An experiment was carried out during winter, spring and summer 1997, on native pastures of the basaltic region of Uruguay to evaluate the effects of different herbage mass levels (high, medium and low) on sward characteristics and on sheep and cattle diet selection and grazing behaviour. Higher herbage masses resulted in increments in herbage accumulation and sheep bite weight: winter (1880, 1513 and 610 kg DM ha-1, P \u3c 0.01; 199, 148 and 128 mg DM/bite, P \u3c 0.01), and summer (3046, 2175 and 1172 kg DM ha-1, P \u3c 0.01; 214, 175 and 143 mg DM/bite, P \u3c 0.01) for high, medium and low herbage mass levels respectively. The nutritive value of diet selected by sheep and cattle was higher than that of the herbage on offer, and higher in sheep than in cattle: winter (10.2, 15.9 and 12.5% crude protein, CP, P \u3c 0.05; 49.2, 32.4 and 38.1% acid detergent fiber, ADF, P \u3c 0.01, and summer (7.4, 10 and 9.5% CP, P \u3c 0.05; 51.1, 36.8 and 43% ADF, P \u3c 0.01) for herbage on offer, sheep and cattle diets respectively. These contrasting results found in the nutritive value were closely associated with differences in the proportions of the botanical components recorded, particularly the differential contribution of dead material. This experiment quantified the importance of diet selection in determining the nutritive value of the forage eaten by sheep and cattle on the native pastures of the basaltic region of Uruguay and established some relationships between diet selection and grazing management, involving animal selectivity ability differences (sheep versus cattle) and seasonal effects
