Residue of bensulfuron methyl in soil and rice following its pre- and post-emergence application

Abstract

Bensulfuron methyl (BSM) is applied in rice to control a wide range of weeds due to low application rate and high efficiency. A study was conducted to evaluate residues of BSM in soil and rice plant at different doses as pre- and post-emergence application in transplanted rice. The quick easy cheap effective rugged safe (QuEChERS) method was evaluated for BSM residue extraction from different matrices. The limit of detection and limit of quantification was 0.005 and 0.01 µg/g, respectively in soil and rice plant. The average BSM recovery of 91.1, 82.8, 84.5 and 88.7% was obtained from soil, rice straw, grain and husk, respectively. Though, BSM residue was detected (0.011 to 0.017 μg/g) in soil at high dose, it was below maximum residue limit (0.01 μg/g) in rice grain at both the doses of BSM. Hence, the study revealed that the BSM can be safely applied to rice at recommended doses for weed control. © 2016, Czech Academy of Agricultural Sciences. All rights reserved.

Full text

Rice (Oryza sativa L.) is the vital staple food for
about 50% of the world’s population that lives in
Asia. Hence, a worldwide research is continuing
to enhance the production and productivity of the
rice crop. A major production constraint in rice
cultivation is the lack of timely weed management
caused by the acute labour scarcity and high manual
weeding cost. Thus, the use of herbicides for weed
control is encouraged in rice production. Asia ac-
counts for vast majority of the global rice herbicide
market and the share continues to grow, since the
multinational agrochemical companies acquire the
advantage of a more liberal trading climate in Asia
(Naylor 1996). Although the use of herbicides has
increased the crop production to cope up with food
demand, there may be an unintentional exposure
of the ecosystem to herbicides residue. Hence, an
inappropriate and indiscriminate use of herbicides
in rice cultivation could cause environmental con-
tamination through longer persistence.
During last 20 years, low application rate her-
bicides viz., sulfonyl urea, sulfonamide, and imi-
dazolinone have been developed and registered
for weed control all over the world. They act by
inhibiting the action of acetolactate synthase, a
key plant enzyme essential for the synthesis of
branched-chain amino acids (Moberg and Cross
1990, Stidham 1991). Among the above classes,
sulfonyl ureas are registered largely for chemical
weed management in rice either as pre- or post-
emergence herbicide for controlling the grasses,
broad leaved weeds and sedges (Russel et al. 2002).
Bensulfuron methyl (BSM) (methyl 2-[(4,6-dimeth-
oxypyrimidin-2-yl) carbamoyl sulfamoyl methyl]
benzoate), one of the environmental-friendly low
dose herbicide belonging to sulfonyl urea is used
highly for weed control in rice. It is marketed
as Londax (single herbicide) and Londax Power
(combination with pretilachlor) in India.
Because of low application rate, its concentration
is expected to be low in water and soils. However,
it depends on the herbicide molecule and for-
mulation properties, application rate, and areas
or type of soils treated with this herbicide. The
solubility of BSM depends on pH of water and
has high solubility (880 mg/L) in water at pH 8.0
Residue of bensulfuron methyl in soil and rice following
its pre- and post-emergence application
P. Janaki1, C. Nithya2, D. Kalaiyarasi2, N. Sakthivel2, N.K. Prabhakaram2,
C. Chinnusamy2
1Soil Science and Agricultural Chemistry, Tamil Nadu Agricultural University,
Tamil Nadu, India
2Department of Agronomy, Tamil Nadu Agricultural University, Tamil Nadu, India
ABSTRACT
Bensulfuron methyl (BSM) is applied in rice to control a wide range of weeds due to low application rate and high
efficiency. A study was conducted to evaluate residues of BSM in soil and rice plant at different doses as pre- and
post-emergence application in transplanted rice. e quick easy cheap effective rugged safe (QuEChERS) method
was evaluated for BSM residue extraction from different matrices. e limit of detection and limit of quantifica-
tion was 0.005 and 0.01 µg/g, respectively in soil and rice plant. e average BSM recovery of 91.1, 82.8, 84.5 and
88.7% was obtained from soil, rice straw, grain and husk, respectively. ough, BSM residue was detected (0.011 to
0.017 g/g) in soil at high dose, it was below maximum residue limit (0.01 g/g) in rice grain at both the doses of
BSM. Hence, the study revealed that the BSM can be safely applied to rice at recommended doses for weed control.
