Introduction: Improved pastures, integrated by forage legumes, are the primary base of agriculture and cattle rearing in Uruguay. The growing demand for cattle products requires higher cattle productivity, which is at the present time limited by the yield and quality of the natural pastures, mainly based in summer grasses. Perennial sown pastures in Uruguay have had a slow but sustained increment in area in the last five decades, from 17.000 has in 1950 to 1.287.000 has in 2003, representing 7% of total pastoral area (DIEA, 2004). In spite of agricultural policies that promoted the sowing of introduced legumes in the sixties, natural grasslands over sown with legumes represents 5% of the total area at the present time (0,7 million has; DIEA, 2004). Diverse factors have influence on the slow adoption of sown pastures, among others the cost of establishment, the uncertainty of the physical result due to poor persistence of the introduced legumes, establishment difficulties, the need of careful defoliation management that require some legumes that produce bloat.