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Nitrogen Management

Simplified Nitrogen Assessment of Orchardgrass Swards

UMR INRA-ENSAT ARCHE, Chemin de Borde Rouge, BP27, 31326 Castanet Tolosan, France

^* Corresponding author (mduru{at}toulouse.inra.fr)

Received for publication September 15, 2003. 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 x 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] (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 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.

Abbreviations: DD, degree-days (0°C base temperature) • DM, dry matter • LAI, leaf area index • N_a, actual sward nitrogen concentration (g N kg^–1 DM) • N_i, sward nitrogen index (varying from 40 to 120) • N_us, nitrogen concentration (g N kg^–1 DM) of approximately the upper 7 cm of sward

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