Improved feed efficiency (FE) is a key parameter for increased sustainability in modern diary production. The project aims to increase the efficiency of converting feed into milk and reducing the environmental impact of dairy production. In order for farmers and breeding companies to include FE as a trait, FE must be scientifically defined and the animal's variation in FE linked to feed and animal parameters be further explored. The project will initially use Nordic and international data (big data sets) with information on animal consumption, production, digestibility and greenhouse gas emissions (GHG). How the FE property correlates with other properties such as digestion, energy metabolism, N-efficiency and health will be calculated to create opportunities for a successful Nordic breeding program. New methods, techniques and calculations will be developed to estimate FE at farm level, which may lead to new methods and tools for use in classical and genomic selection systems.