Cotton crop transpiration reveals opportunities to reduce yield loss when applying defoliants for efficient mechanical harvesting

Chemical defoliation is an essential pre-harvest practice in machine-harvested cotton production in China, in which the timing of spraying chemical defoliants (harvest aids) is the key to ensure the yield and quality without loss due to weather risks at harvest time. This study aimed to find out based on crop transpiration if chemical defoliants could be sprayed early without yield loss. Field experiments were carried out in 2020 and 2021 in Xinjiang, China. The treatments consisted of a defoliant treatment with harvest aids (a mixture of 10% thidiazuron and 40% ethephon) sprayed in mid-September and water as a control, under four typical sowing patterns. Sap flow was measured during the defoliation period using a Heat Ratio Method (HRM) and then calculated as transpiration per unit ground area and per unit leaf area. The boll opening percentage in the defoliation treatment (84.9%) was higher than the control (77.3%), resulting in a 14.8% higher cotton lint yield. Row spacing and plant density did not affect lint yield and the defoliation percentage. Daily transpiration per unit ground area under the defoliants treatment was 1.26 mm d−1 and 39.1% lower than in the control (2.07 mm d−1), and the daily transpiration per unit leaf area was 11% lower than the control. The daily transpiration per unit ground area did not change significantly in the first four to five days after spraying, and then started to decrease rapidly. This indicated that the crop could continue to grow and develop the first days after applying defoliants, thereby reducing yield loss. The results of this study show it is possible to apply defoliants four to five days earlier than farmers’ spraying date in field production to avoid weather risks and ensure timely leaf drop without loss of yield.