In order to clarify plant productivity and the grassland condition in mountain pasture, orchardgrass (Dactilys glomerata L.)-pasture which has been given with compound fertilizer totalling N 171kg, P_2O_5 111kg, K_2O 171kg/ha/yr., and grazed by young cattles for about ten years under rotational system, was investigated throughout 1975 and 1976. The pasture examined lies at the sourthern hillside of Mt. Asama with 1100m altitute, within the campus of Alpine Branch of National Grassland Research Institute, located in middle mountainous region of Japan. And at this area the annual mean temperature and the sum of annual precipitation are 8.6℃ and 916mm, respectively (Fig.1). The experimental pasture had been used by grazing rotationally by 50-60 heifers onto 8.69 ha subdivided into 7 paddocks for ten years since its establishment, with the stocking intensity of 500-700 Cow Days per hectare. During this experiment, 55 castrated Holstein steers, 5-7 months old, and 10 heifers of Japanese Black Breed, 11 months old, were grazed in 1975, and 46 Holstein heifers, 6-7 months old, and 10 heifers of Japanese Black Breed, 10 months old, were grazed in 1976. A paddock of 0.9 hectares of the pasture was used for herbage production and consumption throughout this experiment. Dry matter in both aerial and underground organs of orcharadgrass and surface litter were harvested from early spring, the end of March or the beginning of April, to just before the soil being frozen, early in December. The harvestings were invariably carried out at both pregrazing and postgrazing during grazing season, from the beginning of May to the beginning of November. Twenty quadrats of 1m^2 and forty ones of 0.5m^2 were taken at each sampling to estimate the amount of aerial parts above 5cm from the ground surface, in 1975 and 1976, respectively. On the other hand six quadrats of 50×50cm were done to estimate aerial parts under 5cm above ground surface and underground parts. Results obtained are as follows ; 1. Aerial part attained its maximum weight, 500-600g DM/m^2 in the end of June in both years, then it decreased towards middle summer and recovered the weight towards autumn when measured at pregrazing, higher in 1976 and slightly lower in 1975 (Fig.2). The seasonal pattern of standing aerial biomass changed irregulaly. On other hand the amount of green leaves left did not change largely throughout growing season because the nearly constant amount of them was grazed by cattles during each grazing period (Fig.2). Underground part was about 300g/m^2 in early spring and reached about 600g in the beginning of June in both years and it dropped rapidly towards summer, thereafter increased gradually towards autumn and attained to about 300g before soil was frozen (Fig.3). As the results, it is considered that respiration of the plant comunity for maintenance was very small during winter. 2. Leaf area index (LAI) was 3-6 at pregrazing, with the average of 3.5 in 1975 and 4.7 in 1976. On the other hand LAI at postgrazing was 2-3.5, with the average of 2.1 in 1975 and 2.3 in 1976. 3. Light interception was 87% at pregrazing and 67% at postgrazing in 1976 (Fig.6). 4. Net primary production (P_n) can be expressed by the following equation. P_n=ΔB+NL+G, where ΔB, NL and G stand respectively for the biomass change, plant losses by death and shedding, and herbage intake during a specified period. And NL was caught by capturing shed aerial parts on the slaked lime that had been distributed previously in this experiment. G was biomass difference between pregrazing and postgrazing, included the estimated amount of growth (A). Then, annual net primary production (P_N)=ΣP_n=(B_n-B_1)+ΣNL+ΣG, where B_1 and B_n stand for the biomass at the beginning and the end of the experiment. According to investigated results, ΣG was 882, 870, and ΣNL was 473, 342g in 1975 and 1976, respectively. On the base of these values, it was estimated

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