Determination of optimum nitrogen (N) fertilization rates which maximize pasture growth is challenging due to variability in plant requirements and likely near‐future supply of N by the soil. Remote sensing can be used for mapping N nutrition status of plants and to rapidly assess the spatial variability within a field. An algorithm is, however, lacking which relates the N status of the plants to the expected yield response to additions of N. An algorithm was developed based on a simulation study carried out using the APSIM model. Simulations were performed for an irrigated ryegrass pasture in Canterbury, New Zealand. Nitrogen fertilizer was applied monthly at different rates, ranging from 0 to 150 kg N/ha. To obtain a range of different pasture N contents, a total of 1456 different fertilization rules were set up and evaluated over 20 different years of weather giving 29,120 combinations of pasture herbage N contents and growth responses for each month. The analysis focused on November (spring), a month with generally vigorous growth. A three‐dimensional surface response function, based on the Mitscherlich yield response function, was developed. This function was used to determine required N fertilization rates, which achieve 90% of the maximum yield, based on the antecedent pasture N content. At low pasture herbage N contents (25 g/kg), the required fertilization rate was estimated as 130 kg N/ha, whereas at N contents of 40 g/kg, only 60 kg N/ha was required to obtain the same yield, reflecting the much higher supply of N by the soil.