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Agronomy notes
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Permanent Link: http://ufdc.ufl.edu/UF00066352/00081
 Material Information
Title: Agronomy notes
Uniform Title: Agronomy notes (Gainesville, Fl.)
Physical Description: v. : ill. ; 28 cm.
Language: English
Creator: Florida Cooperative Extension Service
Publisher: Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida.
Place of Publication: Gainesville
Creation Date: October 2006
Subjects / Keywords: Crops and soils -- Florida   ( lcsh )
Crop yields -- Florida   ( lcsh )
Agriculture -- Florida   ( lcsh )
Agronomy -- Florida   ( lcsh )
Genre: government publication (state, provincial, terriorial, dependent)   ( marcgt )
periodical   ( marcgt )
serial   ( sobekcm )
Statement of Responsibility: Florida Cooperative Extension Service, University of Florida, Institute of Food and Agricultural Sciences.
General Note: Description based on: January 1971; title from caption.
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Rights Management: All rights reserved by the source institution and holding location.
Resource Identifier: aleph - 000956365
notis - AER9014
System ID: UF00066352:00081


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Vol. 30:10

October 2006


Cotton D efoliation Problem s in 2006 ........................................ .......................... 2
D efoliatin g C otton ....................................................... ................ .. 2

Herbicide Resistance................................................
Cimarron is replaced by Cim arron Plus .................................. ........................ 2
V ista H erbicide for Pastures ............................................................. ....................... 3
H ow H erbicides W ork A tazine......................................................................... ...... 3

Small Grain and Forage Variety Information
fo r 2 0 0 6 -2 0 0 7 ..................................................... ................ 5
A sian Soybean R ust U pdate............................................................................. ........ 5
Fall Soil Test ..................................... .................. ............... ......... 6

October 3rd Perennial Peanut Field Day, Marianna, FL.
October 11th Weed Control Field Day, Ona, FL.

The Institute of Food and Agricultural Sciences (IFAS) is an Equal Employment Opportunity _Affirmative Action Employer authorized to
provide research, educational information and other services only to individuals and institutions that function without regard to race, color,
sex, age, handicap or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension
Office. Florida Cooperative Extension Service/ Institute of Food and Agricultural Sciences/ University of Florida/ Larry Arrington, Dean.

Cotton Defoliation Problems in 2006

Cotton began to cutout early in many fields
across Florida this year. Some of this was
due to higher heat units experienced during
the growing season and hastened maturity.
However, dry weather was the culprit in
many cases.

Early cutout was also caused by Cercospora,
Alternaria, and Stemphylium leaf diseases in
some fields. These diseases are often seen
when the plant is under stress. Cotton was
under moisture stress most of the season
which resulted in potassium deficiency was
common. Soil samples in these fields may
have shown adequate potassium, but leaf
samples showed deficiencies. These
deficiencies start showing up when high
demand coincides with slower root growth.

David Wright

Defoliating Cotton

The updated cotton defoliation guide can be
found at http://edis.ifas.ufl.edu/AG188 .

Cotton fruit development in late September
will often not result in any additional cotton
yield. In most cases, blooms in July and
early August will make 90% of the cotton
yield or more. Bolls set in mid-summer are
usually larger and mature in 40 to 50 days,
while bolls set in August can take 60 days or
longer to mature. Late flowers look
attractive and may give the appearance of
adding to the final yield of the crop, but
should not be given preference over the fruit
that was set during the first 3 to 4 weeks of
bloom. Therefore, our experience is that
September blooms contribute little to yield.

There are several ways to determine when to
defoliate cotton. An old rule of thumb is to
defoliate when 60% of the bolls are open.
Another method is nodes above cracked
bolls (NACB). Research has shown that

cotton with four nodes above the highest
cracked boll can be defoliated without
significant weight or quality loss. IfNACB
counts average five or more, defoliant
applications should be delayed.

David Wright

Herbicide Resistance

The National Cotton Council has recently
developed a weed resistance on-line learning
module. This program can be accessed

This is a very good program that contains a
great deal of information about herbicides
and weed resistance. There are four parts
that to this module: herbicide mode of
action, herbicide resistance, do I have
resistance, and herbicide resistance
management. I would encourage everyone
to take time to view one or all of these
modules. This was developed to help
educate producers and agriculture managers
and help everyone stay one-step ahead of
these new and developing resistance issues.

Jason Ferrell

Cimarron is replaced by Cimarron

Florida hay producers used Ally Herbicide
for many years to control 'Pensacola'
bahiagrass in bermudagrass. However,
approximately 2 years ago DuPont decided
to change the product name from Ally to
Cimarron. The product was still
metsulfuron-methyl (60%) and everything
remained the same; the only difference was
the new name.

