THE EFFECT OF AN UPPER LIMIT TEMPERATURE THRESHOLD ON HEAT UNIT CALCULATIONS, DEFOLIATION TIMING, YIELD, AND FIBER QUALITY

摘要

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 arecalculated by adding the daily maximum and daily minimum temperature (0F), dividing this value by two, and then subtracting by the base temperature of 600F. This method does not include an upper limit temperature threshold. Where daily temperatures exceed 900F 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. In 2004, a defoliation timing study wasimplemented in Wharton County, Texas to compare the effects of utilizing three upper limit temperature (900F, 950F, 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. 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. In addition to the study conducted in 2004 at Wharton County, this study was also implemented in Burleson County, Texas. In 2003, studies were conducted at Wharton County, Burleson County and Winnsboro, Louisiana as well. Lint yield was increased significantly by the utilizing the 900F upper limit temperature threshold at the 2004 Wharton County location. However, upper limit temperature thresholds failed to explain differences in results of defoliation timing at the other four locations. Accumulated HU from cutout had a significant effect on lint yield and defoliation timing. In 2004, lint yield was significantly increased by waiting to defoliate until 850 accumulated heat units from cutout was reached in Wharton County. However, in 2003 there was no significant increase in lint yield once 750 accumulated HU was reached. The other three locations did not see a significant lint yield increase after 950 accumulated HU from cutout had been reached.