Simplified Nitrogen Assessment of Orchardgrass Swards

A sward N index ( N i ) based on herbage N concentration [ N a in g kg −1 dry matter (DM)] and sward mass (DM Mg ha −1 ) was proposed previously for management of N fertilizer: N i = 100 × N a /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 ( N us ). 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 ( N i ) and simplified ( N us ) 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 N us did not change over time with respect to N i once there were about three leaves per tiller. Two close linear relationships between N us and N i were established, one for samplings made after three leaves had appeared on a tiller [about 500 degree‐days (DD) after defoliation of the sward] ( r 2 = 0.92; n = 62; SE = 2.8) and the other before this stage ( r 2 = 0.94; n = 12; SE = 3.3). We concluded that the N us 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: N i = 2.94 N us − 20.59 and N i = 2.28 N us − 12.07, respectively, after and before 500 DD.