SA Graan April 2016

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April 2016
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Target-site resistance:
Planting time is creeping closer
As the planting window for small
grains is upon us, it is important to
reflect on problems from previous
production seasons/years. Herbi-
cide resistance is still a worrisome fact in
especially the small grain production areas
of the Western Cape.
Many grass weeds may now be able to sur-
vive herbicide applications, which would
have been able to control them in previous
seasons. In the past, the biggest concern
was ryegrass (Lolium spp.), but more and
more weeds are reported as being trouble-
some to control.
The procurement and application of her-
bicides are still the biggest input cost in a
small grain producing system, especially in
no-till/conservation tillage systems, where
herbicides are largely depended upon.
Due to the limited availability of herbicides
from a vast array of different chemical
groups, repeated chemical use in following
seasons occurs way too often. Moreover,
two or three applications of different herbi-
cides at various dosages are often applied
in an attempt to eradicate resistant weeds.
In the end, this leads to resistance to herbi-
cides from several different mode of action
groups in one population, resulting in limit-
ing future chemical control options.
ARC-SGI can screen for
target-site resistance
There are several ways that a weed can
become resistant to herbicides. Two of
these are non-target site resistance and tar-
get-site resistance. Sequestration and me-
tabolisation of herbicides are examples of
non-target site resistance. Target-site resist-
ance is when herbicides are unable to bind
to the target-site it was intended for and
are, therefore, unable to inhibit that specific
enzymatic/biochemical pathway and the
plant survives.
Researchers from Australia and France
have developed mutation markers that can
be used to identify target-site mutations
in weed species. These markers are then
used to genotype specific resistant bio-
types for specific mutations.
Since 2013, the ARC-Small Grains Institute
(ARC-SGI) has been working on the opti-
misation of these markers for South Africa.
Through the screening of multiple samples,
three very distinct mutations have been
identified in South African ryegrass popu-
lations occurring in small grain producing
areas. These markers are the ALS muta-
tion markers 197 and 205, which confer
resistance to all group B herbicides (Su’s,
Triazolopyrimidines and Imidazolinones)
and ACCase mutation 2078, which confers
resistance to many Aryloxyphenoxy-pro-
pionates, all Cyclohexanediones (including
clethodim) and Phenylpyrazolines.
This means that all grass weed samples
that are received, get screened for these
three mutations first, but if none of the mu-
tations are present in the sample, the sample
is screened with all other mutation markers.
This is done to make sure that no mutation
is missed in the screening process.
Mutation markers can also distinguish be-
tween plants that are heterozygous (Aa)
(one copy – resistant allele) or homozygous
(AA) (two copies – resistant allele) resist-
ant for each specific target-site mutation.
Depending on which mutation is detected,
the heterozygous or homozygous state of
resistance can exponentially increase the
needed dosage of herbicide to achieve 50%
mortality. What is also important to remem-
ber is that resistant biotypes, on average,
require ten to 30 times higher herbicide dos-
ages than susceptible types.
With planting season approaching fast,
producers are asked to courier grass weed
samples to ARC-SGI to be screened for
target-site resistance. This service is cur-
rently free, but from later in 2016 screening
fees will be charged. Results will reach the
producer within approximately a week, so
that the necessary herbicide choices can
be made.
Resistance: Which herbi-
cides can the grass weeds
be screened for?
Currently, any grass weed samples can be
sent for resistance screening to herbicides
from the ACCase inhibitor (Group A) her-
bicides and ALS inhibitor (Group B) her-
bicides. ARC-SGI is also looking into the
screening of broadleaved weeds.
How to get your grass
weeds tested for resist-
ance?
Producers are welcome to send grass weed
seedlings or seeds to ARC-SGI, Bethlehem
(ARC-SGI, Blydskap Road S191, Bethlehem,
9700). Please make sure of the following:
Seedlings must be kept moist (prefer-
ably in Ziplock bags) (Photo 1) and must
preferably be couriered, as it will assure
that fresh seedlings arrive in Bethlehem.
Please indicate the GPS-co-ordinate
where the sample was taken.
Seeds must be stored in brown paper
bags (Photo 2) to prevent them from
rotting. GPS-co-ordinates must again
be indicated.
The number of seeds/seedlings must
be high enough to enable an accurate
screening process. Please make sure
that a number of samples of six differ-
ent plants from the problematic field
are sent in; one plant only will not be
adequately representative of the prob-
lematic field. All samples received are
screened in replicates of four to six
times.
Summary
Producers are urged to make use of this
service, as it will allow them to make in-
formed choices of the herbicides needed
for the spraying season. Also, remember
that resistance is able to spread from one
farm to another, meaning that although you
did not have resistance last year, it does
not guarantee no resistance this year.
By being able to make the right herbicide
choices, producers will be saving a vast
amount of time and money in the end.
For further information regarding this top-
ic or how to sample your weeds, please
contact Hestia Nienaber at 058 307 3420
or or
Scott Sydenham at syden-
ON FARM LEVEL Herbicide resistance / Application of herbicides Integrated pest control
HESTIA NIENABER and SCOTT SYDENHAM, ARC-Small Grain Institute, Bethlehem
1: Seedlings must be kept moist – preferably
in Ziplock bags.
2: Seeds must be stored in brown paper bags
to prevent them from rotting.
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