Simplified Nitrogen Assessment of Orchardgrass Swards

A sward N index (Ni) based on herbage N concentration [Na in g kg−1 dry matter (DM)] and sward mass (DM Mg ha−1) was proposed previously for management of N fertilizer: Ni = 100 × Na/48DM−0.32. For the sake of simplification, we developed and evaluated a method based only on the N concentration of approximately the upper 7 cm of sward (Nus). Twenty‐one treatments of pure or dominant orchardgrass (Dactylis glomerata L.) swards, covering a wide range of N fertilizer rates, regrowth periods, sites, and years, were used to establish a relationship between conventional (Ni) and simplified (Nus) sward N status. Detailed studies were conducted on a subset of treatments to determine the N distribution along the length of the leaf and down the sward’s vertical profile. Analysis of N distribution among leaf segments for orchardgrass swards showed that Nus did not change over time with respect to Ni once there were about three leaves per tiller. Two close linear relationships between Nus and Ni were established, one for samplings made after three leaves had appeared on a tiller [about 500 degree‐days (DD) after defoliation of the sward] (r2 = 0.92; n = 62; SE = 2.8) and the other before this stage (r2 = 0.94; n = 12; SE = 3.3). We concluded that the Nus method is good enough to be used for assessing N sward status for a large range of defoliation regimes, without measurement of standing herbage mass: Ni = 2.94Nus − 20.59 and Ni = 2.28Nus − 12.07, respectively, after and before 500 DD.