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A diagnostic method for assessing regional variations in yield was implemented for a network of 24 organic wheat fields located in two neighboring regions, Diois and Val de Drôme (France). We studied: yield and its components; field environmental conditions of the fields; relations between indicators of potential limiting factors and yield components; and linkages between crop management and yield-limiting factors. The yield performances were low and variable (3.5 ± 1.4 t·ha -1 ). The relative kernel number (RKN) was significantly related to nitrogen nutrition and weed density at flowering. The nitrogen nutrition index (NNIf) had a strong and positive effect on relative kernel number, while weed density (1/WF) had a negative effect. The percentage of kernel number variation explained by the regression model (RKN = 0.112 + 0.609 × NNIf + 0.2 × 1/WF) was adequate (Adj. R 2 = 0.53) with a RMSE of 0.09. The duration of the period of grain filling, reflected in the temperature sum, had a strong and positive effect on relative kernel weight (RTKW), while soil compaction and high temperature had a negative influence. Adjusted R 2 for the model was 0.42 with a RMSE of 0.16. The results confirm that legumes preceding crops improve nitrogen nutrition on subsequent winter wheat, e.g. the Nitrogen Nutrition Index at flowering (NNIf) = 0.51 ± 0.12 for a crop rotation with a rate of legumes over 37% vs. 0.41 ± 0.11 for a crop rotation with a rate of legumes under 25%. Alfalfa preceding crops significantly limited the weed density at harvest while stony soils and early sowing (before November 1st) enhanced weed density. organic farming / winter wheat / yield / limiting factors / diagnostic method * Corresponding author: davidc@isara.fr Article published by EDP Sciences and available at http://www.edpsciences.org/agro or http://dx.doi.org/10.1051/agro:2005016 214 C. David et al.for each field, as described by Boiffin et al. (1981) and Meynard and David (1992). For instance, Leterme et al. (1994) analyzed the effect of crop management and environment on wheat yield variation in Thymerais, France (48.3°N, 1.3°E). It was established that the major factors responsible for yield differences were drought stress during stem elongation and meiosis, root and stem diseases and soil structure. Then these factors were explained by differences in soil moisture capacity, rainfall distribution, sowing date and the nature of the preceding crops.This paper focuses on understanding organic wheat yield performance in terms of yield-limiting factors and crop management characteristics. The analysis consisted of four stages: (i) yield and its components were described, (ii) we characterized the environmental conditions of the fields, (iii) we analyzed the relations between indicators of possible limiting factors and yield components, and (iv) determined the relations between crop management and yield-limiting factors.

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