Chemical topping improves the efficiency of spraying harvest aids using unmanned aerial vehicles in high-density cotton

The cotton defoliation strategy is highly appreciated in Xinjiang, where mechanical harvesting is limited by a large proportion of green leaves and unopened bolls at harvest due to an insufficient temperature sum. Because of a high plant density, the application of defoliants (harvest aids) by tractors is less efficient; thus, unmanned aerial vehicles (UAVs) for spraying harvest aids are becoming more and more popular. However, it is unknown if this method affects spraying quality and whether the spray is affected by the cotton plant type that was shaped by chemical topping. This study aims to address if chemical topping could enhance defoliation when harvest aids is sprayed using UAV. Field experiments were carried out in 2019 and 2020 in Alaer, Xinjiang, China. The topping treatments included manual topping (MT) as the control and chemical topping (CT) that inhibit apical growth of the stem by foliar application of mepiquat chloride (MC) at 90 (MC90), 180 (MC180), or 270 (MC270) g ha–1 in mid-July. The harvest aids was sprayed in mid-September using UAVs. The average droplet deposition and deposits of harvest aids within the canopy in CT and MT were not different in 2019. However, the average droplet deposition and deposits of CT were much higher than those of MT in 2020. Both droplet deposition and deposits decreased with canopy height, and the droplet distribution performance of CT was better than that of MT. The droplet deposition and deposits in the upper and middle canopy of CT were significantly higher than those of MT in 2020. At 21 days after harvest aids application, the number of leaves per plant in CT was significantly lower than that in MT, while there was no difference between the amounts of MC. CT did not affect boll opening. Moreover, the yield and quality were not affected by harvest aids application using UAVs and CT. We concluded that spraying harvest aids using UAVs combined with CT improved management efficiency and economic benefits by saving labor, without loss of cotton yield and quality. Our results demonstrate that applying harvest aids using UAVs in cotton with CT could improve the quality of defoliation and provide a reference for optimizing cotton managements globally.