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Defoliation
of Pima Cotton J.C. Silvertooth, Extension Agronomist - Cotton
Defoliation of American Pima (Gossypium barbadense L.) and Upland
(G. hirsutum L.) cotton continues to be an important feature
of the production and management systems in the desert southwest. This
is particularly true with Pima cotton due to it’s more indeterminate
nature and the very high priority given the quality of harvested lint.
Pima cotton has long been recognized (Kittock, et al., 1977) in terms
of the difficulties associated with successful defoliation of this crop.
Pima growers often have to make three to six applications of chemical
defoliants in an effort to prepare the crop properly for harvest (Silvertooth,
1988). This often has been attributed to the robust, perennial nature
which is characteristic of Pima cotton and the difficulties in altering
the basic physiological processes necessary for defoliation (Cathey,
1986) through crop management. A number of experiments have been conducted across Arizona from elevations
of 150 ft. to 2,000 ft. above sea level with the primary objective of
developing management guidelines oriented towards accomplishing a successful
defoliation of a Pima crop from a single application of chemical defoliant
(Silvertooth and Howell, 1988; Silvertooth et al., 1989; Silvertooth
et al., 1990a; Silvertooth et al., 1990b; Nelson and Hart, 1991; Nelson
and Silvertooth, 1991; and Silvertooth et al., 1991b). Considerable
progress has been made from these experiments on the development of
chemical defoliant treatments capable of accomplishing a satisfactory
defoliation (³ 75% leaf drop) from a single application. The information
gained from these experiments has also shown the importance of a number
of other factors which impact the defoliation efficiency besides the
chemical itself. Factors such as plant-water relations, nitrogen (N)
fertility status, the extent of honeydew deposits on the leaves from
insects such as the sweetpotato whitefly (Bemisia tabaci (Gennadius))
or aphids, and weather conditions following the defoliant application
have been recognized as being important in terms of the final defoliation
resulting from a given defoliant material. The purpose of this report is to provide a brief review and summary
of the aforementioned factors concerning the management of a Pima cotton
crop towards optimum production and efficient defoliation. The primary agronomic focus of desert cotton production systems is
the production of optimum amounts of high quality lint, not to develop
a crop that is easy to defoliate. However, the two most important agronomic
inputs provided to a cotton crop for optimum production, water management
(plant-water relations) and N fertility status, also serve to impact
defoliation efforts as well. General agronomic guidelines for Pima cotton
production are outlined in another publication (Silvertooth, 1991),
but water and N management are worthy of being addressed within the
context of both optimum yield and defoliation. The details of plant-water relations and water management which optimizes
yield from a Pima cotton crop extend beyond the scope of this paper.
In terms of late season water management, the timing of the last (terminal)
irrigation and the time interval between the terminal irrigation, and
the application of the chemical defoliant can be very important (Oosterhuis
et al., 1991). The physiological process of defoliation is linked closely with the
natural senescence (aging) of the plant and its individual leaves. During
the senescence of the leaves, a series of hormonal reactions occur which
promote the formation of an abscission layer at the base of a leaf petiole.
In chemically defoliating a crop, an attempt is made to accelerate and
enhance the natural senescence process, leaf abscission, and ultimately
leaf drop (Cathey, 1986). Late in the season some degree of crop senescence will be occurring
naturally and can be enhanced by imposing some degree of water stress.
After the final irrigation has been applied, the crop will progressively
desiccate at a rate dependant upon weather conditions, water holding
capacity of the soil (soil texture), the amount of water applied in
the last irrigation, and the overall condition of the crop fruit load
and canopy. In general, it has been found that for most cases the time
interval between the last irrigation and the application of the chemical
defoliant should be 2X (twice) the normal time span used between the
late-season irrigations. This provides for the water needs of the late
set harvestable fruit intended for harvest and also some degree of crop
dry-down, which in turn promotes crop senescence. The management for optimal plant-water relations has several facets.
A certain degree of crop drying and senescence is desired, but plant-water
status should remain sufficient to maintain adequate physiological activity
necessary to carryout the effects of the defoliant material. In addition,
the plant must maintain satisfactory water relations such that adequate
green leaf weights are maintained to enable the leaves to break through
an abscission layer once it is formed, and actually drop to the ground.
(Cathey, 1986). Attempts to defoliate cotton plants that are excessively dry can often
result in complete leaf desiccation but poor leaf drop. In some cases
an abscission layer may form, but due to very dry conditions the leaves
do not have sufficient green leaf weight to accomplish a shear across
the abscission layer, and consequently the leaves fail to drop from
the plant. In some cases where plants are exceedingly dry at the time
of defoliation, defoliant materials may completely desiccate the leaves
rather than enhance the formation of an abscission layer and defoliation.
