Organic chickens require 25 per cent more energy to rear and produce more carbon dioxide than conventional battery or barn hens, according to the report.
However, the study points out that there are many organic foods with lower ecological impacts than conventional produce. It suggests there is no clear-cut answer as to whether an organic or a conventional trolley of goods has more or less impact.
The report adds: "Organic agriculture poses its own environmental problems in the production of some foods, either in terms of nutrient release to water or in terms of climate change burdens.
"Similarly there is little evidence that the consumption of locally sourced food products generally has a lower environmental impact than those from further afield."
It points out, for example, that organic tomatoes grown in heated greenhouses in Britain generate 100 times more carbon dioxide per kilogram than tomatoes from unheated greenhouses in Spain.
The Soil Association said the report was "not a comprehensive analysis" and did not cover biological diversity. For example, organic farms have been shown to be better for declining farmland birds such as skylarks and partridges.
A spokesman said it recognised that in some areas, such as poultry and growing vegetables out of season, organic was less energy efficient. But that was outweighed by other factors the Defra study had not considered such as animal welfare, soil condition and water usage.
The association is unhappy with the model used for the study which, it says, amplified the amount of nitrous oxide emissions – a greenhouse gas – and increased the land area used by half.
• Illegal genetically-modified rice may still be on sale months after the Food Standards Agency said it had been withdrawn, Friends of the Earth will tell the High Court today.
A judicial review of whether the agency took adequate steps to protect consumers will hear that a strain of GM rice contaminated supplies last year.
Clare Oxborrow, of Friends of the Earth, claimed: "Instead of acting to make sure the public were not exposed to illegal GM rice, the agency sat back and waited for contaminated products to be sold and eaten."
This isn't to say that organic (or locally grown) food is bad, but it will have different levels of effectiveness depending on a lot of factors including the climate of the region you're talking about and the crop people are growing.
