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Abstract
A quadratic nitrogen response model for dryland wheat with moisture stress dependent coefficients is described and validated. The model was used to estimate the effects of moisture stress on the nitrogen levels corresponding to a maximum yield and an economically optimal yield, respectively. For the range of stress values representative of dryland cropping conditions in South Africa the nitrogen level needed for a maximum yield was largely independent of the stress level. In contrast, the nitrogen level needed for an economically optimal yield increased sharply with increasing moisture stress. The implications of these results with respect to the problem of nitrogen fertilization in semi-arid regions are discussed.