Since then, DuPont has decided to phase out
Cimarron. This herbicide will be replaced
with Cimarron Plus. Cimarron Plus has two
active ingredients: metsulfuron-methyl

(48%) and chlorsulfuron (15%).
Considering that the amount of metsulfuron-
methyl per ounce of material has been
reduced from 60% to 48%, it will now be
necessary to convert old Cimarron use rates
to the new Cimarron Plus rates. This can be
done by multiplying the Cimarron rates by
1.25. For example, if you wish to control
'Pensacola' bahiagrass and you have
traditionally used Cimarron at 0.3 oz/A, you
will now need to use Cimarron Plus at 0.375

It is currently unknown what additional
weeds will be controlled by the addition of
chlorsulfuron. However, Cimarron Plus is
safe for use on bermudagrass hay fields and
has a 0 day restriction for grazing and

Jason Ferrell

Vista Herbicide for Pastures

Vista is not a new product, but has been sold
exclusively for vegetation management on
rights-of-ways. Recently, this herbicide was
approved for use in pastures through a
supplemental label. Vista contains
fluroxypyr, which is also a component of
Pasturegard (triclopyr + fluroxypyr). We
have been happy with the results of
Pasturegard over the past couple of years.
So, why has Vista been approved for use in
Florida? At this point in time, I do not
believe that Vista will be a stand-alone
product for weed control in pastures.
However, it can be a good option for tank-
mix partners for hard to control weeds.

The supplemental label for Vista was seen
as an opportunity to increase the weed
spectrum of Milestone and Forefront; two
herbicides recently labeled for tropical soda
apple control in pastures. Milestone is
known to be weak on large dogfennel, but it
was thought the addition of 2,4-D to
aminopyralid (Forefront) would overcome
this issue. However, recent complaints have

shown that large dogfennel were not
controlled with Forefront. Therefore, a test
plot was established to examine potential
tank-mix partners for Forefront to control
dogfennel with one application. The
dogfennel were approximately 40 inches tall
at the time of application. It was found that
the addition of as little as 8 fl oz/acre of
Vista to 2 pints/acre of Forefront provided
>90% dogfennel control 2 months after
treatment (Figure 1). Control with Forefront
alone at 2 and 2.6 pints per acre resulted in
61 and 66% control, respectively. The cost
of Vista is approximately $90/gallon. So,
the tank-mix of Forefront at 2 pt/acre ($16)
plus Vista at 8 fl oz/acre ($6) would cost
approximately $22/acre for excellent
do fennel and TSA control.

Figure 1. Response of dogfennel 2 months
after treatment with 2 pint/a Forefront + 8 fl
oz/acre Vista.

Similar to Milestone, there are no grazing
restrictions for Vista for beef or dairy cows.
However, hay and silage should not be
harvested for 7 days and meat animals
should be removed from treated pastures at
least 2 days before slaughter.

Brent Sellers

How Herbicides Work Atazine

Atazine, which is also the common name, is
one of oldest and most widely used
herbicides. It was first registered for use in
corn, but later registered in other grass crops
such as sorghum and sugarcane. Atrazine
can also be used in certain warm-season

turfgrasses. Atrazine is generally applied
pre-emergence to the soil, but can also be
used postemergence on several weeds. This
herbicide is very effective on broadleaf
weeds and has fair to good activity on
several grasses.

Atrazine is classified as a photosynthesis
inhibitor and is a member of the triazine
herbicide family based on chemical
structure. When atrazine is applied to the
soil, it is taken up by the plant roots and
moved upwards via the water stream. Thus,
it is moved in the xylem tissues and gets into
the leaf through the process of transpiration.
Once inside the leaf, via the leaf veins,
atrazine diffuses into cells and then into the
chloroplasts of cells. Chloroplasts are those
tiny dark green dots within the leaf cells as
shown in figure 1; an expanded view in
figure 2.

