The probability of favorable temperatures for some cotton defoliants decreases importantly in the month of October. More effective and earlier defoliation is crucial to advancing the cotton harvest to reduce rainfall damage and diapausing boll weevil numbers. Probabilities indicate more defoliation should be carried out earlier.

In 1984 Maricopa County produced more acres of upland cotton with lower yields than it had in 1987 but also started harvest later. Weather and insects reduced yield and early maturity of the crop; rainfall delayed harvest in the October-November period less than it had in 1987.

Field experiments were conducted in 1988 to begin an assessment of both the agronomic and entomological implications from differences in irrigation termination dates imposed on both upland and pima cotton. Nine study sites were established with non-replicated treatment arrangements. Two replicated experiments were established at Marana, AZ on both upland and pima comparing early and late irrigation terminations. The upland (DPL 20) field was planted 26 April 1988 and irrigation terminations of 18 August (early) and 5 September (late) were imposed. The pima S-6 field was also planted 26 April and irrigation was terminated on 13 August (early) and 1 September (late). Yield measurements showed no significant differences (P ≤ 0.05) between early or late termination with the DPL 20, but quite substantial and significant differences were observed between treatments in the pima experiment.

A field study was conducted at the Maricopa Agricultural Center to determine the effect of nitrogen fertility level and growth regulators on earliness, yield of cotton, and effectiveness of defoliants. Nitrogen application rates of 80, 120 and 160 lbs N/A did not influence lint yields or the response of cotton to growth regulators. The growth regulator Piz tended to promote earliness but did not affect final yield. The defoliants tested achieved good leaf drop results independent of the plant nitrogen status. Petiole nitrate-N content at the time of defoliation (4 October) did not correlate well with the response of cotton to defoliation treatments.

A field study was conducted at the Maricopa Agricultural Center to evaluate the effectiveness of selected defoliation treatments on pima cotton. Defoliants were applied in October under relatively warm conditions. Treatments containing Dropp generally resulted in the highest leaf drop percentages (over 90 %). Def 6 treatments were ineffective in defoliating pima cotton.

Four field experiments were carried out in several areas of Arizona to evaluate the effects of a plant growth regulator and an array of conventional cotton defoliant treatments on pima cotton. Variable conditions were encountered across locations at the time of defoliant- treatment applications. However, there was a consistent trend observed in terms of treatment effectiveness, and a few distinct treatments appeared to have considerable promise for 1-time applications for satisfactory defoliation of pima cotton.

The analysis of grades of upland cotton picked before and after a 29 October rainfall at Waddell in 1987 reveals significant financial losses for growers. Early harvesters in the area harvested over two thirds of the crop in October; over 98.0% of grades 21 and 31 were harvested before the rainfall. A Buckeye operation also accounted for a difference of $98.60 per acre between an early and a late harvested pima field the same season. The survey indicates that important losses can be reduced by a harvest begun 10 to 15 days earlier than mid- October.

Moduling of cotton has increased strongly since its introduction in 1973. The usage is different from gin to gin due to a variety of conditions. However, the addition of moduling has reduced the pressure on ginning capacity so that some growers can continue using trailers without slowing the speed of the harvest.

A single field experiment was conducted in 1989 on a grower cooperator field to evaluate the response of a medium maturity type Upland cotton (DPL-50) to three dates of irrigation termination. The crop was planted 20 April and managed uniformly in all respects until 2 August when the earliest irrigation termination treatment was imposed. The dates of the second and final irrigation terminations were 17 August and 1 September, respectively. With each subsequent irrigation, the respective plots received an additional six acre inches of water (approximately). Harvest results revealed no significant (P <0.05) differences in lint yield due to irrigation termination treatments. Overall mean lint yield for the experiment was 1,228 lbs. cotton lint/acre, the experimental coefficient of variation (CV) was 11% and the observed significance level (OSL) was 0.34.

A field study was conducted at the Maricopa Agricultural Center to evaluate the effectiveness of selected defoliation treatments on Pima cotton under warm and cool weather conditions. Dropp and other defoliants caused desiccation and freezing of leaves when applied at air temperatures above 100° F. Dropp was an effective defoliant when maximum air temperatures were in the low 90s. The experimental defoliant SN 597 NA218 appears to be a promising defoliant for Pima cotton when temperatures are cool.