In-Season Tissue Testing to Optimize Soft Red Winter Wheat Nitrogen Fertilizer Rates

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 (Neon) 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 Neon was weak (r 2 = 0.43). The relationship between MaxN 30 and N uptake (Neon X 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 Neon 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 Neon 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.