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 m 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 188-243 kg [N] ha-1 , which gave higher yields than the standards at all seed rates in the range 0.36-0.78 t ha-1 and gross margins of £17-60 ha-1. In the second field, variable treatments ranged from 135-197 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 £2 to £20 ha-1. In both fields, the greatest
SummaryTwo experiments were conducted in central southern England between September 1994 and August 1996 to identify the critical weed‐free period in organically grown winter wheat (Triticum aestivum, cv. Mercia). In competition with a mixed weed infestation of predominately Alopecurus myosuroides and Tripleurospermum inodorum it was found that wheat yield decreased as the duration of the weed‐infested period increased and that the crop needed to be kept free of weeds from sowing in order to completely avoid any yield loss. Also, weeds emerging in the wheat crop (predominately T. inodorum) during the growing season had a significant and detrimental effect on yield. The existence of the critical period, therefore, depends on the imposition of an acceptable yield loss. If a 5% yield loss gives a marginal benefit compared with the cost of weed control, the critical period will begin at 506°C days after sowing (November) and end at 1023°C days after sowing (February). This information could be used by farmers to target mechanical weeding operations to control weeds at a time that will have maximum benefit to the crop.
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