Efficiency has replaced productivity as the prime rationale for research and development in agriculture. The reasons for this are to (1) reduce input costs when output is static; (2) to make best use of limited renewable resources; (3) to conserve both non-renewable resources and the quality of the environment. Efficient agriculture and conservation are often seen as opposing arguments because parties are arguing from different, incomplete premises. Blaxter (1975) explored the biological efficiency of agricultural systems using energy as a unit of account. He recognised that it was primarily the use of fossil energy as fuel (0.42) and as fertiliser (0.38) that had increased the productivity of U.K. agriculture but that “in the long term …the world must eventually reach a steady state in which energy provision will not entail a depletion of its capital resources.“