cotton - Greenmount Press

cultivation. Cultivation has been seen as a no-no to the zerotillage
purists, but is now becoming the fall-back position for
many when weeds such as feathertop Rhodes grass become
dominant. Nobody wants to see a return to the old days of fully
cultivated fallows, but the regular use of a strategic cultivation
can be very valuable and will prolong the effective life of the
herbicides. Cultivation is probably the most effective tool for
managing tap-rooted and perennial weeds such as fleabane and
Australian bindweed once they are well established. Cultivation
can also be the most effective option for a double-knock when
weeds become stressed following the first herbicide. A doubleknock
with a herbicide followed by cultivation will control most
glyphosate resistant, tap-rooted and perennial weeds.
Alternative crops
As is the case in fallows, there are also many alternative
herbicides available for use in other crops, but many of these
have the same issues of potential for resistance or prolonged
plant-back periods to cotton.
Cotton growers need to be cautious of using any residual
herbicides in a rotation crop or fallow, even if past experience
has not indicated problems with the herbicide, as a lack of
damage symptoms from a residual herbicide does not necessarily
show that the crop has not been damaged. Work I undertook
with atrazine, for example, showed a 15 per cent yield loss
from atrazine without the crop displaying any obvious damage
symptoms. Smaller yield losses of five or 10 per cent could easily
be caused by herbicide residues without the crop showing any
herbicide damage symptoms. Such losses may seem small but
can add up to a lot of money over a field and a farm. Even
some of the ‘non-residual’ herbicides can have soil activity and a
surprisingly long half-life in the soil.
A management strategy
The primary aim of weed management is to drive down the
weed seed-bank, reducing the competition for crops and the
pressure on herbicides.
Cotton growers need to make use of all their oportunities to
achieve this. Growers need to ensure that in cotton they:
■ Enter the cropping phase with low weed numbers;
■ Use residual herbicides as necessary to reduce the pressure on
in-crop glyphosate applications; and,
■ Always undertake a post-spray audit after every glyphosate
and control any survivors using an alternative tool before they
set seed.
Fields expected to have moderate or high weed numbers
should have residual herbicides applied to manage these weeds
before the problems occur.
In fallows, growers should maintain stubble cover as long as
possible, but:
■ Should not rely on a single herbicide strategy to manage
weeds;
■ Should include a residual herbicide in the system to reduce the
selection pressure on the contact herbicides (provided there
are no plant-back issues with the residual herbicide);
■ Should ensure any survivors are managed using an alternative
tool before they set seed; and,
■ Should use strategic cultivation if necessary to manage
perennial and taprooted weeds and remove stressed survivors
before they set seed.
Allowing weeds to get away in a rotation crop will contribute
to problems in the following fallow, requiring additional inputs
later in the system.
Cotton growers need to think about their expected weed
issues and available herbicides before they consider growing
rotation crops. Some herbicides that appear to be ideal for a
rotation crop may not fit into a cropping system that includes
cotton. Also, the use of some cotton herbicides may damage the
following rotation crop.
In order to manage glyphosate resistant and tolerant weeds,
cotton growers need to be more proactive in their weed
management in all components of their cropping system. They
need to use Roundup Ready Flex cotton not as a ‘cheap’ option
for glyphosate, but as an opportunity to reduce the weed
pressure in their farming system by including residual herbicides
as necessary, regularly monitoring weed pressure and responding
aggressively to ensure weed numbers are driven down.
This season has seen some bad examples of poor field preparation with weeds not adequately managed before planting to
cotton. Much of the sowthistle and fleabane in this field, for example, has survived the first in - crop glyphosate, is setting seed
and is now difficult and expensive to manage.
46 — The Australian Cottongrower December 2012–January 2013