Assessment of yield gap due to nitrogen management in wheat

To assess effect of N fertilizer on yield gap (the difference between maximum attainable yield and actual yield) on wheat farms, a long-term simulation was performed using the CropSyst model and a farm survey study in 2007 - 2008 in Gorgan, Iran. Therefore, the yield simulation was performed after the different N fertilizer treatments (0 to 300 kg ha^sup -1^) under conventional agronomic practices conditions in studied area. The rate of optimum N fertilizer and maximum attainable yield were estimated using a two-segmented regression model as 171 kg N ha^sup -1^ and 5.6 t ha^sup -1^, respectively. The yield gaps between maximum attainable yield and other simulated attainable yields ranged from 0.5 to 3.6 t ha^sup -1^. It seems that yield gaps were a consequence of the deficiency of N fertilizer. The higher the yield gap, the lower the applied N fertilizer. The variation range of yield gap between maximum attainable yield and farmers yield under different N fertilizer applications was -0.4 to 3.4 t ha^sup -1^ and 0 to 2.5 t ha^sup -1^ for 2007 and 2008, respectively. The assessment of applied N fertilizer on farms indicated that nearly all of the farms had applied N fertilizer the lower than 171 kg ha^sup -1^. Therefore, it is expected that there is a yield gap derived N fertilizer on farms. The results showed that in a given level of N fertilizer in both simulation and farm conditions, the yield gaps are different. This can be related to difference between implemented agronomic practices under simulation and farm conditions. Keywords: N fertilizer; wheat; simulation; field survey; yield gap.

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