Crop managers need to determine the most profitable time to defoliate cotton (Gossypium hirsutum L.) in different environments across the U.S. Cotton Belt. In cotton production, delaying defoliation exposes open bolls to a higher probability of rainfall, thus reducing lint yield and fiber quality. However, premature defoliation has detrimental affects on lint yield and fiber quality.Several traditional methods exist to determine defoliation timing, including determining percent open bolls, counting nodes above highest cracked boll, and examining the highest harvestable bolls to determine their maturity. However, these methods rely on subjective judgement; therefore, effectiveness may be reduced.
A more recent method to determine defoliation is based on heat-unit (HU or DD60) accumulation after physiological cutout or five nodes above white flower (NAWF=5). This method recommends initiating defoliation at 850 HUs have accumulated from date of cutout. However, results have been inconsistent across a wide range of field environments when utilizing HU accumulation past cutout; therefore, adoption of this method has been limited. Many regions of the Cotton Belt have maximum day time temperatures during the growing season that are above optimum for maximum growth. In these environments, crop managers may over estimating daily HUs without the use of an upper temperature threshold.
Field studies were conducted in Texas, Louisiana, South Carolina, and Arizona. The purpose of this research was to identify an UTT for calculating degree days for defoliation timing. The experimental design consisted of a split-plot design with four replications. The main plots consisted of two upper temperature thresholds (86°F, and no upper limit) and the subplots were five HU timings (650, 750, 850, 950, and 1050) accumulated from date of cutout.
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