Experiments at two sites growing winter wheat show that in order to manage a wheat canopy more effectively, the use of specific remote sensing techniques both to monitor crop canopy expansion, and to determine variable nitrogen applications at key timings is required. Variations in seed rate were used to achieve a range of initial crop structures, and treatments were compared to standard farm practice. In the first year, the effect of varying seed rate (250, 350 and 450 seeds m(-2)) on crop structure, yield components and grain yield, was compared to the effects of underlying spatial variation. Plant populations increased up to the highest rate, but shoot and ear populations peaked at 350 seeds m(-2). Compensation through an increased number of grains per ear and thousand grain weight resulted in the highest yield and gross margin at the lowest seed rate. In later experiments, the range of seed rates was extended to include 150 seeds m(-2), each sown in 24 in wide strips split into 12 m wide halves. One half received a standard nitrogen dose of 200 kg [N] ha(-1), the other a variable treatment based on near 'real-time' maps of crop growth. Both were split into three applications, targeted at mid-late tillering (early March), growth stages GS30-31 (mid April) and GS33 (mid May). At each timing, calibrated aerial digital photography was used to assess crop growth in terms of shoot population at tillering, and canopy green area index at GS30-31 and GS33. These were compared to current agronomic guidelines. Application rates were then varied below or above the planned amount where growth was above- or below-target, respectively. In the first field, total nitrogen doses in the variable treatments ranged from 188 to 243 kg [N] ha(-1), which gave higher yields than the standards at all seed rates in the range 0.36-0.78 that and gross margins of pound17 to pound60 ha(-1). In the second field, variable treatments ranged from 135 to 197 kg [N] ha(-1) that resulted in lower yields of -0.32 to +0.30 t ha(-1). However, in three out of the four seed rates, variable treatments produced higher gross margins than the standard, which ranged from pound2 to pound20 ha(-1). In both fields, the greatest benefits were obtained where the total amount of applied nitrogen was similar to the standard, but was applied variably rather than uniformly along the strips. Simple nitrogen balance calculations have shown that variable application of nitrogen can have an overall effect on reducing the nitrogen surplus by one-third. (C) 2003 Silsoe Research Institute. All rights reserved Published by Elsevier Science Ltd.