In any case, the retention of desiccated leaves on the plant often results
in excessive trash in the harvested lint. Attempts have been made to quantify and identify critical levels in
plant-water relations by use of infrared thermometry and a Crop Water
Stress Index (CWSI) (Silvertooth et al., 1990b). However, the application
of this technology and CWSI levels identifying critical points in crop
desiccation for defoliation management has not proven to be satisfactory
in providing a reliable, quantitative approach. Alternate technology
such as leaf water potential measurements may be worthy of further study
in this application. At present the general approach using 2X the late
season irrigation interval as the minimal time period between final
irrigation and defoliant application, can serve as a guide for estimating
satisfactory plant-water relations for defoliation. Nitrogen is the mineral nutrient to which cotton crops in Arizona most
consistently respond and is often required as fertilizer N additions
to maintain optimum yield potentials. Excessive N fertility promotes
vegetative growth and delayed senescence. High N fertility levels late
in the season also have a particularly negative effect on defoliation.
Recent research results have shown that N fertility levels corresponding
to 3,000 ppm (or less) NO3--N concentrations in petioles prior to defoliation
will not interfere with defoliation effectiveness. Excessive N levels
(³ 5,000 ppm NO3--N in petioles) will retard effective defoliation. Management of N for optimum yield and late season crop management (including
defoliation) is described more thoroughly in other publications (Silvertooth
and Doerge, 1990; Silvertooth et al., 1991a). It is very important to
minimize N applications past the peak bloom stage of crop development
because it will complicate late season management problems, such as
defoliation. Applications of N fertilizer should be split over the course
of the season and made in response to crop N status (petiole samples
and analyses), crop fruiting patterns (plant mapping), and stage of
crop development (Silvertooth and Doerge, 1990) to accomplish highest
possible efficiency of the N fertilizer applied and to achieve optimum
yield potentials for the crop. Populations of sweetpotato whitefly in cotton fields usually result
in a liberal coating of a sugary exudate (honeydew) on the foliage and
lint. Honeydew buildup on cotton lint results in adverse conditions
for ginning and milling, and is a serious threat to many cotton producing
regions. The presence of honeydew coatings on the leaves of cotton plants
due to whitefly (or any honeydew producing insect) populations can also
diminish the effectiveness of any chemical defoliant application due
to reduced uptake and penetration of the defoliant through the leaf
cuticle and epidermis. Physiological activity of any defoliant is dependant
upon uptake of the material through the leaf surface. Therefore, defoliation
results may be affected by late season whitefly populations and the
extent of honeydew deposited on the leaves prior to defoliant application. As with most management efforts in crop production, weather can have
a very strong influence on the final results obtained from a given defoliant
application. Temperature conditions experienced in the period after
defoliant application on cotton can affect the results. It is often
found that under warmer conditions plant physiological activity is higher
and therefore defoliant effects may be more pronounced and rapid than
under cooler conditions. This pattern of response also impacts the rate
of materials which may be required for obtaining a satisfactory defoliation.
Results obtained in the experimental programs conducted since 1987 (Silvertooth
and Howell 1988; Silvertooth et al., 1989; Silvertooth et al., 1990a;
and Silvertooth et al., 1991b), have shown that a period of 14 days
should be allowed for the full defoliation response for most treatments
and weather conditions. Rates of selected materials should be adjusted
in accordance to current and anticipated (forecast) weather conditions.
Under very warm or hot conditions, lower rates may suffice; whereas
under cooler conditions (late season and/or higher elevations) higher
labeled rates may be required for satisfactory effects from a given
defoliant application. There are several ways of describing prevalent temperature conditions.
A common means of describing temperature conditions with regard to crop
response is by the use of heat units (HU, 86/55°F thresholds; Brown,
1989). Because of the interaction with other factors such as plant-water
relations, N fertility status, honeydew deposits, etc.; it is difficult
to explicitly prescribe rates of defoliants in response to temperature
conditions. A general set of guidelines as shown in Table 1, can be
used to determine the relative rates of a chosen defoliant treatment
which corresponds with anticipated weather conditions. Weather conditions are difficult to predict for any extended period of time. Generally, warm to hot conditions can be expected in Yuma County in early September in contrast to cooler conditions common in Pima County in late October to early November. Under these very general climatic descriptions, one could prescribe rates for selected defoliants at a relatively low rate for the Yuma County case and relatively high (close to top of labeled rate) for the Pima County case as described. The information in Table 1 can be used in combination with predictions of HU accumulations for a given time frame and location based upon historical, long-term weather records listed in another University of Arizona publication (Brown, 1991). A general prediction of HU accumulations expected for a 14-day period after defoliant application could then be applied to Table 1 outlines, and a given defoliant treatment selected as listed in Table 2.
Table 1. Suggested ranges in defoliation
treatment rates in response to expected heat
Table 2. Treatments suggested for Arizona Pima cotton defoliation. * * Treatments listed are based upon
field esperiments conducted under a wide range of conditions in Arizona.
Trade names are provided for the benefit of the
reader and do not imply endorsement by the University of Arizona.
