Grain legumes biologically fix nitrogen from the atmosphere, essentially making their own fertilizer, for free. This reduces both the costs and the environmental impacts of chemical fertilizer use, including the climate change impacts of fossil fuel that is burned to manufacture chemical fertilizer. Improving this trait will improve farming system sustainability while also increasing food supplies and incomes. When grain legumes are added to farming systems at different timings in the seasonal crop cycle, the period of time when the land is protected by leafy cover is extended, reducing soil erosion. Grain legumes added to farming systems also increase the effective capture, productive use and recycling of water and nutrients, such as end-of-season residual soil moisture in maize, rice and wheat fallows. By enriching livestock diets, grain legumes also foster crop–livestock mixed farming systems, which are more sustainable than crop-only or livestock-only systems.