Ruminant production in the UK is largely dependent on grasslands: approximately 52% of UK land is (improved) grassland and rough grazing. Herbage production from improved grassland can be over five times higher than that of indigenous swards (Davies et al., 1984) and these pastures account for the majority of ruminant (meat and milk) production from grassland the UK. They are also amenable to manipulation through the choice of species, variety and mixture of forages sown. Efficiency of production can be influenced at the levels of both the plant and the animal and by the nutrition and genetics of both. This paper examines strategies to increase production efficiency based upon optimising rumen fermentation linked to plant breeding approaches. In this way efficiency can be improved without recourse to diet manipulation with supplements, which is generally impractical at grazing. Although the rumen microbial population allows the ruminant animal to extract energy from otherwise unusable sources (i.e. fibres), the rumen is also the source of greatest inefficiencies in the use of nitrogen and energy. Degraded plant nitrogen that is not captured by the rumen microbes tends to be absorbed as ammonia, much of which is excreted as urea and contributes to ammonia and nitrous oxide emissions. Similarly, methane release from the rumen represents a waste of energy that could otherwise be used for production. Globally, these two processes result in ruminant livestock accounting for approximately a third of the methane emissions (Beauchemin et al., 2008) and half of the nitrous oxide emissions (de Klein and Eckard, 2008) from anthropogenic sources.