| Summary: | Achieving high grain yields and crude protein (CP) standards in organic winter wheat (Triticum aestivum L.) is challenging because ensuring that adequate nitrogen (N) is available at key periods of wheat growth is difficult in organic systems. Split application regimes and in-season N management tests may improve organic production. In field trials conducted over four site-years in Maine and Vermont, USA, N application regimes were analyzed for their effects on organic winter wheat, N uptake, grain yield, and CP. Tiller density and tissue N tests were evaluated as in-season decision tools. Eight treatments arranged in a non-factorial design differed in terms of N application timing (pre-plant (PP), topdress at tillering (T1), and topdress at pre-stem extension (T2)) and N rate. Treatments were: (1) an untreated check, (2) pre-plant N at a low rate of 78 kg N ha<sup>−1</sup>(PP<sub>L</sub>), (3) pre-plant N at a high rate of 117 or 157 kg N ha<sup>−1 </sup>(PP<sub>H</sub>), (4) T1<sub>78</sub>, (5) PP<sub>L </sub>+ T1<sub>39</sub>, (6) PP<sub>L </sub>+ T2<sub>39</sub>, (7) PP<sub>H </sub>+ T2<sub>39</sub>, and (8) PP<sub>L </sub>+ T1<sub>39 </sub>+T2<sub>39</sub>. Responses to N treatments were variable among site-years, however some common results were identified. The PP-only treatments increased grain yields more than they increased CP. The T1<sub>78 </sub>and PP<sub>H </sub>+ T2<sub>39 </sub>treatments were the most effective at increasing yield and CP, compared with the PP-only treatments. Tiller density and tissue N tests were good predictors of grain yield (r = 0.52, p < and CP (r = 0.75, p < 0.001) respectively. Future work should test in-season decision tools using a wider range of tiller densities, and topdress N rates against tissue N measurements.
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