Specialists Speaking -- June 2001
Improving Weed Control
Producers in Alabama are now utilizing the Roundup
Ready weed control system on most of the acres they are planting this
year. This weed control system has been widely adopted on conservation
as well as conventional-tilled fields. While there are many weeds
that this system will control, there are some species that are not
completely susceptible to glyphosate.
Producers should be scouting their fields for various weed species
that may not have been controlled by the initial application of glyphosate.
These species might include morningglories and prickly sida.
Other weeds that would normally be controlled by the initial glyphosate
application can survive if 1) their size was too large for adequate
control or 2) if low rates were used in the initial applications.
After the initial over-the-top treatments (up to the fourth leaf stage),
glyphosate treatments must be directed underneath the crop canopy
to lessen the potential for injury in the form of square shed.
As producers scout their fields for escaped weeds, they should carefully
take note of the specific weed species that are present. Since glyphosate
is more effective on some weed species compared to others, a tankmix
partner may be needed at layby to control them adequately.
The tankmix partner should be chosen in an effort to add to the postemergence
activity and to provide some degree of residual weed control. Producers
also may consider using layby treatments that do not contain glyphosate
in order to rotate herbicide families. This will reduce the long-term
potential for resistance problems.
In most cotton growing areas of Arizona,
June is a month that generally offers tremendous potential for developing
a strong cotton crop. For most of the state, cotton fields are in early
bloom or beyond in June. In the western-most parts of Arizona and the
lower elevations we can commonly reach peak bloom by the latter part
of June. The weather in June is typically dry and hot, which means we
commonly get a wide diurnal (day-night) temperature change, with relatively
cool nights and hot, clear days. This is an excellent time and set of
weather conditions for cotton plants to set fruit, providing they have
strong plant vigor. Therefore, it is in our best interests to set as
much fruit on the plant in the early part of the fruiting cycle as possible,
whether our objective is to produce a crop with a reduced season, early
termination strategy or, whether we plan to go for a top-crop with a
long, full season approach. Accordingly, June is often referred to as
"prime time" for Arizona cotton production.
To maintain strong plant vigor and a high rate of fruit retention, the
most important management factors we must take care of include irrigation
and pest control. Of course, many other inputs of course are important,
but water and pest management are absolutely critical. Accordingly,
it is very important to provide adequate water to the crop on a timely
basis. Water stress should be avoided, since it decreases plant vigor
and generally increases fruit loss and abortion rates.
Scouting is an important component of crop production.
Problems related to pests or the environment which add to or relieve
stress often are visible but are often difficult to quantify. Plants
encounter stress throughout the growing season. Some stress such as
filling a heavy boll load is "good stress" while having too much or
too little moisture is a "bad stress."
Few tools offer the ability to monitor stress as does COTMAN, a computerized
cotton management program which was developed by the University of Arkansas
and provides a very sensitive tool to detect stress. The growth of a
cotton plant is very predictable. Changes in the predictable patterns
of growth such as the development of new nodes can indicate stress if
development occurs below that expected.
A target development curve used in COTMAN represents growth and development
time lines that attempt to maximize yield and earliness. A crop development
curve flatter than the target indicates stress. If growth rates vary
(too slow or too fast), corrective measures can be taken to get plants
back on course. Square retention is a key component in en-suring a balance
between reproductive and vegetative development.
Plants with high retention values will have greater demands as fruit
develops than plants with lower retention values. COTMAN offers the
ability to predict anticipated needs based on interpretation of crop
development curves and fruit retention rates to better time inputs to
avoid plant stress. For more information on COTMAN contact your local
county Extension office.
The convenience of Roundup (RR) Ready
technology has displaced the precision of traditional weed management
programs. The latter requires accuracy in rate selection, timing and
placement. The former requires less. Or does it?
Evidence continues to build that sloppiness with glyphosate in RR cotton
is costing us.
Glyphosate applied over-the-top after the 4-leaf stage can reduce yields
200 pounds/A. Similarly, post-directed treatments of glyphosate which
contact considerable stem and leaf tissue also can result in significant
yield losses. The damage is "invisible." There is no leaf discoloration,
no stunting, nor typically any recognizable square or obvious boll shed.
Nevertheless, yields can suffer. The remedy is accurate post-directed
application of conventional herbicide treatments or glyphosate. The
key is precision-directed applications which minimize spray contact
with the cotton stalk and foliage. Conventional herbicides are the preferred
option. Compared to glyphosate, conventional herbicides provide superior
activity on problem weeds such as morningglories and nutsedge.
If you are using a Roundup Ready or BXN
weed control system, take maximum advantage of it. Several times last
year I observed producers using unnecessary herbicide applications or
cultivations instead of taking advantage of what they had. The Roundup
management system offers an affordable, easy to manage weed control
system for cotton; however, it is critical that you rely on the glyphosate
applications and do not cultivate.
Cultivation will increase weed pressure by allowing the germination
of weeds that would remain in the soil below the depth for germination.
In the case of a BXN system, I found that many times producers will
go ahead and apply other herbicides that are labeled to control the
same weed spectrum as the Buctril.
This is only costing money and reducing your potential for profit. Select
your weed control management system and take full advantage of it. Mixing
weed control strategies often will only cost you money or may even cause
more weed problems.
When there is a severe weed problem,
it is tempting to spray Roundup over the top of Roundup Ready cotton
after the five true leaf stage. Cotton will have its first square on
the fifth to eighth node. At this stage squares can be present but not
be visibly apparent.
The square will be the growing tip on its vegetative branch. Roundup
is translocated to the growing tips of the cotton plant. Sometimes when
Roundup is translocated to the squares, it will cause the flower not
to shed pollen. If the flower does not get enough pollen, it will abort
and be shed.
With a reduced amount of pollen the boll will have its tip distorted
to one side, which is commonly called "parrot beaked." Other bolls will
have to replace these shed bolls which will result in delayed maturity.
The manufacturer would certainly like to be able to recommend using
Roundup over the top after five true leaves, but the boll shed is an
excellent reason for prohibiting use in this manner.