A list of some defoliant treatments are shown in Table 2 which include
only materials tested under experimental conditions described earlier.
This list is not all inclusive and does not describe all possible materials
and/or combinations. The list in Table 2 does provide a group of treatments
which have proven to be capable of providing a satisfactory defoliation
(³ 75% total defoliation) from a single application. The ranges
in rates listed in Table 2 should be considered in light of expected
weather conditions as previously described (Table 1). An important point to recognize is that all treatments listed in Table
2 represent a combination of two or more materials. For example it has
been consistently found with Pima cotton defoliation that combinations
of materials such as DROPP + DEF/FOLEX or DROPP + Accelerate are much
more effective than applications of any of these materials individually
at a given rate. Also, under warm conditions (more than 200 HU in 14
days expected) the combination treatments at low to medium rates are
more effective and consistent than single material applications at higher
rates. In the use of any chemical defoliant, label specifications should always be followed closely.
Defoliation often represents the final step in the production of a
cotton crop. With the emphasis on premium quality associated with Pima
cotton production, efficient defoliation is a matter of paramount concern
in late season crop management. In managing a Pima crop for defoliation,
a number of important factors such as plant-water relations, N fertility
status, extent of insect honeydew deposits present on the foliage, and
weather conditions must be taken into account before a defoliant treatment
is selected and applied. General descriptions of these factors and their
contributing influence on Pima cotton defoliation have been described
in this report for consideration towards achieving a satisfactory defoliation
from a single treatment application. Successful defoliation from a single
application can be accomplished under a wide range of conditions. Best
defoliation results have a higher probability of occurring when the
applicator is conscious of the prevailing circumstances and manages
the crop accordingly.
Brown, P. W. 1989. Heat units - Report 8915. The University of Arizona, College of Agriculture. 12 pp. Brown, P. W. 1991. Normal values of heat unit accumulation for southern Arizona - Report 190041. The University of Arizona, College of Agriculture. 61 pp. Cathey, G. W. 1986. Physiology of defoliation in cotton production. In J. R. Mauney and J. M. Stewart (ed.), Cotton Physiology. No. 1, Cotton Foundation, Memphis, TN. p. 143-153. Kittock, D. L., H. F. Arle, L. A. Barriola, and T. J. Henneberry. 1977. Chemical termination of cotton fruiting. Cotton, A College of Agriculture Report. University of Arizona, Series P-40:83. Nelson, J. M. and G. Hart. 1991. Defoliation research at the Maricopa Agricultural Center in 1990. Cotton, A College of Agriculture Report. University of Arizona, Series P-87:33-35. Nelson, J. M. and J. C. Silvertooth. 1991. Defoliation research on Pima cotton at the Marana Agricultural Center in 1990. Cotton, A College of Agriculture Report. University of Arizona, Series P-87:36-38. Oosterhuis, D. M., R. E. Hampton, and S. D. Wallschleger. 1991. Water deficit effects on the cotton leaf cuticle and the efficiency of defoliants. J. Prod. Agric. 4:260-265. Silvertooth, J. C. 1988. Preparing a Pima crop for harvest. Proc. Western Cotton Production Conference. Las Cruces, NM. p. 61-63. Silvertooth, J. C. and D. R. Howell. 1988. Defoliation of Pima cotton.
Cotton, A College of Agriculture Report. University of Arizona, Series
P-72:117-120. Silvertooth, J. C., D. R. Howell, S. W. Stedman, G. Thacker, and S. S. Winans. 1989. Defoliation of Pima Cotton, 1988. Cotton, A College of Agriculture Report. University of Arizona, Series P-77:77-81. Silvertooth, J. C. and T. A. Doerge. 1990. Nitrogen management in Arizona cotton production - Report 9024. The University of Arizona, College of Agriculture. 4 pp. Silvertooth, J. C., D. R. Howell, G. Thacker, S. W. Stedman, and S. S. Winans. 1990a. Defoliation of Pima Cotton, 1989. Cotton, A College of Agriculture Report. University of Arizona, Series P-81:20-22. Silvertooth, J. C., S. W. Stedman, and J. Tollefson. 1990b. Interaction of Pima cotton defoliation and crop water stress index. Cotton, A College of Agriculture Report. University of Arizona, Series P-81:32-34. Silvertooth, J. C. 1991. Agronomic guidelines for Pima cotton production - Report 190038. The University of Arizona, College of Agriculture. 3 pp. Silvertooth, J. C., L. J. Clark, J. E. Malcuit, E. W. Carpenter, T. A. Doerge, and J. E. Watson. 1991a. 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., S. H. Husman, G. W. Thacker, D. R. Howell, and S. S. Winans. 1991b. Defoliation of Pima cotton, 1990. Cotton, A College of Agriculture Report. The University of Arizona is an Equal Opportunity/Affirmative
Action Employer. Any products, services, or organizations that are mentioned,
shown, or indirectly implied in this publication do not imply endorsement
by the University of Arizona. |
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