| The control of volunteer
seedlings before planting cotton in spring is usually done as part of
a knockdown program. While Roundup is not labeled for control of conventional
volunteer cotton, there have been many mixtures applied that included
Roundup, targeting a range of weed species.
The challenge for some growers was controlling Roundup Ready volunteers
before the emergence of the new crop. The degree of this challenge is
often related to the amount of rain over winter and the number of workings
that a field received. The plant-back period of many potential control
products is another limitation which reduces the options for growers.
This article looks at three years of Monsanto data. The size of the
volunteer seedlings ranged from just emerged through to the 8-leaf stage.
What did we find?
A number of chemical options gave commercially acceptable control (above
85 per cent). The economics of the respective products was not in the
scope of the trials — the focus was identifying potential control
options.
Flumioxazin across a number of rates provided very high commercial control.
While we did see some phytotoxicity at the higher rates evaluated, this
was not evident at lower rates.
Bromicide 200 provided good commercial control at the rates trialed.
The lower rates (750mls per hectare and one litre per hectare) worked
well on smaller seedlings but struggled on larger seedlings. The higher
rate of Bromicide (1.5 litres per hectare) provided robust control.
Basta applied at three and four litres per hectare gave consistent commercial
control.
The higher rate of Hammer provided very good commercial control. The
lower rates were not as robust and consistent. There was also some trash
coverage on some replications that impeded control.
Sprayseed 250 applied at a rate of 0.8 litres per hectare provided commercial
control on smaller seedlings. When applied at 1.6 litres per hectare,
good consistent commercial control was observed. The addition of Diuron
to Sprayseed did not have a significant effect on final control. Indeed,
at the higher rate of Sprayseed, there was some drop off in the level
of control.
Amitrole T at five litres per hectare also provided commercial control.
Moderate levels of control were observed with four litres per hectare
of Amitrole T, 500 mls per hectare and 750 mls per hectare of Striker,
and both 375 and 750 mls per hectare of Starane.
Poor control of seedling volunteers was observed with three, four and
five litres per hectare of Daconate.
Get them early
Volunteers, whether they are conventional or Roundup Ready plants are
best controlled when they are small. Control past the six leaf stage
becomes far more unpredictable because of the mode of action of most
pre emergence herbicides, and requires considerably greater coverage
to achieve commercial control.
Consideration needs to given to trash levels and soil clods when making
decisions on controlling volunteers. Both can impede coverage and efficacy.
There is a range of products that can potentially control volunteer
cotton. There should be at least two more chemical options for volunteer
control in the spring of 2003.
Interrow cultivation can eradicate volunteer numbers from shoulder to
shoulder when timed correctly.
Risk assessment checklist
A subjective assessment of whether volunteers will be an issue can be
made at the end of August by asking a series of questions;
• Has the winter been dry? Volunteers will germinate over
winter, more so in a mild, wet winter.
• How many passes have I put over the field in the winter?
This will help in reducing volunteer numbers.
• Is there a lot of lint and seed in/on the beds?
• Is the field back-to-back or fallow?
• Am I going to water up or pre water? Monsanto recommends
only watering up fallow fields where volunteers are thought not to be
an issue. Pre irrigation is an effective means of germinating and eradicating
volunteers.
Controlling Ratoon or Stub Cotton
Cotton is a perennial plant, commercially grown as an annual plant.
Once the fully mature fruit has been harvested, the plant moves from
a physiological hiatus in reproductive growth, to grow vegetatively
initially, then reproductively again. This growth is known as ratoon
growth. This occurs because of the huge root mass present (from the
previous crop) producing photosynthate and the plant having no ‘sink’
or area to deposit this photosynthate.
These ratoon plants are inherently difficult to control chemically due
to their large root mass and the lack of non-persistent herbicide options.
Monsanto established replicated trials in the 2002–03 season in
NSW and Queensland under permit to evaluate several herbicide options.
The ratoon cotton plants in this trial had regrowth up to 10–20
centimetres in length at the time of treatment. There was some mechanical
damage to the plants that could be attributed to the planting process
at one site, but the plants were actively growing at the time of application.
Application volumes were 75 litres per hectare and the spray pressure
was 200 kpa. Sicot 189rr and Sicala V2rr were the two varieties evaluated.
Brownout (Figure 2) is very different to control (Figure 3), and in
these trials commercially acceptable brownout or control was not achieved
with any of the products evaluated.
Those herbicides that are contact inhibitors of photosynthesis are really
limited in their effectiveness. The huge resources provided by the existing
root mass cannot be influenced by this mode of action.
The suppressing effect of Starane is typical of what is seen in commercial
experience. They do offer some control of regrowth until cultivation.
Recommendations
Ratoon plants are notoriously hard to control due to the large root
mass they have accumulated in their first crop. Commonly these ratoon
plants are controlled by being ‘root cut’, and/or ‘stalk
pulled’.
During these operations there is always the opportunity for plants to
be half cut or pulled leaving them to grow when they receive adequate
moisture. It is these plants that growers find hard to control at a
later date.
Limited choices
These plants can only normally be controlled chemically by a residual
herbicide which may have an extended plant-back period. These herbicides
act predominantly via root uptake, hence their effectiveness. This method
is not favoured due to the uncertain effect of the persistent herbicide
on subsequent crop germination and growth.
Other alternate chemistries like the photosynthetic inhibitors are really
the only choices left to the farmer.
Monsanto Australia has conducted trial work this past season demonstrating
the ineffectiveness of these shoot uptake herbicides in controlling
ratoons. Indeed Roundup is quite ineffective at anything other than
minimal suppression of non- Roundup Ready cotton growth.
The recommendation is to use a phenoxy based herbicide to suppress growth
of the ratoons, until a supplementary cultivation pass is undertaken.
Experience has shown that a very harsh frost has a good chance of killing
ratoon plants over the winter period, before the next spring planting.
This in combination with a low persistence phenoxy herbicide and a cultivation
is the best way to control ratoon cotton, whether they be transgenic
or non transgenic.
Always consult the respective labels and be aware of the plant back
issues and soil persistence.
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