Evaluating In-Season Tests to Guide Nitrogen Topdressing Rates for Organic Winter Bread Wheat.

Nitrogen (N) management is a key challenge for winter bread wheat production, especially for organic producers.  Readily available N applied at planting is subject to over winter losses, and mineralization of organic N sources is difficult to predict and lags behind crop demand in the early spring.  Failure to provide the correct rate of N at the proper times reduces grain yields and protein. Spring N topdressing, a recommended practice for conventional winter wheat production, is a viable strategy for organic producers to boost yields and grain protein (Mallory and Darby, 2013), however, methods are needed to determine if and how much topdress N is cost effective.  In-season diagnostic tests are used successfully to guide topdress decisions for conventional winter wheat.  The N topdress rate at spring green-up (Zadok growth stage (GS) 25) is adjusted based on tiller density to optimize yield; the rate at jointing (GS30) is adjusted based on tissue N concentration to optimize grain protein.  The objective of the current research was to evaluate these in-season N tests under organic production.  Treatments were a combination of pre-plant dairy manure application rate (0, 78, and 118 kg ha^-1 available N) and spring topdressing.  Topdress treatments included no topdress and 39 kg N ha^-1, as Chilean nitrate, applied at GS25, GS30, or both stages.  In all cases, splitting N applications between fall and spring or topdressing additional N increased both grain yield and protein.  Tiller density at GS25 was strongly affected by the manure treatments (p < 0.001), but was not a good predictor of grain yield (adjusted R² = 0.45).  Likewise, tissue N at GS30 was affected by manure and GS25 topdress treatments (p < 0.001), but correlated weakly with grain protein (adjusted R² = 0.30).  Further testing with treatments that produce a wider range of tiller densities and tissue N concentrations is needed to adequately evaluate the in-season N tests.