Evaluation of a Feedback Approach to Nitrogen and Pix Applications, 1997

Abstract

A single fiield experiment was conducted in 1997 at Marana, AZ to compare a scheduled approach (based on stage of growth) versus a feedback approach (based on growth parameters) to both nitrogen (N) and mepiquat chloride (PIXtm) applications on Upland cotton (Gossypium hirsutum L.). PIX feedback treatments were based upon fruit retention (FR) levels and height to node ratios (HNRs) with respect to established baselines for Arizona growing conditions. Scheduled and feedback PIX applications were made for a total of 0.75 and 1.50 pt./acre, respectively, with the scheduled treatments being initiated earlier in the fruiting cycle (early and peak bloom). Feedback PIX treatments consisted of a single 0.75 pt./acre application near peak bloom (approx. 2000 heat units after planting, HUAP, 86/55°F threshold). Scheduled applications of fertilizer N totaled 150 lbs. N/acre from two applications and feedback N treatments received a total of 100 lbs. N/acre from two applications. Treatments consisted of all combinations of scheduled or feedback applications of both N and PIX. The highest lint yields were from treatments receiving PIX applications, with significant differences (Pł0.05) between a check treatment (with no PIX applications) and several other treatments that did receive PIX applications. If PIX was applied, there were no significant differences between the scheduled or feedback approach. Applications of PIX in relation to increasing HNRs (feedback approach) are demonstrated and reinforced in this study.

Introduction

The balance between reproductive components (squares, flowers, bolls) and vegetative components (leaves, stems, roots, etc.) is a critical aspect of cotton (Gossypium spp.) management. In irrigated agriculture it is commonly recognized that water and N serve as the two strongest growth stimulants. However, on occasion cotton plants develop vegetative tendencies at the expense of yield components. Mepiquat chloride, or PIXtm, is a growth regulator that has been used in commercial cotton production to reduce and control vegetative growth. This gibberellic acid suppressant is absorbed by green parts of the plant, reduces cell elongation, and thus overall plant height. Theoretically, this allows the plant to direct more energy towards reproductive structures. Accordingly, PIX has become a general management tool in many cotton production regions, although its use and management varies considerably. A considerable amount of research has been conducted over the past 20 years in an attempt to determine optimal application guidelines. To date, application strategies that result in consistent increases in lint yield from PIX have not been identified.

During the past several years numerous studies have been conducted in Arizona (Silvertooth et al., 1989, 1990b, 1991c, 1992a,1993b; and Fletcher et al.,1994) in an attempt to determine optimum rates, timings, and strategies of multiple PIX applications for Upland (G. hirsutum L.) and Pima (G. barbadense L.). Results from this work have contributed to the development of a feedback-type approach to PIX applications, in response to actual crop conditions. Therefore, a feedback approach to crop input management, such as with PIX, requires a reference to an established criteria for optimum or excessive vegetative growth patterns. Accordingly, guidelines relative to crop fruit retention (FR) levels and height to node ratios (HNR) have been developed for this purpose (Silvertooth et al., 1991a; Silvertooth et al., 1992a; Fletcher et al., 1994; Silvertooth et al., 1995b; and Silvertooth and Norton, 1996b). Management guidelines for fertilizer N inputs to cotton have developed along a similar line regarding feedback vs. scheduled approaches (Silvertooth et al., 1991b, 1992b,1993a, 1994, 1995a, 1996a, and 1997a). In the case of N, FR levels represent the sink or demand of N by the plant, which can also be evaluated by a simple HNR measurement. Traditionally, petiole NO₃-N concentrations have been used in a feedback type of approach in terms of actual in-season N fertility status assessment (Silvertooth and Doerge, 1990). It has been found that the petiole N evaluation can be enhanced by the use of FR information and/or the vegetative/reproductive balance (HNR) for in-season decisions concerning N management.

In 1993 a field experiment was initiated in central Arizona with the objective of evaluating the effects of a scheduled and feedback applications of both N and PIX (Fletcher et al., 1994; Silvertooth et al., 1995b; 1996b, and Silvertooth and Norton, 1997b). From 1993 through 1996 the experiment was conducted at the Maricopa Agricultural Center. In 1997 the experiment was conducted at the Marana Agricultural Center where growth conditions more commonly justify the use of plant growth regulators such as PIX.

