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Prior to the 1960s, symbiotically fixed nitrogen (N) in forage legumes in most temperate grassland systems was the main input of nitrogen other than that recycled in manures. However, as inorganic N fertilizer became more readily available in the 1950s, especially in Europe and North America, reliance on forage legumes declined. For example in Britain, between 1964 and 1974, forage legume seed sales declined by 50%, while sales of N fertilizers more than doubled. Despite this trend, optimistic predictions about its reversal were based on results from studies at that time, showing the relatively high potential DM production and N fixation rates of the commonly grown forage legumes and their generally higher feeding value than that of grass (Aldrich, 1974). The energy crisis, precipitated by instability in the Middle East in 1973 threatening ammonia supplies, contributed to forecasts about the decline in N fertilizer use. However, N fertilizer usage in grassland systems in the Northern Hemisphere continued to grow at the expense of reliance on forage legumes. Grassland in the UK in the 1980s was estimated to exploit the equivalent of <10% of the symbiotically fixed N of New Zealand pasture systems.