In‐Season Tissue Testing to Optimize Soft Red Winter Wheat Nitrogen Fertilizer Rates: Influence of Wheat Biomass

In the southeastern USA, soft red winter wheat ( Triticum aestivum L.) N fertilizer recommendations are based on growth stage (GS) 30 tissue testing and models that assume that the relationship between tissue N concentration (N con ) and optimum N fertilizer rates (MaxN 30 ) is stable across fields differing in GS‐30 biomass. However, previous research has indicated this may not be the case. Consequently, it was critical to re‐evaluate these models. Using a split‐split plot design, six experiments were conducted in North Carolina between 2002 and 2004. Main plots were planting date–seeding rate combinations that produced wheat with different GS‐30 biomass. Subplots and sub‐subplots were five N rates applied at GS‐25 and GS‐30, respectively. Wheat yield was responsive to fertilizer N at all site‐years. The overall relationship between MaxN 30 and N con was weak ( r 2 = 0.43). The relationship between MaxN 30 and N uptake (N con × biomass) was weaker ( r 2 = 0.27). However, when the data were divided into different biomass classes, the overall model improved ( R 2 = 0.75). For biomass < 340 kg ha −1 , the N con at which no additional N fertilizer was required (N critical ) was 70.0 g N kg −1 . As biomass increased, N critical decreased to 33.2 g N kg −1 . Intermediate classes had slopes of MaxN 30 versus N con and N critical values that were similar to those previously reported. This study indicates that to use tissue testing to determine N fertilizer recommendations across a range of GS‐30 biomass conditions requires information regarding dry matter biomass.