Figure 1. Adapted from:


t......i .~PLORIU G COm
www. caribbeanedu.com

Figure 2. Adapted from:

Once inside the chloroplast, atrazine moves
to the thylakoids, which are membranes
within the chloroplast (Figure 2). Thylakoid
membranes contain a series of complex
proteins with figure 3 showing a cross-
section of a thylakoid membrane. These
proteins are embedded within the thylakoid
membrane and function to direct the light
energy captured by the chlorophyll
molecules. This energy is captured in the
form of electrons (e- in the figure below)
which are passed through the proteins to
form intermediates used for carbon fixation.
Atrazine binds to a protein in photosystem
II, thereby blocking the flow of electrons.

photosystem II
photosystem I

Figure 3. Adapted from: www.icb.ufmg.br

When this electron flow is blocked,
intermediates are not formed and the plant
cannot fix carbon. Without carbon, the plant
cannot continue to grow and perform normal
functions. In addition to blocking electron
flow, another important problem arises
within the chloroplast. Sunlight continues to
shine and the chlorophyll molecules
continue to absorb light energy. There are
approximately 300 chlorophyll molecules
feeding energy into each complex shown
above in figure 3. With electron flow
blocked by atrazine the chlorophyll
molecules cannot release the energy
absorbed from sunlight. Unable to dissipate
energy, they self-destruct.

Atrazine can also be applied postemergence,
over the top of both weeds and crops. It is
absorbed by the leaves and diffuses into

cells and then into the chloroplasts of cells.
Basically, atrazine follows the same path as
the arrow for sunlight as shown in figure 1.
When atrazine is applied postemergence, it
does not move within the plant and acts like
a contact herbicide.

This cascade of events helps to explain the
symptoms observed after applying atrazine.
First, the plant begins to show yellowing of
the leaves as the chlorophyll molecules
disintegrate. Once the chlorophyll begins to
destruct, oxygen radicals are formed. Then
the chloroplast membranes are destroyed
and eventually outer cell membranes
become ruptured. This leads to necrotic
tissue, taking on the appearance of a brown
paper bag.

The images above show 2 types of injury
observed with atrazine. The first image is
injury symptoms from a carryover situation,
where atrazine was used in the previous crop

and there was sufficient residue to affect the
soybean plant above. Notice the chlorotic
(yellow) tissue. In the second image, the
plants are dead from atrazine applied to the
soil before emergence. As the plants
emerged, the atrazine moved to the young
leaves, causing rapid chlorosis and necrosis.
Plants that are killed from atrazine in this
manner are often called "mummies".

Atrazine has a moderate amount of
persistence in many soils, but rarely causes a
carryover problem in Florida due to our
warm and humid climate.

Greg MacDonald

Small Grain and Forage Variety
Information for 2006-2007

Information on ryegrass and small grain
varieties for Florida and Georgia can be
found at www.griffin.uga.edu/sevt.

David Wright

Asian Soybean Rust Update

Asian soybean rust (ASR) did not spread as
fast in 2006 as we anticipated since infected
kudzu survived throughout the winter in
North Florida, Georgia, and Alabama. The
reduced spread of ASR was mainly due to
dry weather conditions from late February to
late July. Tropical storm Ernesto swept
through Florida and appeared as if it could
carry the disease into the corn/soybean belt,
but turned and hit the Carolinas which has
seen higher infection levels on soybeans
than most of the country.

Rust has been identified in many sentinel
plots in Florida, but not in 100% of the plot
areas. We have monitored these plots
throughout the season and have watched the
disease progress. Only a few of the early
planted sentinel plots have been defoliated
from the disease. We have not observed any

commercial fields with high levels of
infection to this point and it is probably too
late to influence yield. Fungicides did an
excellent job of controlling rust where we
had early planting and relatively early
infection on the research station. We would
like to thank counties extension faculty who
worked with us again this year on plots in
their counties and it is also appreciated by
farmers from the mid west watching where
rust spread. Asian soybean rust has stayed
in the south this year being found widely on
both soybean and kudzu. Much research
effort is being expended to find out about
this disease and NFREC in Quincy has been
the center for much of the work in the U.S.

David Wright and James Marois

Fall Soil Test

Soil tests taken immediately after harvest of
crops in the fall can be used to determine
fertility requirements as nematode levels to
develop a plan of action for the coming year.
If soil pH needs adjusting, fall is a good time
of the year to apply needed lime since it
takes several months to change pH. Most of
the fertilizer should still be applied in the
spring prior to planting the crop. Knowing
nematode levels can also help growers to
determine what crop should be planted in
certain fields as well as what variety or
nematicides should be used. This may make
the difference between making a good yield
and losing money on the crop.

David Wright

The use of trade names does not constitute a guarantee or warrant of products named and does not signify approval to the exclusion of similar
Prepared by: J.M. Bennett, Chairman; J.A. Ferrell, Extension Agronomist ii, ii ,i l ,l ...hI F.M. Fishel, Pesticide Coordinator
I.I. i, i ,,11 ..,, C.R. Rainbolt, Extension Agronomist .11 ,1...11, .I i ,,l ..ih B.A. Sellers, Extension Agronomist
ii i., ii1, ,,1-i .i D.L. Wright, Extension Agronomist (dlw@ifas.ufl.edu).