The aim of this study was to use the data envelopment analysis for determining the energy efficiency and find the optimum energy consumption in dairy farms of Qazvin city of Iran. In this study have been used from two approaches constant returns to scale and variable returns to scale model of data envelopment analysis for determining the degrees of technical efficiency, pure technical efficiency and scale efficiency. Moreover, the effect of optimum energy consumption on greenhouse gas emissions

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... has been studied and also the total amount of greenhouse gas emissions. The results showed that from total number of dairy farms 42.55% and 53.19% were efficient based on constant returns to scale and variable returns to scale model, respectively. Accordingly, the average score of technical, pure technical and scale efficiencies of farmers were calculated 0.9, 0.94 and 0.953, respectively. The total optimum energy required was estimated 129,932 (MJ cow À1 ). Energy saving target ratio for dairy farms was calculated as 12%. According to results feed intake had the highest share (85.44%) from total saving energy, followed by fossil fuels (11.19%). The total greenhouse gas emission was calculated as 5393 (kgCO 2eq . cow À1 year À1 ) in dairy farms that this amount can be reduced to 4738 (kgCO 2eq . cow À1 year À1 ) with optimum energy consumption. The enteric fermentation had the highest potential to reduction of total GHG emissions with 47% that has a direct connection to the amount of feed intake.