Keywords: Oryza sativa L.; weed management; herbicide; contamination; persistence
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(25°C) and very low solubility (1.1 mg/L) at pH 5.0
(Roberts et al. 1998, Vencill 2002). It is highly stable
under slightly alkaline aqueous solutions (pH 8)
and degrades slowly under acidic conditions. It is
stable to direct photolysis in sterile buffer solu-
tions and degrades rapidly in natural water under
sunshine radiation by cleavage of the sulfonylu-
rea linkage to methyl 2-(sulfomethyl) benzoate
and (4,6-dimethoxypyrimidin2-yl) urea, with a
degradation half life (DT50) of 3–4 days (Roberts
et al. 1998). Bensulfuron methyl degrades also
through chemical hydrolysis and microbial pro-
cesses in moist soils and is classified as immobile
to moderately mobile depending on the soil organic
matter and pH (Roberts et al. 1998). The major
degradation product under aerobic metabolism is
CO2; however, under sterile conditions the non-
volatile compounds were also produced, namely
sulfonamide and pyrimidine amine [2-amino-4,6-
dimethoxypyrimidine]. This showed that the mi-
crobial degradation is not obligatory for BSM
degradation (CDFA 1989).
Since sulfonyl ureas act upon a specific plant
enzyme acetolactate synthase that is not found
in mammals or other animals, their toxicities to
animals is very low (Brown 1990). As they are very
active at low concentrations, residual phytotox-
icity of sulfonyl ureas to rotation crops such as
corn, sunflowers, sugar beets, and dry beans has
been already reported in literature (Anderson and
Humburg 1987, Curran et al. 1991). The rice plant
metabolizes the BSM rapidly with a DT50 of 4–6 h.
The crop selectivity is due to the slower rate of
translocation from roots to shoots and an increased
rate of metabolism in rice (Takeda et al. 1986).
The available literature on BSM is limited to bioef-
ficacy, laboratory dissipation studies (Langeland and
Lorache 1994) and very few under field conditions
(CDFA 1989, EFSA 2008). Similarly, the influence
of time of application on the persistence of BSM
has not been reported. In view of these facts, the
present study was undertaken to study the persis-
tence of BSM in soil and its terminal residues in
rice as influenced by the time of application under
subtropical arid agro-climatic conditions.
MATERIAL AND METHODS
Experimental details. Field experiments
were conducted during the kharif season (June
to September) of 2012 and 2013 at a wet land
farm of the Tamil Nadu Agricultural University,
Coimbatore with rice (cv. ADT 48) as a test
crop. As herbicide bensulfuron methyl (60%
dry flowable (DF)) was applied at two different
doses (100 and 200 g active ingredient (ai)/ha)
as pre- and post-emergence along with control
(no herbicide). The experiment was conducted in
randomized block design and the treatments were
replicated thrice. The pre- and post-emergence
application of bensulfuron methyl was done on 3rd
and 14th day after transplanting, respectively, using
flat fan nozzle and knapsack sprayer with the spray
volume of 400 L/ha. All the management practices
were followed as prescribed for rice cultivation
in the crop production guide. The experimental
farm is located at 77oE, 11oN latitude and 426 m
a.s.l. Experimental field soil was clay loam in tex-
ture and belongs to Typic Chromusterts group
and Noyyal soil series. The experimental field
soil was medium in organic carbon status (0.60%)
with the available nutrient status of low nitrogen
(226 kg/ha), medium phosphorus (18.3 kg/ha)
and high potassium (458 kg/ha) and has alkaline
soil reaction (8.27) and the electrical content of
below 0.43 dS/m.
