The Influence of Defoliation Timing on Yields and Quality of Two Cotton Cultivars

The timing of certain cotton (Gossypium hirsutum L.) management practices varies according to the yield potential and quality characteristics associated with a variety. A defoliation timing study was performed to (i) determine if certain cultivars respond differently to defoliation timings and (ii) compare the use of the open boll percentage at defoliation (OBPD), nodes above cracked boll (NACB), and micronaire readings at defoliation for their effectiveness in timing defoliation. The study was conducted in 1999, 2000, and 2001. Treatments consisted of two proprietary cultivars (ST 474 and DP 5409), each defoliated on the basis of OBPD measurements. At the time of defoliation, NACB was recorded and lint samples were retained for later high volume instrumentation (HVI) analysis. Neither variety produced consistently higher yields than the other in this study. In 2000, delaying defoliation from 40 to 60 OBPD would have resulted in a significant addition of approximately 75 kg lint ha⁻¹ for either variety. Stoneville 474 micronaire was highest in all years suggesting that timely defoliation is more critical to ST 474 cotton compared with DP 5409 in years when overall conditions are favorable for high micronaire. DP 5409 fiber length (UHM) values were consistently higher than ST 474 and UHM was unaffected by changes in OBPD values regardless of variety. Stoneville 474 had higher uniformity index (UI) values in all three years and delaying defoliation produced mixed results. The data demonstrate that proper defoliation timing strategies aimed at optimizing quality can vary across varieties. Proper defoliation timing in the two varieties examined in this study varied little with respect to yields. Both NACB and micronaire readings taken at defoliation were more effective for timing defoliation to optimize micronaire readings than OBPD.