The 10 key commodities studied were bread wheat, potatoes, oilseed rape, tomatoes, beef, pig meat, sheep meat, poultry meat, milk and eggs and the main conclusions are as follows:

• Nitrous oxide (N2O) is the single largest contributor to global warming potential (GWP) for all commodities except tomatoes, exceeding 80% in some cases.
  
• Organic field crops and animal products generally consume less primary energy than non-organic counterparts owing to the use of legumes to fix N rather than fuel to make synthetic fertilisers. Poultry meat and eggs are exceptions, resulting from the very high efficiency of feed conversion in the non-organic sector.
  
• The relative burdens of GWP, acidification potential (AP) and eutrophication potential (EP) between organic and non-organic field-based commodities are more complex than energy and organic production often incurs greater burdens.
  
• More land is always required for organic production (65% to 200% extra).
  
• All arable crops incur smaller burdens per t than meats, but all commodities have different nutritional properties and energy requirements beyond the farm, so care must be taken in comparisons.
  
• Ruminant meats incur more burdens than pig or poultry meats, but ruminants can derive nutrition from land that is unsuitable for the arable crops that pigs and poultry must eat.
  
• Heating and lighting dominate the burdens of tomato production; but maximising the national use of CHP could reduce the primary energy consumption by about 70%.
  
• Non-organic, loose classic tomatoes incur the least burdens and they increase progressively and definably towards organic, on-the-vine specialist types.
  
• The model has been used to inform other research projects and is well placed to analyse variations in existing production systems as well as being readily developed for new systems or commodities.
  
• The model can be accessed via the Cranfield University web site