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Agronomic
Guidelines for Pima Cotton Production in Arizona J.C. Silvertooth, Extension Agronomist - Cotton American Pima (Gossypium barbadense L.) cotton production has
historically been a very important feature of Arizona cotton production.
The first commercial crop of extra-long staple cotton in Arizona was
produced in 1912 (McGowan, 1961). A total of 11 extra-long staple varieties
have been developed and released for production in Arizona and the desert
Southwest. Pima S-1 was released in 1951 after development of a selection
from a series of crosses involving Sea Island, Pima, Tanguis, and a
variety of Stoneville Upland (G. hirsutum L.) cotton (Bryan, 1955). These crosses provided a broad germplasm base for the development of
improved fiber and yield characteristics for further Pima selections.
Subsequently, five additional Pima varieties have been released: Pima
S-2, 1960; Pima S-3 and S-4, 1966; Pima S-5, 1975; and Pima S-6, 1983
(Turcotte and Feaster, 1988). Improvements in each successive release
have provided higher yield potentials through increasing heat tolerance
and earliness. Improved heat tolerance and earliness have been beneficial
to lower elevation areas (below 2,500 ft.), and improved earliness has
been a benefit to higher elevation areas (above 2,500 ft.) (Niles and
Feaster, 1984). Continued improvements in yield potential through increasing
heat tolerance and earliness, while maintaining or improving fiber properties,
is an ongoing goal within the Pima breeding program for the development
of future variety releases (Turcotte and Feaster, 1988). With the interest and activity in Pima cotton production in Arizona,
perhaps an outline of some of the major points concerning some of the
agronomic factors involved in the production of Pima cotton in Arizona
would be in order. Newcomers and veterans to the Pima production process
recognize that differences do exist between Upland and Pima in terms
of growth habits and management. Even with the release of Pima S-6 in
1983, Pima remains to be more inde-terminate than its Upland relatives,
a factor which figures very predominantly in several aspects of its
management. A series of experiments have been conducted in the past several years,
where Pima S-6 was included as a variety planted at four locations,
with four to five planting dates at each. Locations included Yuma, Maricopa,
Marana, and Safford. Dates of planting ranged from late February to
early April at Yuma, late March to early June at Maricopa, and very
early April to early June dates at both Marana and Safford. The planting
dates were separated by about 14 days in each case. The results are
fairly consistent among these tests with regard to the yield response
of Pima S-6 to date of planting. Even though Pima seeds are generally
regarded as being more cold tolerant, they emerged and became an established
stand best when planted in suitable warm soil conditions. Such conditions
could be generally described as 60-65°F in the zone of seed placement
for several days prior to and after planting, with a well-prepared seedbed.
We also found from these experiments that Pima S-6 tended to perform
best when planted early, and realized rather consistent declines in
yield with delayed plantings as used in these experiments. The point
to be taken is that planting of Pima S-6 is probably best for as early
a date as conditions will allow. This is to say generally that late
March to April plantings are best for most areas, and that delays past
that time may be quite costly in terms of yield potentials. For Yuma,
this possibly could be extended to say that Pima plantings should optimally
occur from late February to the first of April. Another way of describing optimum planting dates is by the use of heat
units (HU) using 86 and 55°F upper and lower limits. A range of
about 300 to 900 HU accumulated since January 1 at any given Arizona
location, can best describe the time frame for Pima (or full-season
Upland varieties) plantings that provide an optimum for yield potential.
This range of accumulated HU may vary by calendar date from year to
year, and by location; but can provide a better measure of seasonal
patterns. By the time 300 HU have been accumulated, adequate soil temperatures
likely will have been reached. This does not eliminate the need for
monitoring soil temperatures and weather forecasts, but serves as a
guideline for planting. Other things learned from these experiments included the fact that
as Pima was planted later (greater than 900 HU accumulated since Jan.
1) plants grew taller, more vegetative, and less productive. This is
seen as a plant response to greater amounts of heat (heat units) being
accumulated in shorter periods of time, which causes greater internode
length, larger leaves, and a lesser tendency to begin fruiting. So a
delayed planting of Pima will tend to bring about taller, more vegetative
plants, with lower yield potential, after causing a grower a degree
of concern and difficulty in management. In fact, as HU accumulations
since January 1 exceed 700, yield potentials from subsequent Pima plantings
become increasingly marginal as vegetative tendencies under warm weather
conditions become more likely. Acceptable plant populations for Pima cotton range from approximately
20,000 plants per acre (ppa) to 50,000 ppa. Optimum populations range
from about 25,000 ppa to 40,000 ppa. Pima plants typically have long
fruiting branches, with as many as eight fruiting sites per branch.
