Understanding the Differences Between GHG Modelling Tools
Farmers, dairy processors, and meat processors now have access to a range of greenhouse gas (GHG) modelling tools. Some of these tools focus on emissions at the farm level, while others assess emissions associated with final products - like a kilogram of milk powder or a leg of lamb.
Three of the most referenced tools are OverseerFM, Ag:LCA, and the Farm Emissions Model (FEM). These tools will produce different results - understanding why is important. The differences reflect deliberate design choices made to suit each tool’s intended purpose.
Farm Emissions Model (FEM)
Managed by MPI | Released 2024
The FEM is a simple and accessible tool designed to estimate agricultural GHG emissions at the farm level. Its primary purpose is to support New Zealand's international climate commitments, and it is built with scalability and consistency in mind.
To keep the tool practical and easy to use, FEM combines farm-specific information with regional and national averages. It does not include embedded emissions - that is, emissions associated with products or inputs brought onto the farm (e.g., fertiliser) - because these are handled elsewhere in national accounting frameworks.
Ag:LCA
Managed by AgResearch | Release 2024
Ag:LCA is a full life cycle assessment (LCA) tool used primarily by processors. It maps emissions across the entire value chain - from upstream inputs through on-farm emissions and downstream activities such as transport and processing, right up to the end consumer.
This tool is designed to support market-facing reporting obligations, especially for international supply chains and sustainability claims. By accounting for both embedded and post-farm-gate emissions, Ag:LCA gives processors a comprehensive view of product-level emissions.
OverseerFM
Managed by Overseer Ltd | GHG Modelling since 2003
OverseerFM models the flow of nutrients around and beyond a farm system, and the associated impact on both nutrient losses to water and GHG emissions. The latter includes both on-farm emissions (e.g., enteric methane) and embedded emissions (e.g., the emissions associated with imported feed or fertilisers).
This design allows farmers and their advisers to understand the impact of management decisions and avoid “pollution swapping.” For instance, replacing local grain with imported soy might appear beneficial until the full emissions context is assessed by the Overseer model.
OverseerFM does not model emissions beyond the farm gate (such as transport or processing), so it is best suited to farmers, advisers, and policy stakeholders who need a tool to support long-term, farm-focused sustainability decisions. For a detailed explanation of its emissions modelling, refer to Wheeler, Ledgard and deKlein (2008)* .
Summary
| Tool | Primary User | Scope of Emissions | Key Strength |
| FEM | Government reporting | On-farm only (simplified approach) |
Simple, scalable, aligned with NZ’s inventory method |
| Ag:LCA | Processors | Full supply chain (cradle to consumer) |
Market-ready emissions reporting |
| OverseerFM | Farmers & advisers | On-farm + embedded (stops at farm gate) |
Strategic farm system decisions & nutrient insights |
* Wheeler, Ledgard and DeKlein, (2008), Using the OVERSEER nutrient budget model to estimate on-farm greenhouse gas emissions. Australian Journal of Experimental Agriculture 48(2) P.99-103