Optimizing time distribution of water supply and fertilizer nitrogen ratesin relation to targeted wheat yields

Long-term field experiments were conducted at sis different sites (represen ting dryland and irrigated environments) with wheat (Triticum aestivum) on sandy loam soils to generate a database relating available water supply and fertilizer nitrogen (N) rate ro grain yield. Stepwise multiple regression analysis showed that water supply and fertilizer N at more than 53 cm a(-1) and 103 kg ha(-1) a(-1) were unproductive. A method, based on the principl e of equi-marginal productivity of water in different periods of crop growt h, has been used to estimate the optimum allocation of the variable amounts of water supply over the growing season. The optimized quantities of water in different periods of crop growth were then employed to compute fertiliz er N requirement to achieve a given yield target. For medium grain yield ta rgets (4-5 t ha(-1)), the range of substitution between fertilizer N and wa ter for efficient resource use was quite wide.