In forage production areas forage is periodically removed by grazing/cutting and there is a close interdependence between shoots and roots. Root mass and shoot:root ratio help explain how environmental factors affect and modify plant growth. The objective of this experiment was to quantify and discuss root mass and shoot:root ratio of three forage genotypes submitted to two clipping frequencies, either irrigated or rainfed. The experiment was carried in Piracicaba, Brazil. Data were collected during the summer (December to March) of 2012 and 2013 from both an irrigated and a rainfed experiment. The experimental design of each experiment was a randomized complete block with a 3x2 factorial arrangement, combining three grasses (Brachiaria hybrid Convert HD 364 ("cv. Mulato II"), Brachiaria brizantha (A. Rich.) Stapf cv. Marandu and Tifton-85 bermudagrass) harvested at two frequencies (28 and 42 days) and four replications, (4- x 5-m plots). . Forage was harvested mechanically at 10 cm and roots were collected in a frame of 0.50 m by 0.20 m to 0.20 m depth, washed and dried at 65ºC to constant weight. Treatment means were estimated and compared by "Student’s t" test at 5%. Tifton-85 had higher shoot:root ratio under the 28-d clipping schedule (P,0.0001), mainly due to low root mass. Under the 42-d schedule, Marandu palisadegrass accumulated high amounts of forage, resulting in highest shoot:root ratio. Irrigated forage had a higher shoot:root ratio when harvested every 28 days, as the root mass was lower than that of the rainfed plots Under rainfed conditions, higher shoot:root ratio was found for the 28-d clipping frequency, where root mass was lower.