Materials and Methods

A field study was conducted in 1997 at the University of Arizona Marana Agricultural Center on a Pima clay loam soil (Typic Torrifluvent). Upland cotton (Gossypium hirsutum L.,var. DP NuCOTN 33b) was planted in moisture with a dry soil mulch (cap) on 15 April (Table 1). Treatments were structured to utilize all combinations of N and PIX feedback and scheduled applications (Table 2). Fertilizer N application rates and dates are outlined in Table 3 with PIX application rates and dates shown in Table 4. All treatments were arranged in a randomized complete block design with four replications. Plots extended the full length of the irrigation run (600 ft.) and were 4, 40 inch rows wide. PIX treatments were applied by the use of ground rig applicator with 20 gallons/acre carrier. On approximately 14 day intervals a complete set of plant measurements (plant height, mainstem node numbers, bloom counts per 167 ft.² area, nodes above the top white flower (NAWF), percent fruit retention, and percent canopy closure) were taken from each plot. Plant maps were also made from composite samples of each PIX treatment. Nitrogen fertility levels were also monitored during the season by sampling petioles and analyzing their NO₃-N concentrations on approximately 14 day intervals. Management of the study area was carried out in a uniform manner with regard to irrigation and pest control. Final irrigations were made on 19 August to provide sufficient soil water to accomplish full boll development of fruit set up to cut-out. Defoliants were applied on 20 September. A mechanical picker was used to harvest the entire four rows of each plot (experimental unit) on 9 October in order to obtain yield estimates. Data was analyzed in accordance to procedures outlined by Steele and Torrie (1980) and the SAS Institute (1988).

Results

Patterns for FR and HNRs are presented in Figure 1 for all treatments. The treatments consisting of the scheduled approaches to PIX management utilized two applications of 0.75 pt. PIX/acre with each application. The first PIX application was made to the scheduled treatments pre-bloom (1074 HUAP), at which time vigor conditions were not excessive, but HNRs were increasing slightly. Based on FR and HNRs relative to the baselines, PIX applications were not imposed on the feedback treatments until early bloom (7 July, 1688 HUAP) in response to a distinct increase in HNRs. Treatments associated with the scheduled approach to PIX management received a second application on 7 July also. Subsequent plant measurements revealed higher HNR levels for treatment one plots (no PIX) relative to all others that did receive PIX.

Nitrogen applications in the feedback treatments were structured to provide approximately 100 lbs. fertilizer N/acre. This followed from a projected yield goal of three bales, assuming a N requirement of about 60 lbs. N/bale, and crediting 50 lbs. residual soil NO₃-N/acre (approximately 14 ppm NO₃-N from a 12inch, pre-season soil sample). Applications of fertilizer N were split from pinhead square formation through peak bloom (~2000 HUAP) in an effort to optimize efficiencies of fertilizer N uptake and utilization by the crop. Applications of PIX and N in the scheduled treatments followed a predetermined approach in an effort to "push" the crop. It is also interesting to note that all treatments began the season with a range of 10 to 15 ppm NO₃-N concentrations (residual) in the top 12 inches of soil profile, which equates to approximately 40 lbs. NO₃-N / acre in this top layer of the soil. Water sources for this study were used with negligible levels of NO₃-N concentrations.

Petiole NO₃-N concentrations are outlined for each treatment in Figure 1. Based upon the petiole information, adequate N fertility was maintained all season. This information serves to reinforce earlier findings indicating that fertilizer N increments of approximately 50 lb. N/acre/application, split over the early stages of the fruiting cycle, are sufficient in maintaining plant N requirements for cotton.

Yield results are shown in Table 5 with means separated according to single degree of freedom orthogonal contrasts. The results of several meaningful contrasts that were conducted are shown in Table 6. Significant differences among treatments (P less than or equal to 0.05) revealed a general benefit from PIX applications for either regime (scheduled or feedback). The highest yields (arithmetically) were realized with treatments four and five with the higher rate of N and both approaches to PIX management. Therefore, a definite benefit was realized from the PIX applications with a trend towards yield enhancement with the higher rate of N. In this study, it appears that a single application of PIX (7 July) at a rate of 0.75 pt. PIX/acre was sufficient to provide adequate growth control and improved yield.

The use of a feedback approach to management offers the opportunity to improve upon the efficiencies associated with important crop management inputs. A feedback approach requires site-specific management, regular field evaluation, and well established references or baselines. For example, it is probable that cotton crops would not respond positively to scheduled PIX applications every season. Accordingly, the use of a feedback approach provides the ability to better determine when the probability of a positive yield response to PIX is greatest.

Acknowledgment

The support provided by the Marana Agricultural Center staff, particularly Glen Barney, is gratefully acknowledged. Also, we want to acknowledge the valuable technical assistance provided by illustrious crew associated with the University of Arizona Cotton Agronomy Program.

References

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