Therefore, Pima plants have a very flexible nature in terms of compensating
for varying plant populations, and maintaining high yield potentials.
High plant populations (greater than 50,000 ppa) can lead to plants
that are tall, vegetative, and generally poor in total fruit (boll)
retention per plant. Therefore, high populations contribute to plants
that are difficult to manage for high fruit retention, and delay maturity
(decrease earliness). Pima cotton typically has been grown on a 40 inch row spacing, which
is generally accommodating to the wide branching nature of the plants.
At present, insufficient data are available to determine the advantages
or disadvantages associated with the use of 30-inch-row spacings. Further
research with more determinate Pima selections may prove to be of benefit
under narrow (30-inch) row systems in an effort to improve earliness
with this type of cotton. Probably one of the most difficult decisions to make in a Pima production
season is the time to initiate the first irrigation. Plant water stress
itself is a tool that many veteran Pima growers have found useful with
pre-S-6 varieties, and also sometimes with S-6, to control vegetative
growth. However, most successful Pima S-6 growers currently do not intentionally
water stress a Pima crop at any time except during very early periods
in the season. This is usually just prior to the first irrigation. Just
how much to stress a Pima crop at this time is a very good question
and a delicate one. It does not appear that anyone has established some
easily measured point at which enough stress has been incurred and irrigation
is needed before serious harm is done. This still remains a somewhat
“artistic” act that Pima cotton producers must carry out.
Most Pima growers and researchers agree that an imposed water stress
should be avoided up to the time one is preparing the crop for termination.
Substantial data on consumptive use patterns of Pima versus Upland
cotton are not available. However, on a daily basis, Pima cotton requires
essentially the same amount of water as its Upland counterparts do.
Any differences in total water used on a Pima crop probably occurs later
in the season when an additional one (or two) irrigation(s) may be provided
to mature later-set bolls. In terms of water management throughout the
course of the season, Pima S-6 is usually managed very much like full-season
Upland varieties. With the abundance of HU that Arizona cotton crops commonly accumulate,
the two main controls an Arizona cotton grower has are water and nitrogen
(N) fertilizer. Keeping the plant in good condition with regard to water
relations through the season is an obvious objective. Since the control
of the vegetative/reproductive balance is particularly critical in Pima,
one must also consider the N fertilization of Pima in a unique light.
Many producers and researchers agree that Pima is sensitive to excessive
N levels, and can convert extra growth to pure vegetation without any
trouble. In fact, many Pima growers purposely avoid fields where a high
level of residual available N (NO3- - N) will be present. This should
be a point of consideration for growers placing Pima in a field after
vegetable or alfalfa crops. Guidelines have been developed for managing N fertility in Pima by
use of petiole sampling in-season. The Arizona Cooperative Extension
publication 8373: The Cotton Petiole: A Nitrogen Fertilization Guide
(Pennington and Tucker, 1984) outlines procedures and ranges in petiole
nitrate -N (NO3- -N) levels for management through the growing season.
In comparison to Upland levels of petiole nitrates, Pima levels should
be somewhat lower throughout the season (approximately 40 percent lower).
