本研究于2019-2020年开展了田间试验，研究DPC化控封顶时期和剂量对喷施脱叶催熟剂（50%噻苯•乙烯利悬浮剂，简称TE，2250 g hm^-2）前吐絮率（9月中下旬，药前吐絮率）及TE施用后14天脱叶性状和吐絮率的影响。结果表明，2019年（棉株长势相对正常）晚封顶（T3期，接近生理终止，白花以上节位约为5.0，较当地人工打顶晚7天）与早封顶（T1期，盛花期，较当地人工打顶早7天）和中期封顶（盛花期后7天，与当地人工打顶时间相同）相比，药前吐絮率显著降低5.9%-11.2%；高剂量DPC（270 g hm^-2）封顶与低（90 g hm^-2）、中（180 g hm^-2）剂量DPC封顶相比，药前吐絮率显著降低22.0%。2020年（棉株后期长势偏旺）T3期封顶的药前叶片数虽然少于T1期和T2期，但药后14天的残留叶片数较多，脱叶率较T1和T2期封顶降低23.2%-27.2%；高剂量DPC封顶的药前叶片数与中、低剂量DPC封顶相似，但药后14天的残留叶片数最多，脱叶率较低、中剂量DPC封顶降低15.0%-21.7%。此外，2020年晚封顶主要影响果枝叶片（包括主茎叶）的脱落，而高剂量DPC封顶对果枝叶（包括主茎叶）和营养枝叶片的脱落均有影响。综上，花铃期应用DPC进行化控封顶对棉花熟性和脱叶催熟效果有一定影响，且具体结果与植株生长状态有关。为避免延迟成熟和降低叶片对脱叶催熟剂的敏感性，DPC化控封顶时间不宜过晚，也不宜采用高剂量DPC进行封顶。在黄河流域棉区，建议于盛花期~盛花期后7天左右（当地常年人工打顶时间）应用90-180 g hm^-2 DPC进行化控封顶。

Early maturity, complete defoliation and boll opening are essential for the efficient machine harvesting of cotton.  Chemical topping, involving one extra application of mepiquat chloride (MC) in addition to its traditional multiple-application strategy, may be able to replace manual topping.  However, it is not known whether this chemical topping technique will influence maturity or cotton responses to harvest aids.  In this 2-yr field study, we determined the effects of the timing of chemical topping using various rates of MC on boll opening percentage (BOP) before application of harvest aids (50% thidiazuron·ethephon suspension concentrate, referred to as TE), and the defoliation percentage (DP) and BOP 14 days after TE application.  The results indicated that late chemical topping (near the physiological cutout, when the nodes above white flower is equal to 5.0) significantly decreased BOP before TE by 5.9–11.2% compared with early (at peak bloom) or middle (seven days after peak bloom) treatments in 2019, which was a relatively normal year based on crop condition.  Also, a high MC rate (270 g ha^–1) showed a significantly lower (22.0%) BOP before TE than low (90 g ha^–1) or medium (180 g ha^–1) rates.  In 2020, which was characterized by stronger vegetative growth in the late season, the late chemical topping reduced the number of leaves before TE application relative to early or middle treatments, but had lower DP (23.2–27.2%) 14 days after TE application.  The high MC rate showed a leaf count before TE application that was similar to the low and medium rates, but it showed the most leaves after TE and much lower (15.0–21.7%) DP in 2020.  These results suggest that late timing of chemical topping and a high MC rate decreased the sensitivity of leaves to harvest aids.  Further analysis indicated that the late chemical topping mainly affected the leaf drop from the mainstem and fruiting branches where the late regrowth occurred, and the high MC rate reduced leaf shedding from these parts and also from the vegetative branches.  In conclusion, chemical topping with MC during the bloom period affected cotton maturity and responses to harvest aids in different ways according to the crop condition.  To avoid the risks of delayed maturity and poor defoliation after the application of harvest aids, chemical topping should not be performed too late (i.e., near the physiological cutout) by using MC at more than 180 g ha^–1.  The optimum timing of chemical topping probably varies from peak bloom to around seven days later, and the safest MC rates for chemical topping should be less than 180 g ha^–1.