overall, the odor modeling impact assessment showed that the addition of composting operations to a typical farm (based on flux chamber measured emission rate data) would have a negligible impact on overall emissions. “However, this is based on the assumption that the windrows would be placed and then disturbed infrequently, and managed appropriately,” said Eugene.