Caution should particularly be taken to avoid excessive levels of NO3-
-N in the Pima petioles early in the season, to avoid excessive vegetative
growth before fruit set begins. Further details describing N management
for Upland and Pima cotton are provided in Arizona Cooperative Extension
publication 9024: Nitrogen Management in Arizona Cotton Production (Silvertooth
and Doerge, 1990). In terms of insects, weeds, and diseases; the general cases that pertain
to Upland cotton can be transferred to Pima. However there are a few
points of difference. Pima bolls seem to be more susceptible to pink
bollworm (Pectinophora gossypiella (Saunders)) damage than Upland. This
is primarily due to Pima bolls maintaining a higher moisture content
and softer boll walls that are prone to pink bollworm damage over a
longer period of time. Pima bolls are not sufficiently hardened until
35 to 40 days after bloom (800 to 900 HU). The sweet potato whitefly (Bemisia tabaci (Gennadius)) is becoming
a pest of concern in both Upland and Pima cotton production. Concern
is particularly associated with the diminished quality that results
from the sooty mold and the stickiness of the lint that may be caused
by the honeydew secreted by whiteflies. Pima cotton has appeared to
some to be more attractive to whitefly populations. Regardless of the
degree and cause of attractiveness, we know that whiteflies and Pima
are not good companions. Pima is an indeterminate plant, often grown
in a long, full-season setting. When whitefly populations characteristically
grow at nearly exponential rates late in the season, very little remedy
appears to be available at present. Termination, defoliation, and quick
removal of the crop has proven to be a good alternative to growers faced
with this problem. This is one point causing interest in evaluating
the length of season a Pima cotton crop requires for optimum economic
return, and the option of early termination. Particularly considering
the strong emphasis that quality has in Pima production systems. In terms of disease control, Pima cotton has demonstrated several differences
to Upland. Pima cotton is often thought of as being more susceptible
to Texas Root Rot than Upland cotton. Probably due to a deeper rooting
tendency, and due to the fact that Pima plants often don’t accomplish
a significant fruit set until later in the season. Many growers avoid
fields known to have substantial Texas Root Rot kill patterns. Pima
also has been cited as being more prone to developing Alternaria Leaf
Spot, a fungal pathogen which attacks the leaves of the plant. This
however, has not yet developed into a problem of any broad extent in
Arizona. In order to obtain the highest possible returns on a Pima crop, one
must maintain the highest quality lint as possible. The quality of harvested
lint is often a result of the preparation and picking process at the
end of the season. Accordingly, the late season management of the crop,
and the defoliation of the crop affect the timeliness in which the cotton
is harvested from the field. The choice and timing of defoliant chemicals
that are applied certainly are important in achieving satisfactory defoliation.
But other factors such as the late season plant-water status, the N
fertility status, and the boll load that the crop plants are carrying
have a definite impact on the way a cotton crop defoliates. This is
true of both Upland and Pima due to their perennial nature, but particularly
Pima with its robust and indeterminate growth pattern. By using chemical defoliants, one is attempting to enhance the natural
physiological process of plant senescence and leaf abscission. Defoliation
requires a degree of natural senescence which can be brought along to
some extent by the development of water stress late in the season. Plants
carrying a good boll load also naturally senesce a little more rapidly.
A certain degree of physiological activity is needed to realize the
effects of chemical defoliants, and also to have a sufficient green
leaf weight to actually drop the leaf from the plant once an abscission
layer is developed. Otherwise, leaves may be burnt but not dropped,
leading to a possible trash problem. Recent research conducted in an effort to develop better guidelines
for Pima cotton would reinforce these points. Exceedingly dry Pima plants
are difficult to defoliate (usually leaving intact, burnt leaves), while
fresh, lush growth also is very difficult to slow down and defoliate,
and has strong regrowth tendencies. Developing a slight water stress
following termination encourages senescence, but too much will hinder
defoliation efforts. Allowing three to four weeks (depending on soil
water holding capacity) after the final irrigation before defoliant
applications, generally provides adequate plant senescence to accommodate
defoliation. Late season N levels that are high also can cause the plant to maintain
strong vegetative growth. Based upon the guidelines mentioned previously,
petiole NO3- -N levels should be drawn down below 3,000 ppm prior to
defoliation. This will not cause a yield-limiting decline in N fertility,
while allowing for a stronger trend in plant senescence. Not every aspect of Pima cotton production has been addressed in this
bulletin. However, a discussion of the points in Pima production where
principal differences exist in comparison to Upland cotton production
has been attempted. Basic agronomics are essential for producing both
the quantity and the quality required for successful Pima production
in Arizona. Bryan, W. E. 1955. “Prospects for Pima S-1 cotton. Progressive
Agric. in Arizona, p.6. McGowan, J. C. 1961. “History of extra-long staple cottons.”
M. S. Thesis, University of Arizona. 170 p. Niles, G. A. and C. V. Feaster. 1984. “Breeding.” In R. J.
Kohel and C. F. Lewis (ed.) Cotton. Agronomy Number 24:201-231. American
Society of Agronomy, Madison, WI. Pennington, D. and T.C. Tucker. 1984. The Cotton Petiole: A Nitrogen
Fertilization Guide. Bulletin No 8373, The University of Arizona, College
of Agriculture. Silvertooth, J.C. and T.A. Doerge. 1990. Nitrogen Management in Arizona
Cotton Production. Bulletin No. 9024, The University of Arizona, College
of Agriculture. Turcotte, E. L. and C. V. Feaster. 1988. “Pima variety improvements — What can we expect?” Proc. Western Cotton Prod. Conf. p.58-60. 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. |
