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Nitrogen (N) deficiency and weed infestation are main factors limiting yield and yield stability in organic wheat. Organic fertilizers may be used to improve crop performance but off-farm input costs tend to limit profitability. Instead, forage legumes may be inserted into the crop rotation to improve the N balance and to control weed infestation. In opposition to simultaneous cropping, relay intercropping of legumes in organic winter wheat limits resource competition for the legume cover crop, without decreasing the performance of the associated wheat. The aim of this study is to evaluate the effect of spring organic fertilization on the performance of intercropped legumes and wheat, and on services provided by the legume cover. Two species of forage legumes ( Trifolium pratense L . and Trifolium repens L . ) were undersown in winter wheat ( Triticum aestivum L. cv Lona) in five organic fields during two consecutive crop seasons. Organic fertilizer was composed of feather meal and applied on wheat at legume sowing. The cover crop was maintained after the wheat harvest and destroyed just before sowing maize. Spring organic nitrogen fertilization increased wheat biomass (+35%), nitrogen (+49%), grain yield (+40%) and protein content (+7%) whatever the intercropping treatment. At wheat harvest, red clover biomass was significantly higher than white clover one (1.4 vs. 0.7 t ha −1 ). Nitrogen fertilization decreased forage legume above-ground biomass at wheat harvest, at approximately 0.5 t ha −1 whatever the specie. No significant difference in forage legume biomass production was observed at cover killing. Nitrogen accumulation in legume above-ground tissues was significantly higher for white clover than for red clover. Both red and white clover species significantly decreased weed infestation at this date. Nitrogen fertilization significantly increased weed biomass whatever the intercropping treatment and decreased nitrogen accumulation in both clover species (−12%). We demonstrated that nitrogen fertilization increased yield of wheat intercropped with forage legume while the performance of legumes was decreased. Legume growth was modified by spring fertilization whatever the species.

Advances in scientific understanding of the plant and soil behaviour in a cultivated field led to the design of numerous soil-crop models simulating crop growth. The frequent low predictive quality of these models is linked to uncertainties in inputs, parameters and equations. The AZODYN crop model predicting wheat grain yield and grain protein content was previously developed to support decision for N management of conventional and organic wheat crops. This paper outlines a sequential approach to improve the predictions of the AZODYN model by testing various formalisms. This study is based on the comparison of 38 versions of the model assessed in multi-environment trials carried out under conventional or low-input conditions. This paper describes and discusses the methodology. The results show that the predictive value of grain yield and grain protein content could be largely improved without increasing model complexity.

In organic wheat, nitrogen is one of the key limiting factors responsible for irregular productivity and low quality. On arable farms, the decreasing use of N-organic sources such as forage legumes, manures and composts relative to mixed-farms requires the development of suitable fertility strategies based on the use of off-farm organic fertilizers. The Azodyn crop model was previously adapted to organic fertilization and evaluated on various field trials in order to predict grain yield, grain protein content and soil mineral N at harvest. This paper analyses the value of Azodyn-Org as a decision-tool for managing N fertilization by (1) evaluating the economic benefits of an organic N application and (2) selecting optimal fertilization strategies according to farmers’ constraints and conditions. It appears that Azodyn-Org is relevant for selecting appropriate strategies in various conditions only if the multiple limiting factors (e.g. weeds, diseases, soil compaction) effect on yield was well predicted. In these conditions, its use could allow to identify the fields where an organic N application is profitable, through rule-based simulations. The development of an early risk-assessment method to predict the effect of major limiting factors is required before providing this tool to users. Other methodological elements of model-based explorations are addressed in the discussion.

In agriculture, differences between the kinetics of crop requirements and the rate of soil mineralization are known to affect wheat yield and quality. The high cost of organic nitrogen fertilizer may result in nitrogen availability being a limiting factor during the crop cycle. It is therefore necessary to improve fertilizer application conditions, to limit losses, to ensure that grain quality exceeds a certain threshold, and to optimize economic efficiency. The aim of this study was to improve nitrogen management in organic winter wheat crops by optimizing spring fertilization. We validated and used the Azodyn model [Eur. J. Agron. 10 (1999) 129] to determine the effect of various fertilization strategies on winter wheat production under N constraints. We first adapted the Azodyn model by constructing an organic fertilizer submodel. The resulting Azodyn–Org model was tested under several sets of conditions for the rate and timing of organic fertilizer applications, in three experiments. Preliminary results confirmed that the Azodyn model accurately predicted N soil availability in a wide range of conditions. We assessed the performance of the Azodyn–Org model for predicting grain yield, grain protein content and mineral N in the soil. We found that grain yield was correctly predicted whereas grain protein content was slightly overestimated. Thus, the Azodyn–Org model has potential for use as a decision-making tool for managing spring N fertilization in organic wheat crops.