One week after rice harvest, the green gram (Vigna
mungo) cv. CO 6 was grown in strips without dis-
turbing the layout and observation on germination
percent, plant height, number of pods per plant and
seed yield at harvest were recorded to assess the car-
ryover effect of BSM applied to the main crop (rice).
Collection of samples. e soil samples were col-
lected for residue analysis at the time of harvest from
0–15 cm depth. Five core soils were randomly taken
using soil auger from each treated and untreated plot
avoiding outer 20 cm fringes of plot. e core soils
were pooled from each plot, air dried, powdered and
sieved through 2 mm sieve and stored for analyses.
e paddy grain and straw samples were collected
from BSM treated and untreated plots at harvest.
Grains of rice were removed from husk and crushed
using a mechanical blender, while straw was chopped
into small pieces using knife.
Meteorogical conditions. Weather parameters
prevailing during the cropping period were record-
ed (Figure 1) at weekly intervals. It was observed
that the maximum and minimum temperatures
ranged from 18.4–33.3°C during kharif 2012 and
from 21.6–32.8°C during kharif 2013, respectively.
The total rainfall per day recorded ranged from
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6.1–165.2 mm during kharif 2012 and 0.6–1.8 mm
during kharif 2013.
Determination of bensulfuron methyl residues
in soil and plant samples. The BSM residue was
extracted by quick easy cheap effective rugged
safe (QuEChERS) technique (Janaki et al. 2015)
using 1% HOAc in MeCN + dichloromethane and
anhydrous MgSO4 and NaOAc · 3 H2O in vertex
mixer. An aliquot was cleaned up using the Bond
Elut C18 SPE cartridge with acetonitrile as eluent
and then concentrated in rotary vacuum evapora-
tor for LC-DAD analysis.
Validity of the method was tested by conducting
the recovery experiments using spiked samples of
soil and rice plant parts from the control treatment
with the known BSM standards concentration of
0.01, 0.05, 0.1 and 0.5 µg/g in three replicates.
After spiking, the residue of BSM was extracted
and cleaned up as described for samples above. The
concentration of BSM was determined by compar-
ing the peak area of the samples and calibration
curves of five levels of standards. A quantification
limit of 0.01 g/g was used for the calculation. The
blank soil and rice plant parts reference was used to
establish the limit of quantification. A calibration
curve was prepared by plotting concentrations of
BSM on x-axis against the average peak area on
y-axis (Figure 2).
Instrumentation. The BSM residues were de-
termined using Agilent HPLC (1200 series) with
Diode Array Detector (DAD) detector, binary pump
and auto sampler having rheodyne injection sys-
tem. The compound was separated using Agilent
Eclipse XDB-C 18, 5 µm, 4.6 × 150 mm column
kept in thermos-stated oven maintained at 25°C.
The instrument was connected to a computer that
recorded the response in terms of peak area and
height using the EZChrom software (USA). The
acetonitrile:water (50:50% v/v) with orthophos-
phoric acid (pH 3.0) was used as a mobile phase
for the separation of BSM with the flow rate of
0.5 mL/min. The injection volume of sample was
20 µL. Detection was performed at 234 nm for all
the standards and unknown samples.
