The use of environmentally friendly diets (e.g., low-protein diets supplemented with synthetic amino acids for balancing amino acid contents) in animal production systems is receiving attention as a method to reduce greenhouse gas (GHG) emissions. The objective of the present study was to quantify how dairy and pig farmers have the preference for such environmentally friendly diets by conducting a discrete choice experiment, one of econometric methods, with 29 dairy farmers and 30 pig farmers. In order to evaluate the farmers' preference for environmentally friendly diets, farmers were offered a series of choice sets including the five attributes as follows : quantity of the product, ability to reduce GHG, quality of the product, palatability of the diet and price of the diet. As the results, both dairy and pig farmers showed great preferences for "quantity of the product" and "quality of the product", i.e. the attributes directly related to their income, and they also showed the positive preference for "ability to reduce GHG". Moreover, dairy farmers showed larger preference for "quantity of the product" than for "quality of the product", whereas pig farmers showed large preferences for both "quantity of the product" and "quality of the product" with similar levels.

To quantify what attributes of wastewater treatment systems are preferred by pig farmers, a discrete choice experiment was conducted by a questionnaire survey of pig farmers in four prefectures, Japan, in the present study. Farmers were offered a series of choice sets including the following five attributes of wastewater treatment systems: ability to improve water quality, ability to remove odor, technical difficulty, cost, and ability to reduce greenhouse gas (GHG) emissions. As the results, the farmers showed great preference for "ability to improve water quality", indicating that pig farmers are strongly aware of the temporary effluent standards under the Water Pollution Prevention Act as expected. Moreover, farmers showed slightly positive preference for "ability to reduce GHG emissions" that is not currently subject to regulation. In the future, in order to improve farmers' willingness to pay for the attributes of wastewater treatment systems related to environmental improvement, it is suggested that it is essential to further inform farmers of the importance of environmental protection and present the specific measures.