Impact of Early Defoliation on California Cotton

Defoliation is an integral pre-harvest step in cotton (Gossypium hirsutum L.; Gossypium barbadense L.) production; improper defoliation timing will compromise both cotton yield and quality.  Objectives of this research were to evaluate the impacts of the rate and timing of early applications of Ginstar (thidiazuron + diuron) or Ginstar plus Finish (ethephon + cyclanilide) on defoliation, yield, and select fiber quality characteristics.  In 2008 and 2009, Thidiazuron and Ethephon were applied at rates of 0.053-0.079  and 0.079-0.670 kilograms per hectare (kg/ha), respectively, at the 4 nodes above cracked boll (NACB) stage, followed by a secondary treatment at 0.079-0.105 and 1.12 kg/ha, respectively. These treatments resulted in significantly greater defoliation and desiccation, and improved open boll percentages when compared to similar treatments with an earlier application beginning at the 6 NACB stage. There were no significant yield differences between treatments initiated at 6 or 4 NACB, however, differences did occur in micronaire and other lint quality attributes. Defoliation results varied from earlier pre-treatment studies conducted in 2006 and 2007; these studies differed in that when initiating defoliation at the 6 NACB, the earlier application resulted in improved defoliation, desiccation, and boll opening, but had a greater impact in reducing micronaire. Similar studies were conducted from 2003 to 2005 on Pima cotton comparing the 10, 8, 6, and 4 NACB stages. Generally with Pima, the results were similar, including no significant yield differences between the 4 through 8 NACB, improved defoliation and desiccation, and a slight reduction in micronaire.