Herbicide impact
Cropping
Herbicide link to low legume nitrogen fixation
Acetolactate synthase (ALS) herbicides (such as Broadstrike, Glean and Spinnaker) are widely used by farmers
throughout Australia in cereal and legume crops and pastures. This article outlines the results of preliminary research,
which shows ALS herbicides could have a negative impact on legumes.
by
Steve Rogers and
Jeff Baldock,
FA R M I N G A H E A D
No. 134
February 2003
FIGURE 1 Influence of ALS herbicides on chickpea nitrogen fixation
A
2000
Shoot dry matter
Amount of N fixed
1600
20
1200
15
800
10
400
5
0
0
5
10
20
Residual level of Glean
(% of recommended application rate)
0
B
2400
25
Shoot dry matter
Amount of N fixed
18
2000
15
1600
12
1200
9
800
6
400
3
0
0
S
B
(F ro
lu ad
m st
et rik
su e
la
m
)
(Im S p
i
az n n a
et k e
ha r
py
r)
• Additional nitrogen fertiliser may be
needed to ensure an adequate
supply for cereal crops grown after
legumes treated with ALS
herbicides.
Herbicide impact on productivity
If weeds are not controlled, they reduce
crop yields, hinder harvest operations and
contaminate produce.
ALS herbicides are widely used throughout
Australia and are an important tool in onfarm weed management.
Amount of nitrogen fixed (kg N/ha)
• Research shows that common ALS
herbicides such as Glean,
Broadstrike and Spinnaker can
reduce the amount of nitrogen fixed
by chickpea crops by up to
70 per cent.
Where legume growth and biological
nitrogen fixation are sufficient, biologically
fixed nitrogen can more than offset the
nitrogen removed from paddocks in the grain
of a subsequent wheat crop.
Amount of N fixed (kg/ha)
• Preliminary CSIRO research
indicates acetolactate synthase
(ALS) herbicides can have a
negative impact on the growth
and nitrogen-fixing ability of
legume crops.
The remaining 75% of the nitrogen in the
legume residues enters the soil organic
nitrogen pool and replaces soil-derived
nitrogen, which is mineralised and taken
up by crops and pastures during the
growing season.
Using legumes in crop rotations also has
other benefits including improved soil
structure, particularly when legume residues
are incorporated into the soil, breaking
disease and cereal pest cycles and increased
soil microbial activity following the addition
of legume residues.
AL
At a glance
biological fixation can impact on the growth
and productivity of the legume and the
subsequent cereal crop.
No
Legume benefits in crop rotations
One of the important roles of legumes in
crop rotations is to provide an inexpensive
source of nitrogen, which reduces reliance on
nitrogen fertiliser during production of the
legume and subsequent cereal crops.
Cereal crops can use up to 25% of the
nitrogen returned to the soil in preceding
pasture or grain legume residues.
Since a significant proportion of legume
residue nitrogen is biologically fixed, this
New CSIRO research shows acetolactate synthase (ALS) herbicides such as Glean, Broadstrike and
Spinnaker can significantly affect the nitrogen-fixing ability of legumes. The picture shows a comparison of
chickpea plants with Glean (chlorsulfuron) present during rhizobial growth, seed germination and in the soil
(right) with chickpea plants grown in the absence of Glean (left), six weeks after planting. The chlorsulfuron
concentration in the soil is equivalent to 10 per cent of the recommended agricultural application rate.
Chickpea shoot dry matter (kg/ha)
pplications or residues of acetolactate
synthase (ALS) herbicides can reduce
the growth and nitrogen-fixing ability of
legumes, according to preliminary research.
CSIRO Land and Water trials indicate that
common ALS herbicides such as Spinnaker,
Broadstrike and Glean can reduce the
amount of biologically fixed nitrogen in
chickpeas by up to 70 per cent.
The results showed the herbicides
slowed chickpea growth and reduced grain
yield. This was particularly evident in
alkaline soils.
Reduced productivity has also been
observed for field peas, medic, subclover
and lucerne.
Shoot dry matter (kg/ha)
A
CSIRO Land and Water
CSIRO
Type of ALS herbicide applied
Note: These figures demonstrate the influence of residual levels of Glean (a) and in-crop applications
of Broadstrike and Spinnaker (b) on chickpea shoot dry matter and the amount of nitrogen fixed
at harvest.
Source: CSIRO Land and Water.
39
Herbicide impact
Chickpea field trial results
Paddock trials assessed the influence of
residual levels of Glean (chlorsulfuron)
applied in the preceding growing season and
the influence of in-crop application of
Broadstrike (flumetsulam) and Spinnaker
(imazethapyr) to chickpeas.
Residue levels of Glean were set to
0, 5, 10 and 20% of the recommended
application rate to simulate a range of residue
carry-over levels.
The results showed chickpea dry matter
(shoot) and amount of nitrogen fixed at
maturity declined progressively with
increasing levels of residual Glean
(see Figure 1, page 39). In-crop application of
Broadstrike and Spinnaker also reduced
chickpea dry matter and amounts of nitrogen
fixed at maturity (see Figure 1).
Impact on chickpea nodulation
Laboratory studies investigated the reasons
for poor nitrogen fixation in ALS-treated
chickpeas.
Possible reasons include a direct effect of
the ALS herbicide on the growth of the plant
or rhizobia; an impact on the ability to form
or sustain nodules; and a reduction in the
activity of rhizobia in nodules.
