maine12, your local range management specialist should be able to tell you which grasses or grasses legume mix can produce the most dry matter over the year. it will also depend on how it is managed, as ground cover not closely grazed is underestimated as a means to preserve moisture in the soil to be utilized for evapotranspiration through the stoma on the plants leaves. root depth is also a consideration, which might mean perrenial grasses. an interesting site for grasses is the land institute. notice their logo is roots. they've been around for at least 25 or so years. i couldn't find it, but somewhere on the site, they've done some research digging up entire plants root systems and compared annuals and perrenials, their mass, abilitity to take up water over a longer period of time and how this affects when what type of plant flowers and seed fills. it's not a simple answer, and plants that flush during the rainy season are ok with heavy grazing, but the perrenials, you will have to study how much length, leaf area is required to not be eaten for them to keep growing and stay in the field. if they are overgrazed, annuals or other plants with shallow root systems tend to take over. also, shallow rooted plants tend to favor compaction of the soil, interfering with water penetration. also, clovers and deep rooted plants make "waterways" for water to penetrate deeper in the soil, particularly in clay soils. you can see this by planting different types of plants in the same soil in different spots and doing spot digs and compare how easy it is to dig a hole with a shovel in the different areas. since we don't see roots, we tend to ignore them. the nice thing about perrenials, is that they actually thrive when grazed, in the sense that if you don't, they tend to decay as their carbon to nitrogen ratio intensifies, and sometimes the only way to reduce it is through a prairie fire when they get so bad. conservationists are finally figuring this out as they are letting cattle back in to federal lands where they have eliminated grazing.