Efficacy of cotton harvest aids applications with unmanned aerial vehicles (UAV) and ground-based field sprayers – A case study comparison

The opportunity of using of Unmanned Aerial Vehicles (UAVs) for applying harvest aids in ultra-low volumes (ULV) in cotton is an interesting perspective to overcome common problems associated with traditional ground-based applications like crop mechanical damage, yield losses and soil compaction. Moreover, as cotton harvest aids induce a prominent effect on the appearance of the crop, their efficacy can be conceptualized in wide scale remote sensing observations to provide general insights on the performance of UAV plant protection applications. The scope of the present study was to evaluate the efficacy of UAV spraying in cotton in comparison with traditional ground-based applications. Two field trials were established for that scope and each field included eight treatments. Two treatments involved ground based, field sprayer applications with and without air assistance on the boom and six treatments, combinations of multirotor UAV applications involving two spray altitudes, (2 m and 3 m above the canopy), two spray volumes (16 l ha⁻¹ and 10 l ha⁻¹) and the addition of a spray adjuvant on the mixture. The efficacy of the alternative treatments was evaluated through UAV remote sensing and by ground truth measurements. The evaluated parameters were the differences obtained in the Normalized Difference Vegetation Index - NDVI and the Cotton Fiber Index – CFI, from time of spraying to harvest, the defoliation rate, the boll opening rate and the final yield. The results revealed that UAV sprayings were more efficient from both field sprayer applications (with and without air assistance). The higher efficacy was accompanied by an improvement in cotton yield. The best results were obtained from the low altitude, 2 m UAV operation. There were no clear differences between the high volume and the low volume UAV applications implying that the lower 10 l ha⁻¹ volume might be sufficient and provide the potential to increase field capacity. The addition of the spray adjuvant was beneficial only when spraying was followed by rainfalls. The results of the present study implicate that UAV applications can support a wide range of plant protection and plant care applications in cotton.