Spring Weed Management
‘Stopping seed set’
Peter Newman, Alex Douglas and Sally Peltzer
Weeds Research Officers, Geraldton, Katanning and Albany
Welcome to the third state wide edition of Weed Management. This edition focuses on stopping weed set in
spring. The articles that follow focus on spray topping pasture, crop topping and computer simulation to see
what difference it makes in the long run.
Crop topping
Peter Newman, Weeds Research Officer, Geraldton
Stopping weed seed set in crop just prior to harvest is a key IWM tool that leading growers are using to keep
their weed seed banks low. For some, crop topping is used to stop a weed “blow out” in its tracks. For others,
who have been practicing a fully integrated weed management package for many years, crop topping is
something that is placed in the budget every year and is seen as just another tool to keep a paddock in crop.
Lupins are currently the main crop that are crop topped in W.A..
Croptopping is the knockdown applied six months in advance
A grower in the Mullewa area thinks of crop topping lupins as a means to be able to dry seed some wheat the
following year. Last year his lupins didn’t really need crop topping as they were relatively clean but he went
ahead and did it anyway. His ryegrass numbers are now so low that this allowed him to dry seed some wheat
this year without any knockdown herbicide. So, in his words, “crop topping lupins last year was really just
applying my knockdown well in advance”.
Kitchen sink approach can fix a blow out in one year
At a field walk near Mingenew recently a chemical company had chosen a site for a pre-emergent ryegrass
herbicide trial. The grower explained that last year there were so many ryegrass in that area that he could not
see his lupin crop. So he crop topped the lupins, burnt the windrows, and the paddock was knocked down
three times prior to seeding wheat this year. The end result was that the nil herbicide plots in the trial this year
had just 10 ryegrass / m2. Plots sprayed with pre-emergent herbicide had 1 to 5 ryegrass /m2.
Glyphosate in wheat
Some glyphosate labels allow for glyphosate to be applied to wheat from the time the grain has 28% moisture
onwards. This is very late in the crop stage (ie. near maturity) so the weed seed set control may be quite
limited. This practice will also put undue pressure on glyphosate. However, there may be some situations
where it is warranted.
Radish pod drop – should I crop top if I have radish and ryegrass together?
Lupin crops that contain both ryegrass and wild radish may benefit from crop topping. However, crop topping
wild radish can dry out the wild radish pods, causing them to drop off the plant. This limits the ability to
manage the weed seeds by windrow burning or chaff cart collection. This warrants the question, “If I have
both of these weeds is it better to crop top or manage the weed seeds at harvest?” If the wild radish pods are
very small (ie. flowers to pencil lead thickness) at the time of crop topping then crop topping will do a fantastic
job and not cause any pod to fall to the ground. If the wild radish is loaded with mature pods at crop topping
timing then it is likely that crop topping will not be effective on the wild radish seed and will reduce the ability to
place these pods in a windrow or chaff cart. This is where it really comes down to the judgement by the
grower / agronomist as to which will end up with the greatest total seed set reduction.
Croptopping in a nutshell
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Ideal timing is to crop top lupins that are 80%+ leaf drop
Annual ryegrass should be flowering to soft dough for best results. .
Apply 800mL/ha paraquat by ground rig at a minimum of 70 L water / ha (fine to med droplets).
Wild radish control depends on the pod stage at the time of spraying. Wild radish seeds that contain a
dark green embryo in the middle are viable and will not be affected by herbicide. .
Paraquat is registered for crop-topping of Chickpeas, Faba Beans, Field Peas, Lentils, Lupins and Vetch at
400 to 800 mL/ha.
Should we spray-top in a dry season?
Alex Douglas, DAFWA Katanning
As 2010 has had a very dry start in many areas it is likely that this year most of the weed control in pastures
will be deferred until spring. Spray-topping may be the only weed control some pastures get this season.
Waiting until spring will enable a full germination to occur, maximising the amount of pasture available for
stock utilization.
If you must restrict the number of paddocks that are spray-topped this season it is most important to target
those paddocks that will be cropped next year. This may make the difference between seeding a paddock dry
or needing to wait for a germination to be controlled by a knockdown. It is also a good idea to target paddocks
with large populations of grasses that can be difficult to control in crops, eg. barley and brome grass.
To manage herbicide resistant or dense populations of weeds, controlling weed seed set is a high priority.
Neglecting weed control could mean that this is the year that ‘blow’s out’ the soil seed bank and sets back your
weed management program by several years.
What to use
The best time to spray-top is when the grasses are flowering, however treatment can be made anywhere from
seed head emergence to the dough stage of the seed. When you spray-top will depend partly on the herbicide
that is chosen.
Glyphosate often gives better results if applied from head emergence until flowering, while paraquat may be
applied as late as the dough stage. The different application windows are due to the different modes of action
of the two herbicides.
