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Weed control for
herbicide resistance
H
erbicide resistance is a well-known
occurrence in many crops in South
Africa. Since the first confirmed
case of herbicide resistant wild
oats in 1986, various cases of herbicide re
sistance have been confirmed annually.
High levels of weed control are necessary to
obtain the maximum yield possible. Accord
ing to literature, the presence of weeds in a
wheat field can reduce wheat yield with up
to 33%. This high percentage reduction in
yield will cause economic losses.
Various publications focussed on the con
trol/management of herbicide resistance,
but herbicide resistance seems to be ever
increasing. One of the major reasons for the
development/increase in herbicide resist
ance is the limited availability of herbicides
from different mode of action groups in
South Africa. This leads to the repeated use
of herbicides from the same mode of action
groups, as well as unregistered mixtures
being used.
During a visit from an Australian study
group, an Australian extension officer made
the comment that they are starting to re-
introduce ploughing into their management
systems. This may come as a shock to pro
ducers in South Africa who are mainly prac
tising conservation/no-till agriculture. The
truth however, is that the Australian produc
ers also practise CA, but the herbicide re
sistant weeds have become so problematic,
that they needed to consider other control
measures.
According to Dr Michael Renton (senior
lecturer, University of Western Australia),
mouldboard ploughing is the best way to
control weeds. He suggests that it should
not be done every year, as it will lead to
other environmental problems. Moreover,
it is expensive and time consuming. He
ran numerous simulations and found that
mouldboard ploughing once every four
years was a good option for weed control.
He found that even once in every eight
years still had some benefits. Occasional
mouldboard ploughing can be used to bury
herbicide resistant weed seeds, effectively
removing them from the gene pool.
Some more research done in Western
Australia found that inversion ploughing
can also be used with success. Inversion
ploughing is used to fully invert the soil to
ensure weed seeds that were on or just be
low the soil surface are placed at a depth
where they cannot germinate. This can
be practised every eight to ten years, with
conservation tillage used in the intervening
years.
In Western Australia, annual ryegrass seeds
failed to establish and eventually died when
soil was fully inverted to a depth greater
than 20 cm using a specialist mouldboard
plough fitted with skimmers. This single soil
inversion event reduced annual ryegrass
numbers by more than 95% at Katanning
and Beverley, Western Australia, for a pe
riod of two years (
www.agric.wa.gov.au
).
Other methods of
controlling weeds
Resistance can be prevented or slowed
down by keeping to the following instruc
tions:
Rotate your crops if possible, as this
will help suppress weeds through allel
opathy (i.e., a biological phenomenon
by which an organism produces one or
more biochemicals that influence the
germination, growth, survival, and re
production of other organisms).
Herbicides must be rotated so that the
same herbicide is not used year after
year.
Implement crop rotation if possible.
Crop rotation allows for using different
herbicides on different weeds in the dif
ferent crops that are planted. Herbicides
with different modes of action can be
utilised.
Never make unregistered mixtures.
Stick to label recommendations at all
times.
Keep records of herbicides used and
also the rates at which the herbicides
were applied.
Regularly monitor your crops so that re
sistant patches can be observed in time
to be controlled with more specific in
terventions, like spot spraying or physi
cal removal.
Apply the herbicide at the correct leaf
stage of the weed and the crop.
Make certain that the correct rate is al
ways used, because rates that are too
high or too low can select for herbicide
resistance.
Make sure that the spray equipment is
correctly calibrated and cleaned thor
oughly and that the person in charge of
the spraying knows what he/she is do
ing. This helps to prevent or reduce the
introduction of weed rhizomes, stolons
and seeds into the fields.
Always follow the instructions of the
herbicide label.
Conclusion
Producers must realise that the herbicide
resistance challenge in South Africa is not
getting better and that they will have to
adapt to make sure that they can keep on
farming. Research concludes that produc
ers will have to adapt and that part of that
adaptation may include a return to mechani
cal ploughing.
For further information on the topic, feel
free to contact the author at
058 307 3420 or
deweth@
arc.agric.za
.
References
https://www.agric.wa.gov.au/grains-research-devel-
opment/crop-weeds-reduce-weed-seed-numbers-
soil?page=0%2C3
(Accessed: Friday, 20 July 2018).
http://www.abc.net.au/news/rural/2015-08-12/
herbicide-resistance- could-bring-back-the-
plough/6690656
(Accessed: Friday, 20 July 2018).
https://phys.org/news/2015-08-weed-problems-
occasional-plough.html#jCp
(Accessed: Friday,
20 July 2018)
Focus
Integrated pest control
Hestia Nienaber,
ARC-Small Grain, Bethlehem
Grain SA/Sasol photo competition