Cotton Defoliation Evaluations, 1997

Abstract

Three field experiments were conducted near Yuma, Coolidge, and Marana, AZ in 1997 to evaluate the effectiveness of a number of defoliation treatments on Upland (var. DP NuCotn 33b) cotton. All treatments consisted of materials commercially available in Arizona. Results reinforce general recommendations regarding the use of low rates (relative to the label ranges) under warm weather conditions and increasing rates as temperatures cool.

Introduction

Due to the rather indeterminate nature of the cotton (Gossypium spp ) plant, crops are often still actively growing late in the growing season. As a result, many cotton growers have experienced difficulty in satisfactorily defoliating the crop in preparing for harvest. Ideally, growers would like to accomplish a complete and satisfactory defoliation with a single application of defoliant. Historically, it has often required two applications and sometimes even three or four applications to accomplish defoliation. This can be further complicated later in the fall and at higher elevations due cooler weather conditions, which serves to slow down the physiological activity of the plant and the resultant activity of chemical defoliants on both Upland (G. hirsutum L.) and Pima (G. barbadense L.) fields.

Defoliation work in this program began in 1987, when a single field experiment was conducted in the Yuma Valley to compare several defoliation treatments on a field of Pima cotton (Silvertooth and Howell, 1988). That experiment was followed by a series of at least four similar experiments each year from 1988 (Silvertooth et al., 1989), 1989 (Silvertooth et al., 1990) and 1990 (Silvertooth et al., 1991) in an effort to expand locations, and treatment comparisons. Some treatment consistencies were identified from the 1987, 1988, and 1989 experiences, which were then used for the 1990, 1991, 1992, 1993, 1994, and 1995 experimental projects (Silvertooth et al., 1992; Silvertooth et al., 1993; Silvertooth et al., 1994; Silvertooth and Norton, 1995; Silvertooth, 1996, and Silvertooth and Norton, 1997). Nelson and his associates have also conducted a number of experiments concerning defoliation factors and refinement (Nelson and Hart, 1991a; Nelson and Hart, 1991b; Nelson and Silvertooth, 1991; Nelson and Hart, 1992; Nelson and Hart, 1993; Nelson and Hart, 1994; Nelson and Hart, 1995; Nelson and Hart, 1996; and Nelson and Hart, 1996). Common treatments resulting from this earlier work include Dropp + DEF and Dropp + Accelerate combinations, with increasing rates as temperature conditions cool. The 1997 experiments represent an extension of this general project in terms of evaluating some new treatments and combinations, and attempting to refine recommendations and guidelines.

Methods

Field experiments were conducted in the Yuma Valley on the University of Arizona Agricultural Center; near, Marana AZ; and Coolidge, AZ in 1997 as outlined in Tables 1, 2, and 3. Treatments employed are listed in Tables 4, 5, and 6. In all three cases, treatments were made to Upland cotton (var. DP 33b). All treatments were applied with a ground rig, with treatments arranged in a randomized complete block design with four replications. Plots were 18, 24, and 4 rows wide at Coolidge, Marana, and Yuma respectively; and extended the full length of the irrigation run in each case.

After treatments were applied, visual estimates of percent defoliation, and the regrowth/topgrowth control ratings were made 14 days after the treatment date. Weather conditions following the defoliant treatment applications are described in terms of heat units (HU, 86/55 °F thresholds) accumulated during the 14 day period following defoliant applications. Measurements and ratings were made at multiple locations within each plot. Regrowth ratings were made on a scale of 1 - 10, with a rating of 1 indicating excellent regrowth and topgrowth control and 10 indicating very poor control.

The primary objective of this study was to compare and evaluate a relatively new defoliatiant material (CottonQuick^TM) with a set of conventional defoliants (Dropp^TM and Ginstar^TM), with Ginstar serving as a standard treatment. Integrate, another relatively new harvest aid was evaluated at the Coolidge location only. An additional objective was to consider the results in terms of current guidelines and recommendations for cotton defoliation in Arizona.

Results

Defoliation and regrowth (topgrowth) ratings from the Yuma test (approx. 150 ft. elevation) are found in Table 4. With the early application date at Yuma (6 September), a relatively high number of HU (437) were accumulated 14 days after application of defoliants (20 September). The results from the Marana location (Table 5) represent a slightly higher elevation ( approx. 1,975 ft.) and the application was made on 26 September, with 279 HU being accumulated 14 days following treatment applications (8 October reading). Coolidge results (approx. 1,400 ft. elev.) are presented in Table 6, where 184 HU were accumulated 14 days after defoliants were applied (13 November reading)

Summary

One aspect of interest in this study was to compare and evaluate two relatively new defoliation materials (CottonQuick^TMM and Integrate) with a set of conventional defoliants (Dropp^TM and Ginstar^TM). Results from these two tests indicate that treatments including Dropp and Ginstar performed very well and were consistent. The most substantial difference among the treatments compared in these two studies was that of regrowth/topgrowth control.

These results also serve to reinforce current defoliation guidelines and recommendations relative to treatments and rates in accordance to expected weather conditions (i.e. HU accumulations).

Acknowledgements

The authors would like to express their appreciation for the support provided by the Entek and Agrevo Companies. Also, the friendly and valuable cooperation provided by Prechel Farms (Coolidge), Post Farms (Marana), and the University of Arizona Yuma Agricultural Center is greatly appreciated.

References

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