| Many methods are currently used in determining proper timing of harvest aids. Among these methods is the use of growing degree-day units (heat units, HU). The growing degree-day concept bases harvest aid application on accumulated heat units after physiological maturity (i.e., ‘cutout’; nodes above white flower equal five, NAWF = 5). From ‘cutout’, HU are accumulated to determine timing of harvest aid applications. Currently, 850 HU after ‘cutout’ is the guideline set for harvest aid application. However, in some areas this benchmark appears to result in early application of defoliants resulting in decreased lint yield. Field studies were conducted, in 2000 and 2002, using two cotton varieties treated with harvest aids at 650, 750, 850, 950, and 1050 HU after ‘cutout’. Both varieties exhibited the same response, over years, with respect to defoliation and percent open bolls at day of harvest aid application and on the day of harvest. Increasing heat units accumulated after ‘cutout’ significantly improved overall harvest aid performance. Treatments having less than 950 HU exhibited 50 percent or less open bolls on day of harvest aid application. At 14 DAT, 650, 750, and 850 HU had less than 84 percent open bolls where 950 and 1050 HU showed greater than 90 percent open bolls. The overall effect of defoliation and percent open bolls at harvest was more apparent in lint yields. Yield ranged from 789 (DP 422B/RR) and 803 (DP 20B) to 1392 (DP 422B/RR) and 1447 (DP 20B) kg. per hectare; 650 HU yielded the least and 1050 HU the most. As HU increased after ‘cutout’, lint yields and micronaire values also increased significantly. Increased micronaire could potentially lead to lint quality deductions decreasing overall profit. However, the overall financial influence in these studies resulted in significantly increased net returns with increasing accumulated HU. These data suggest that NAWF = 5 is not indicative of cotton ‘cutout’, harvest aid application at 850 accumulated HU will result in decrease in lint yield, and an upper limit threshold may need to be implemented in calculating HU for cotton. |
