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Heat, Maturity, and Defoliation

T.K. Witten and J.T. Cothren


ABSTRACT

Harvest aid timing is essential for timely harvest. Early harvest aid application can lead to delayed harvest, need for multiple harvest aid applications, decreased yield, reduced lint quality, and ultimately reduced cotton profitability. Harvest aid timing is based upon boll maturity, with many methods currently existing for determining the proper application timing of harvest aid materials. One method of determining time of harvest aid application is based on accumulated heat units after physiological maturity. Currently, 850 heat units after cutout is the guideline set for defoliant application without observing a reduction in yield. However, this recommendation may be premature in some cotton growing areas. A study was conducted in 2000 and 2001 to address the effects of early and late applications of harvest aids based upon varying accumulated heat units after cutout. A field study was conducted at the Texas Agricultural Experiment Station in Burleson County, in 2000, and a controlled environment (i.e greenhouse) study at the Norman Borlaug Crop Biotechnology Center at Texas A&M University in College Station. Two cotton varieties, DP 20B and DP 422 B/RR, were planted with each being treated with defoliants at 650, 750, 850, 950, and 1050 HU accumulated after cutout. In the field experiment, percent open boll counts at the day of harvest aid application ranged from 1 to 68 percent, in 650 and 1050 HU treatments, respectively. Treatments having less than 950 HU exhibited 25 percent or less open bolls. At 14 DAT, 650, 750 and 850 HU had less than 67 percent open bolls where 950 and 1050 HU had greater than 87 percent open bolls. At 14 DAT, all treatments had greater than 76 percent defoliation with 950 and 1050 HU showing greater than 90 percent leaf removal. These effects were furthermore realized with lint yields ranging from 354 to 867 lbs. lint /A with 650 HU yielding the least and 1050 HU being the most. In the greenhouse DP 422 B/RR yielded numerically more lint yield than DP 20B. However, the effects of HU treatments were identical and representative of the observed results from the field study in 2000. These data lead to three possible conclusions: 1) the definition for cotton 'cutout' of 5 NAWF cannot be applied uniformly to all cotton growing areas; 2) the use of 850 HU after 'cutout' is inadequate for timing harvest aid application without reducing lint yield; and 3) an upper limit threshold temperature of 90oF is needed for calculating HU after physiological maturity.





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Document last modified May 20, 2002