RESULTS AND DISCUSSION
Recoveries and detection limit. Equations of
analytical calibration graphs, obtained by plotting
peak areas on the y axis against concentrations of
BSM on the x axis showed good linearity (Figure 2)
with the correlation coefficient of 0.979. The re-
tention time of BSM standards and samples was
4.50 ± 0.2 min (Figures 3 and 4) under the given
instrumental conditions of HPLC-DAD. The av-
0
20
40
60
80
100
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Weather parameters
Kharif 2012
0
5
10
15
20
25
30
35
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Weather parameters
Crop growing period (in weeks)
Kharif 2013
Maximum temperature (°C) Minimum temperature (°C)
Sunshine (h/day) total rain fall (mm)
Figure 1. Distribution of
weather parameters dur-
ing the growing period of
rice
0
20
40
60
80
100
12345678910 11 12 13 14 15 16
Weather parameters
Kharif 2012
0
5
10
15
20
25
30
35
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Weather parameters
Crop growing period (in weeks)
Kharif 2013
Maximum temperature (°C) Minimum temperature (°C)
Sunshine (h/day) total rain fall (mm)
0
20
40
60
80
100
12345678910 11 12 13 14 15 16
Weather parameters
Kharif 2012
0
5
10
15
20
25
30
35
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Weather parameters
Crop growing period (in weeks)
Kharif 2013
Maximum temperature (°C) Minimum temperature (°C)
Sunshine (h/day) total rain fall (mm)
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erage recovery of BSM obtained from soil, rice
straw, grain and husk was 91.1, 82.8, 84.5 and
88.7%, respectively. The results of the study are
presented in Table 1. The limit of detection and
quantification of BSM in all the matrices was found
to be 0.005 and 0.01 µg/g, respectively, with the
signal to noise ratio of 3:1. The recovery of BSM
from different matrices was found to be accept-
able up to fortification level of 0.01 g/g (Table 1)
and was also found satisfactory at different con-
centration levels. Niell et al. (2010) also reported
the BSM recovery of 91% from rice grain by modi-
fied QuEChERS extraction using 1% acetic acid in
acetonitrile, MgSO4 and sodium acetate.
Terminal residues of bensulfuron methyl in
rice grain, husk and straw. Irrespective of time
(pre- or post-emergence) and dose (100 or 200 g/ha)
of application, the BSM residue was below the quan-
tification limit of 0.01 µg/g in the rice straw, grain
and husk at the time of harvest in both years (Kharif
2012, 2013). This could be due to the selectivity
of rice crop to the BSM residue by slower rate of
translocation from roots to shoots and through an
increased rate of metabolism. Takeda et al. (1986)
reported that the rice shoots metabolized BSM with
a DT50 of 4–6 h while sensitive broad-leaved and
sedge weeds did not degrade BSM (DT50 > 50 h).
According to Priester (1985), the metabolism pro-
ceeds initially through -demethylation of the
pyrimidine ring to yield methyl α-(4-hydroxy-6-
methoxypyrimidin-2-ylcarbamoyl sulfamoyl)-o-
toluate and the hydrolysis of the parent methyl
ester to produce bensulfuron (Usui et al. 1993).
Similar results of less than 0.03 mg/kg of BSM
residue in rough rice, husk and rice straw were
also reported by Wu et al. (2000).
The residue of BSM in rice grain, husk and straw
was below the maximum residue limit (MRL) in
rice parts set by the EFSA (0.02 mg/kg) for the
European union (EFSA 2008), FSSAI (0.01 mg/kg)
for India (FSSAI 2015), FSCA (0.02 mg/kg in rice
and 0.05 mg/kg in rice bran) for Australia (FSCA
2014) and Japan (0.1 mg/kg) by Clever and Sato
(2011). Wei and Chen (1995) also reported a below
MRL residue of BSM (0.0116 g/g) in rice after 98
days of application. The present results suggest
that the normal rate (100 g/ha) of BSM applica-
tion is environmentally safe as its residues were
found below the MRL in rice grain and straw sets
by different agencies of various countries.
Terminal residues of bensulfuron methyl in
field soil. The sulfonyl ureas degrade in soil pri-
marily by the microbial or chemical metabolism
and conversion through photochemical occurs
only in the presence of UV light, which is a minor
decomposition mechanism (Singh et al. 2010).
The dissipation of sulfonyl ureas in soil is gener-
ally influenced by pH and other minor factors
are temperature, moisture and organic matter.
