GROWTH ANALYSIS OF A TALL FESCUE SWARD FERTILIZED WITH DIFFERENT RATES OF NITROGEN

Nitrogen fertilization increases yield of perennial grasses, but little knowledge exists on the effect of N fertilization on growth processes at the sward level. Growth of an irrigated tall fescue (Festuca arundinacea Schreber) sward fertilized with different rates of N was analyzed during two summer regrowths (1987 and 1988) and two spring regrowths (1987 and 1989). When no N was applied, cumulative shoot growth at the end of each regrowth period (summer 1987, summer 1988, spring 1987, and spring 1989) was 29, 15, 36, and 42% of that obtained with the highest N rate used in that period (160, 240, 200, and 180 kg N ha-1), respectively. Analysis of the N nutrition status of the sward based on the concept of an optimal N content indicated that N was not limiting shoot growth at the highest N rates used in any of the regrowths except during the summer of 1987. Radiation-use efficiency at the highest N rate was 1.68 and 1.82 g DM MJ-1 in the summer regrowths of 1987 and 1988, and 1.83 and 2.16 g DM MJ-1 in the spring regrowths of 1987 and 1989. Cumulative intercepted photosynthetically active radiation (PAR) and radiation-use efficiency both increased with an increase in N rates. The effect of the N deficiency on cumulative intercepted PAR was, for the most part, limited to situations where the relative N concentration was <0.5. Radiation-use efficiency, however, was affected through most of the range of relative N concentrations.