Rhizobia were grown in the presence and
absence of chlorsulfuron and imazethapyr in
a preliminary experiment.
The presence of either herbicide did not
alter the rhizobia growth rate, suggesting a
direct effect of the herbicides on rhizobial
growth was unlikely to account for the
reduced yield and nitrogen fixation observed
in the paddock.
The influence of Glean (chlorsulfuron)
and Spinnaker (imazethapyr) on the
formation of nodules was studied in a second
laboratory trial.
40
Number of nodules per plant
FIGURE 2 Chickpea nodulation
30
25
20
15
10
5
ex N
p o ot
se
d
epr
ex
po
se
d
0
e-
Three different types of ALS herbicides
exist including sulfonylureas (Glean, Ally,
Logran), imidazolinones (Spinnaker) and
sulfonamides (Broadstrike).
Sulfonylureas are used for controlling
broadleaf weeds and some grasses in
cereal crops, while imidazolinones and
sulfonamides are used for weed control in
some legume crops and pastures.
But reductions in grain and pasture legume
productivity and nitrogen fixation have
been observed following the application of
ALS inhibiting herbicides or when ALS
residues are present following application to
previous crops.
These herbicides possibly could be slowing
the symbiotic nitrogen fixation of legume
crops or pastures.
CSIRO Land and Water investigated the
effects of ALS herbicides on the growth
and production of chickpea and medic and
the nodulation and nitrogen fixation
of chickpea.
Pr
Cropping
Rhizobium inoculant herbicide pre-treatment
Note: The figure shows the number of nodules
on chickpea plants inoculated with rhizobia
pre-exposed to Glean before inoculation, with
no other herbicide present in the soil. Similar
results were observed for Spinnaker.
Source: CSIRO Land and Water.
Results showed the presence of both Glean
and Spinnaker in the soil reduced the
nodulation of chickpea plants by 90%.
But the presence of the herbicides at
germination had little effect on nodulation.
When rhizobia were grown in the presence
of the herbicides before inoculation, fewer
nodules formed on chickpea plants (about
50% less) for both herbicides in the absence
of herbicide in the soil or at germination.
The results for Glean are shown in
Figure 2. This result has not been reported
previously and highlights that reductions in
nodulation and nitrogen fixation may not be
related solely to reduced growth of the host
legume plant.
The results suggest the presence of Glean
or Spinnaker in the soil could interfere with
the rhizobia nodule formation process but
further CSIRO Land and Water studies are
needed to confirm this finding.
Other research has shown the influence of
ALS herbicides on legumes is not confined to
chickpeas but may occur in a variety of
common grain and pasture legumes.
Recommendations
The results have major implications for the
use of ALS herbicides in farming systems.
In-crop application of ALS herbicides or the
presence of ALS herbicide residues from
previous applications have the potential to
reduce the nitrogen benefits associated with
using legumes in crop rotations.
The negative effects can potentially occur
on all soil types but residual herbicide
impacts may be enhanced on alkaline soils.
The reduction in nitrogen fixation
potentially limits yield and protein content of
grain from legumes as well as subsequent
cereal crops.
Decreased input of nitrogen to the soil
organic pool could also occur.
The extent that the use of ALS herbicides
will impact on agricultural systems depends
on the reason why legumes are being grown.
Where legumes are being used to provide
a disease break or an alternative weed
management strategy, the impact of ALS
herbicides will be minor as these objectives
can still be achieved regardless of the ability of
the legumes to fix nitrogen.
But where legumes are used to provide an
inexpensive nitrogen source, be aware of the
potential reduction in nitrogen fixation
associated with applying ALS herbicides.
Additional nitrogen beyond that usually
applied could be required to ensure an
adequate supply to cereal crops when
they follow legumes treated with ALS
herbicides.
Further studies
If the inputs of biologically fixed nitrogen
are to be maintained in crop rotations, ALS
herbicide applications need to have a minimal
impact on symbiotic nitrogen fixation and
the return of nitrogen to the soil and
plant system.
A new CSIRO Land and Water project is
under way to quantify the influence of ALS
herbicides on growth and nitrogen fixation in
legumes commonly planted across southern
Australia. This includes medic, subclover,
lucerne, lupin, faba beans, field peas
and vetch.
The assessment will be carried out in a
glasshouse trial where plants will be exposed
to ALS herbicides at the manufacturer’s
recommended rates.
Measurements will include the extent of
nodulation, symbiotic nitrogen fixation, plant
dry matter accumulation and leaf area.
ALS herbicides may directly or indirectly
affect nitrogen fixation. Using molecular
methods, the impact of ALS herbicides on
microbial nitrogen fixation mechanisms will
be assessed.
The project also aims to improve
understanding of how ALS herbicides affect
the symbiotic relationship between rhizobia
and their host legumes. It is hoped the
research will increase farmers’ awareness of
the potential losses in nitrogen fixation
associated with the use of ALS herbicides.
This could result in more strategic use of
ALS herbicides.
Acknowledgements: The chickpea ALS
herbicide studies were supported by a
Co-operative Research Centre for Weed
Management PhD scholarship awarded to
Annette Anderson. The current CSIRO project is
supported by the Grains Research and
Development Corporation.
For more
information contact Steve Rogers,
CSIRO Land and Water, by email
on [email protected], phone
(08) 8303 8407 or fax (08) 8303 8550.
FA R M I N G A H E A D
No. 134
February 2003