Though the residue of BSM was not detected in
the rice straw, husk and grain at doubled dose of
200 g/ha, its residue was detected in field soil at
harvest during kharif 2012 under both the pre-
and post-emergence application. It was below
y = 1E+06x + 51950
R² = 0.9796
0
200 000
400 000
600 000
800 000
1 000 000
1 200 000
1 400 000
1 600 000
0.0 0.2 0.4 0.6 0.8 1.0
Area (mAU)
Concentration of bensulfuron methyl standards
(mg/L)
Figure 2. Calibration of bensulfuron methyl standard
at concentration levels of 0.01–5.0 g/mL
Figure 3. Bensufuron methyl standard 0.01 µg/mL de-
tected by high-performance liquid chromatography
with diode-array detection (HPLC-DAD)
(min)
0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0
(mAU)
22
20
18
16
14
12
10
8
6
4
2
0
–2
–4
–6
–8
Bensulfuron methyl 4.527 28138
DAD: Signal A,
234 nm/
Bw: 4 nm
– Bensulfuron methyl
0.01 ppm
BSM std. dat
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the quantification limit of 0.01 µg/g during kharif
2013 (Table 2 and Figure 5). The quantity of BSM
residue detected in soil at post-emergence applica-
tion is higher (0.017 µg/g) than at pre-emergence
(0.011 µg/g) application. The presence of bensul-
furon residues in soil even at the time of harvest
suggests that being a weak acid (pKa of 5.2), it would
be relatively stable in the observed high pH of the
present experimental field soils (Langeland and
Lorache 1994) and hence it persists until harvest.
The presence of residue in soil during kharif 2013
below the detectable level might be the results of
variation in rainfall and other weather variables
(Figure 5). The amount of rainfall received on the
first month of bensulfuron application during kha-
rif 2012 was low when compared to kharif 2013.
This might have enhanced BSM sorption to soil
and reduced its leaching or runoff. Hence, it was
detected above the quantification limit in soil at
harvest. The enhanced dissipation of BSM residue
during kharif 2013 from soil could be the result
of enhanced hydrolysis and microbial degradation
by even distribution of rainfall throughout the
cropping period (Figure 1). Afyuni et al. (1997)
indicated that 1.1% to 2.3% of applied sulfonyl
ureas was lost in runoff during a simulated rainfall
event 24 h after herbicide application.
Bensulfuron methyl had medium organic carbon
normalized distribution coefficient (Koc) values
(205–567), which indicates that it is moderately
mobile (EFSA 2008) and more adsorption could
be anticipated for clay soils depending on the soil
organic matter and pH. The chemical hydrolysis
of BSM to ortho carbo methoxy group and further
microbial breakdown is the important route of its
degradation in soil (Langeland and Lorache 1994).
However, in the present study the possibility for
chemical hydrolysis is low due to the alkaline pH
of the field water (> 8.5) in which it is highly stable.
Hence in the present study, the BSM degradation
might have been enhanced by the increased sun-
shine radiation and day temperature independent
of pH as suggested by CDFA (1989) through the
formation of photo degradation products like beta-
lactic acid via the complete breakdown of phenyl
ring at low concentrations. Yordy (1987) reported
that in moist field soils, BSM has degraded via
chemical hydrolytic degradation and microbial pro-
cesses with a DT50 of 3–4 weeks by producing the
compounds like methyl 2-(aminosulfonyl methyl)
benzoate and 4,6-dimethoxy-2-aminopyrimidine
and [1H-2,3-benzothiazin-4(3H)-one 2,2-dioxide]
and CO2. Wei and Chen (1995) reported the half-life
of 28 days for bensulfuron in soil with the residue
Figure 4. Bensufuron methyl standard 1.0 µg/mL de-
tected by high-performance liquid chromatography
with diode-array detection (HPLC-DAD)
Table 1. Recovery (%) of bensulfuron methyl from soil, rice grain, husk and straw
Fortified concentration
(g/g) Soil Rice grain Rice husk Rice straw
0.01 79.0 ± 2.10 78.0 ± 1.19 87.0 ± 2.92 72.0 ± 2.87
0.05 91.6 ± 2.36 78.8 ± 2.45 89.2 ± 3.01 71.2 ± 4.17
0.10 97.4 ± 2.89 86.7 ± 1.79 87.4 ± 3.12 90.8 ± 3.16
0.50 96.2 ± 3.19 94.4 ± 2.91 91.0 ± 3.94 97.2 ± 4.27
Average recovery (%) 91.1 84.5 88.7 82.8
Mean of three replications; Values ± are % standard deviation
(min)
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
35
30
25
20
15
10
5
0
–5
–10
–15
DAD: Signal A,
234 nm/
Bw: 4 nm
– Bensulfuron methyl
1.0 ppm
BSM std. dat
(mAU)
Bensulfuron methyl
4.547 132 5294
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of 0.0015 g/g in postharvest soil when it was ap-
plied at normal rate. Hence, the continuous and
indiscriminate use of BSM may be circumvented to
overcome the biomagnifications of its residues in
soil environment due to its medium to low mobil-
ity nature in soil with organic carbon normalized
freundlich coefficient (Kfoc) of 205–561 mL/g
(EFSA 2008).