Herbicide
Operation
Diquat/paraquat
Spray-topping
(115/135 g/L)
Grass seed set
Paraquat (250 g/L)
Spray-topping
Grasses generally
annual ryegrass
Glyphosate (450 g/L)
Rate/ha
800 mL/ha
400 mL/ha
– particularly
Pasture topping for annul grass and
capeweed seed set reduction
240 – 360 mL/ha - Barley grass, Brome
grass, Capeweed and Silvergrass
360 mL/ha – annual ryegrass
Pasture legumes
Research has demonstrated that sub-clover seed yield was reduced by up to 75% when spraytopping
coincided with flowering of the sub-clover. Try to avoid the period of early legume seed development, i.e. when
the seed is still soft. Waiting until the seed is firm will result in less damage.
For most pasture legumes a late spray-top application will be less damaging.
Attempt to delay the development of the grass so that you can spray-top when the legumes have finished
flowering. Do this by grazing the paddock closely for several weeks in spring.
What’s best to spray-top?
The best candidates for spray-topping are ryegrass and brome grass. Try to avoid spray-topping barley grass
and silver grass.
Barley grass has a long flowering period and may have several tillers at different stages of development on the
same plant. Silver grass flowers over a short period and the best time for herbicide application is easily
missed
Spray-topping will improve the nutritive value and palatability of the grass residue. This may mean that stock
will selectively graze sprayed areas. While this will improve the control of grass seeds it can also lead to a
higher soil erosion risk as feed is removed quickly and thoroughly from the pasture
Monitoring grazing after treatment will become imperative this summer as it is important to maintain some
ground cover over summer to prevent soil loss.
Stopping seed set and catching seed pays
Sally Peltzer, Weeds Research Officer, Albany
The Weed Seed Wizard is a computer simulation tool which models the biology of a range of cropping weeds.
It simulates important interactions between weather, paddock management and seed biology to predict the
numbers of seeds in the soil and their germination. It uses actual weather data and paddock management
events.
We used this new model to simulate annual ryegrass in a wheat / lupin / wheat rotation in Northam where a
post-emergent application of Hoegrass in the first year of wheat failed due to herbicide resistance (Scenario
1). We then added a crop-topping event in the second year sown to lupins (Scenario 2) to reduce the annual
ryegrass seed set. A seed-catcher was used in all three years in the final scenario (Scenario 3). We chose the
2002 to 2004 dates as these were average to above average years (The wizard must use real weather data).
Scenario 1 – wheat / lupin / wheat rotation. The wheat crops were sown after a double-knockdown and an
application of trifluralin (80% control) while the lupin rotation was sown with a pre-sowing application of
simazine and atrazine and a post-emergent application of clethodim (Select®) (90% control). A post-emergent
application of diclofop-methyl (Hoegrass®) in the first year of wheat failed due to herbicide resistance.
There were 500 annual ryegrass seeds/m2 in the seed bank after harvest in 2001.
2002
2003
2004
05 May
11 May
11 May
15 June
05 Dec
22 April
23 April
07 June
04 Dec
30 May
3 June
4 June
5 Dec
glyphosate + triasulfuron (eg. Logran®)
Spray.Seed® + trifluralin (eg. Treflan®)
Westonia wheat sown with knifepoints
diclofop-methyl (eg. Hoegrass®) FAILED due to resistance
Harvested with spreaders
simazine + atrazine
Tanjil lupins sown with knifepoints
clethodim (eg. Select®)
Harvested with spreaders
glyphosate + triasulfuron (eg. Logran®)
Spray.Seed® + trifluralin (eg. Treflan®)
Wyalkatchem wheat sown with knifepoints
Harvested with spreaders
The annual ryegrass density graph (on the left) depicts the annual ryegrass emergence from 2002 to 2004. In
2002, there were over 100 annual ryegrass plants that set seed. In 2003, over 2000 ryegrass plants emerged
in late April and May of which most were controlled by the applications of simazine, atrazine and clethodim.
Nearly 500 of these remained to set seed (lupins are poor competitors) and cause problems in the 2004 wheat
phase. This is reflected in the loss of potential yield shown in the graph on the right. The potential yield
represents the weed free yield. So in this scenario, the weeds in 2004 reduced wheat yield by about 2.2 t/ha.
Scenario 2 – The same wheat /lupin / wheat rotation but with an addition of a crop-topping event in the lupin
phase. (Note: the crop-topping controlled 90% of the annual ryegrass seed set but with a 10% lupin yield
penalty)
2003
4 October
crop-topping with gramoxone
The addition of a crop-topping event reduced the annual ryegrass numbers setting seed in 2003. This led to a
reduction in the number of plants emerging in 2004 which reduced yield loss in the 2004 wheat crop.
Scenario 3 – The same wheat /lupin / wheat rotation but with an addition of a crop-topping event in the lupin
phase and seed-catching at harvest in all three years. It is assumed that seed catching will remove 70% of the
annual ryegrass seeds that are set.
2002
05 Dec
Harvested with seed-catcher
2003
04 Dec
Harvested with seed-catcher
2004
05 Dec
Harvested with seed-catcher
The addition of both the crop-topping and seed-catching reduces the annual ryegrass to long-term
manageable levels and reduces the yield loss substantially. This paddock can stay in a cropping rotation
and the potential yield of the crop can be realised.