Evaluation of a Feedback Approach
to Nitrogen and Pix Applications, 1997
J.C. Silvertooth, Plant Sciences Department
E.R. Norton, Plant Sciences Department
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.
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).
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.
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.
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- SAS Institute. 1988. SASSTAT:Procedures. Release 6.03 ed. SAS Inst., Cary, NC.
- Silvertooth, J. C. and T. A. Doerge. 1990. Nitrogen management in Arizona cotton production. Report 9024. The University of Arizona, College of Agriculture. Tucson, AZ.
- Silvertooth, J.C., D.R. Howell, C.R. Farr, and J.E. Malcuit. 1989. Evaluation of PIX multiple application treatments on Upland and Pima cotton in Arizona, 1988. Cotton, A College of Agriculture Report. University of Arizona, Series P-77 : 104-109.
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- Silvertooth, J.C., J.E. Malcuit, D.R. Howell, and C.R. Farr. 1990b. PIX multiple application evaluations in Arizona on Upland and Pima cotton. Cotton, A College of Agriculture Report. University of Arizona. Series P-81 : 50-56.
- Silvertooth, J. C., P. W. Brown, and J.E. Malcuit. 1991a. Basic crop development patterns. Cotton, A College of Agriculture Report. University of Arizona. Series P-87:43-49.
- Silvertooth, J. C., L. J. Clark, J. E. Malcuit, E. W. Carpenter, T. A. Doerge, and J. E. Watson. 1991b. Nitrogen management experiments for Upland and Pima cotton, 1990. Cotton, A College of Agriculture Report. University of Arizona. Series P-87:209-221.
- Silvertooth, J.C., J.E. Malcuit, S.H. Husman, W.S. Winans, and L. Hood. 1991c. Cotton response to multiple applications of PIX, 1990. 1991 Cotton Report. College of Agriculture, University of Arizona. Series P-87 : 55-58.
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- Silvertooth, J.C., J.E. Malcuit, L. Hood, and S.H. Husman. 1993b. Cotton Reponse to Applications of PIX, 1992. Cotton, A College of Agriculture Report, University of Arizona. Series P-94 : 84-99.
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- Steel, R.G.D., and J.H. Torrie. 1980. Principles and procedures of statistics. McGraw-Hill, New York.
This is a part of publication AZ1006:
"Cotton: A College of Agriculture Report," 1998, College of Agriculture,
The University of Arizona, Tucson, Arizona,
85721. Any products, services, or organizations that are mentioned, shown, or indirectly
implied in this publication do not imply endorsement by The University of Arizona.
The University is an Equal Opportunity/Affirmative Action Employer.
This document located at http://ag.arizona.edu/pubs/crops/az1006/az10068b.html
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