Obrigawitch et al. (1998) found that the presence
of 0.1 g/ha active ingredient of sulfonyl ureas is
the threshold dose in soil and would be enough
to decrease the yields of even the most sensitive
non-target plants. Hence, in the present study, sen-
sitive indicator species namely green gram (Vigna
mungo) was grown as succeeding crop to assess
the phytotoxicity of BSM residue in soil. During
both years of study, the residual phytotoxicity was
not observed on green gram and the parameters
viz., germination percent, plant height, number
of pods per plant and seed yield were found to be
unaffected by the time and dose of BSM application.
The present results suggest that the risk of BSM
carryover to succeeding crops planted following
pre- or post-emergence application to rice is low in
tropical environment at the normal rate as single
application. However, the effect of repeated and
continuous application needs to be investigated
in wetland rice growing environment.
In conclusion, it can be concluded that the
QuEChERS method could be well applied for the
BSM extraction from soil and rice parts to the
level of below MRL prescribed by different coun-
tries. It is observed that the rice grown in soil of
neutral to alkaline pH, high sorption capacity and
even distribution of high rainfall might lead to
less persistence of BSM residues after its pre- or
post-emergence single application at normal rate
of 100 g/ha. The terminal BSM residue in rice
grain, husk and straw was found to be below the
maximum residue limit set by the Japan, FSSAI
and some European countries when it is applied
at the normal rate ranged from 100–200 g/ha.
However, the effect of repeated and continuous
application on its dissipation in soil and residues
in rice needs to be investigated in wetland rice
growing environment as there is a chance for the
biomagnifications of herbicide residues in soil
and crop.
Acknowledgements
The authors are grateful to the Directorate of
Weed Research, Jabalpur and the Tamil Nadu
Agricultural University, India for providing nec-
essary research facilities to carry out the work.
The authors declare that they have no conflict of
interest regarding this study.
Table 2. Residue of bensulfuron methyl (µg/g) in soil at harvest
Method of
application
Kharif 2012 Kharif 2013
pre-emergence post-emergence pre-emergence post-emergence
Dose 100 200 100 200 100 200 100 200
(g ai/ha)
Soil < LOQ 0.011 ± 7.17 < LOQ 0.017 ± 6.89 < LOQ < LOQ < LOQ < LOQ
LOQ – limit of quantification (0.01 µg/g); Values ± are % standard deviation
Figure 5. Detection of bensulfuron methyl residue in
post-harvest soil during Kharif 2012 by high-perfor-
mance liquid chromatography with diode-array detec-
tion (HPLC-DAD)
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14
(min)
260
240
220
200
180
160
140
120
100
80
60
40
20
0
–20
(mAU)
Bensulfuron methyl 4.460 15382
DAD: Signal A,
234 nm/
Bw: 4 nm
– Bensulfuron methyl
POE - soil - R3T4.dat
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Received on April 16, 2016
Accepted on August 8, 2016
Corresponding author:
Assist. Prof. Dr. P. Janaki, Ph.D., Tamil Nadu Agricultural University, Department of Agronomy, 641 003 Coimbatore,
India; e-mails: janakibalamurugan@rediffmail.com; janaki.p@tnau.ac.in
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Vol. 62, 2016, No. 9: 428–434 Plant Soil Environ.
doi: 10.17221/294/2016-PSE