Development of an algorithm for relating pasture nitrogen status to yield response curves

Abstract

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.

Development of Algorithm for Determining N Fertiliser Requirements of Winter Wheat Based on N Status Using APSIM Modelling

“…where Y is the grain yield (kg DM ha −1 ), Y max is the maximum yield under the climatic and edaphic conditions (kg DM ha −1 ), N r is the rate of N applied (kg N ha −1 ), Y 0 is the yield when no external N is applied (N r = 0), and β is an 'activity' coefficient, which is a measure of the availability of the applied nutrient to the crop. Following Vogeler et al [31], it is assumed that both Y 0 and β are dependent on the N uptake during early development:…”