Legumes increase the yield and quality of grass-dominant pastures. They fix atmospheric nitrogen to feed the pasture system and provide highly digestible, protein-rich feed in their own right. This feed in turn fuels growth and productivity in the highest-feed-demanding stock classes
Nitrogen is very commonly a limiting factor to grass growth. You can apply nitrogen fertiliser directly (and sometimes this is necessary in order to gain an instant boost) but biologically fixed nitrogen is a valuable and free mainstay. It is slow-release and helps to build soil biology and fertility. It offers a more sustainable nutrient supply and reduces the risk of nitrate leaching, a potential consequence of using nitrogen fertiliser.
In a pasture, enough legume looks like a lot. A target may be at least 30% of the pasture mass. This is because the benefits of nitrogen fixation are only realised if there are enough legume leaves and plants grown to produce tonnes of dry matter. Every tonne of above-ground biomass can be roughly equivalent to 25 kg of nitrogen. A low percentage of legume means few tonnes and fewer benefits.