Nitrogen management of organic winter wheat: decision-making through model-based explorations.

In organic wheat, nitrogen is one of the key limiting factors responsible for irregular productivity and low quality (David et al, 2005b), 5 to 50 % less than conventionally managed crops (Nieberg and Schulze Pals, 1996). 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. Numerous mechanistic crop models simulating the dynamics of crop requirements and N supply in the soil (e.g. CERES, EPIC, APSIM, ARCWHEAT STICS) have previously been developed (Whisler et al, 1996). Although these models are highly used in research, their complexity and input requirements have limited their practical use for farmers and advisers. The aim of this study was to develop an engineering approach (Passioura, 1996) by the development of a decision-making tool for assessing N management of organic wheat on commercial farms. Azodyn-Org crop model was developed in organic agriculture to predict the influence of spring N fertilization strategies on grain yield, grain protein content, mineral N in the soil at harvest and gross margin (David et al., 2004). This simpler model requires little input data, which are easily measured in farmers’ fields (soil characteristics, climatic data, crop biomass and mineral N in the soil at the end of winter). The performance of Azodyn-Org was relevant for selecting appropriate strategies in a large range of environment and crop management conditions (David et al., 2005a). This paper focuses on the potential contribution of model-based explorations from Azodyn-Org for managing N fertilization in organic wheat crops at the regional scale.