Variable-rate application of high spatial resolution can improve cotton N-use efficiency and profitability

Abstract

Spatial crop nitrogen (N) management has advanced over the years by using canopy reflectance data to make N recommendations. But the value of an automated system for variable-rate applications (VRA) has not been extensively demonstrated in cotton. This study evaluated the performance of an integrated system for on-the-go VRA at a 1-m spatial resolution in two different field trials in 2016 and 2017. The VRA system applied consistently less fertilizer N than the farmer practice, 42% and 33% less total N in the 2016 and 2017 field trials, respectively. Despite the reduced N inputs, increased energy-use efficiency (EUE) by 16% and return to N cost by €81/ha in 2016, VRA still over-applied N because of lack of yield response to N fertilizer. In the 2017 field, VRA yield gains by 5–8% over the farmer practice were not statistically significant and came about after cotton regrowth due to a bollworm infestation. With or without a back-off function in effect, VRA delivered an Economic Optimum N Rate, greatly increased N recovery by 12–22%, EUE by 26% and net return to N by €190–248/ha in comparison to the farmer. The reduction of N inputs also improved coupling of the soil-crop N cycle by reducing the effects of excess farmer application on soil acidity, electrical conductivity and residual nitrates in the root zone. Under the field conditions of this study, implementation of the high-resolution VRA can result in environmental and economic benefits especially in non-problematic fields where the crop responds to N fertilization.