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The Effect of an Upper Limit Temperature Threshold on Heat Unit Calculations, Defoliation Timing, Yield and Fiber Quality

D.D. Fromme, J.T. Cothren, and J.B. Bynum

ABSTRACT

Researchers from across the Cotton Belt have come to different conclusions on the optimum defoliation time based on COTMAN and the accumulated heat units from cutout (NAWF=5) method that the program utilizes. Currently daily heat units for cotton are calculated by adding the daily maximum and daily minimum temperature (ºF), dividing this value by two, and then subtracting by the base temperature of 60ºF.This method does not include an upper limit temperature threshold. Where daily temperatures exceed 90ºF and night temperatures are also high, calculated heat units may be overestimated and could possibly explain the differences in defoliation timing recommendations based on accumulated heat units from cutout. A defoliation timing study was implemented to compare the effects of utilizing three upper limit temperature (90ºF, 95ºF, and no upper limit) thresholds to calculate heat unit (HU; growing degree units) accumulation past cutout and the subsequent impact on defoliation timing, yield, and fiber quality. The study was conducted at three locations during 2003. Treatments consisted of the three different upper limit temperature thresholds in calculating daily heat units and defoliation at five maturity stages based on 650, 750, 850, 950, and 1050 accumulated HU from date of cutout. Nodes above white flower (NAWF) counts were recorded during bloom until cutout was reached. At the time of defoliation, nodes above cracked boll (NACB) and percent open bolls were recorded. Percent open boll and lint yield were determined at harvest. Lint samples were retained for HVI analysis. There were no differences in lint yield between the three upper limit temperature thresholds at all three of the locations. Two locations saw significant lint yield increases by waiting to defoliate until 950 accumulated heat units from cutout. The other location did not see a significant lint yield increase after 750 accumulated HU from cutout had been reached. Upper limit temperature thresholds failed to explain differences in results of defoliation timing at the three different locations.





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Document last modified 04/27/04