How to improve model-based decision rules for nitrogen fertilization

Numerous mathematical models have been developed for calculating optimal decision rules for nitrogen fertilization. The purpose of this paper is to describe and apply methods for studying how model-based fertilizer recommendations could be improved. Three criteria are presented for evaluating the monetary gain that would result from an improvement of model-based decision rules. Different approaches for improving decision rules are then identified on the basis of past studies: adding more data, including more explanatory variables in the model, taking into account measurements correlated with the model parameters, and taking into account information about the functional form of the response. The proposed methods are used for evaluating several decision rules calculated using a yield response model developed from 112 winter wheat (Triticum aestivum L.) experiments. The results show that, compared to the application of fixed nitrogen recommendations calculated independently of site-year characteristics, the application of nitrogen doses calculated with models including end-of-winter mineral soil nitrogen can increase the farmer's gross margin up to 136 F ha(-1) (20.7 Euro ha(-1)). However, the usefulness of modelling the response to applied nitrogen as a function of end-of-winter mineral soil nitrogen depends on the amount of data used for estimating model parameters. When the data set used for parameter estimation includes only 2 site-years of data, fixed nitrogen fertilizer recommendations are more profitable by 109 F ha(-1) (16.6 Euro ha(-1)) than decision rules calculated with models including end-of-winter mineral soil nitrogen. Models including end-of-winter mineral soil nitrogen give more profitable nitrogen fertilizer recommendations only if more than 10 site-years of data are used for parameter estimation. Another result shown in this paper is that, compared to the application of decision rules calculated with models including end-of-winter soil nitrogen, the application of perfect decision rules can at best increase the farmer's gross margin by 331 F ha(-1) (50.5 Euro ha(-1)).