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 = 2.94N us − 20.59 and N i = 2.28N us − 12.07, respectively, after and before 500 DD.Please view the pdf by using the Full Text (PDF) link under 'View' to the left.
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