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2-Plants: RR-cotton needs two sprays "to knock out resilient weed"

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TITLE:  Managing Roundup Ready(R) crops
SOURCE: Cottonworld, Australia, by Donald Turner
DATE:   November 28, 2001

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Managing Roundup Ready(R) crops

GROWERS looking to maximise the benefits of Roundup Ready(R) varieties in 
early-planted crops would have already sprayed first applications of 
Roundup herbicide over the top. They will maximise the benefits by getting 
a second application on before the four true-leaf stage, to knock out 
resilient weeds.

Management tips up to four true-leaf stage:

Dr Grant Roberts of the Australian Cotton CRC, Narrabri specialises in 
management of cotton varieties with Roundup tolerance. He reports 
"excellent situations" where growers have applied Roundup herbicide early, 
based on recent field trips.

Says Grant: "I am thinking particularly of one field with a serious nut 
grass problem, where Roundup Ready cotton was planted with plenty of 
moisture. Roundup was applied early and it has an excellent job.

"The grower in that situation will get a second spray on, because the crop 
is only just coming up to four true-leaf stage. At this stage, a lot of 
earlier planted crops are coming to four true-leaf.

"From now on, it is important to get an indication of how many plants in 
the metre are at four true-leaf stage. Once the fifth leaf unfurls -- at 
about the size of a 10-cent piece or a little larger -- then we must stop 
applications over the top because you run the risk of yield reductions no 
matter what rate of Roundup herbicide you use."

Roundup(R) label rates:

The label indicates a maximum of 1.5kg of Roundup herbicide over the top up 
to two nodes, followed by directed or shielded sprays.

Says Grant: "The plant is very tolerant vegetatively -- some growers report 
having sprayed Roundup over the top of volunteer plants and it hasn't 
affected them, as we would expect.

"But we do have to watch the reproductive components of the plant, so that 
in the period around flowering it is crucial not to get drift on flowering 

Herbicide on lower leaves:

"You don't want any drift if you can help it, but we do know the plant is 
vegetatively tolerant. This technology really has become a drift safety 
net, from this stage of the season.

"So we need to have shielded sprays set up and to be out in the field 
looking at wind and drift conditions on those particular days.

"In theory, we can look at direct sprays as long as we go to the base of 
the plant -- the woody component."

Late applications and yield reductions:

Grant Roberts notes there have not been any documented or reported cases of 
yield reductions in commercial crops due to applications of Roundup 
herbicide over the top after four true-leaf stage.

Says Grant: "In our own research we have seen yield reductions of 40% to 
50% when we went at flowering. That rate of yield reduction decreases the 
earlier you go.

"So the later you apply over the top, the greater the impact. You can see 
where Roundup takes fruit off in the zone for those fruiting nodes where it 
has been applied.

"In dryland situations I haven't recorded cuts in yield from late 
applications, but it's a risk you take. You don't want to take that risk 
with the way the cotton price is at the moment."

Interview by Cotton Seed Distributors extension and development team


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