Phytotoxicity and soil dissipation of preemergence herbicides applied to beetroot

Ramprakash T, Madhavi M and Leela Rani P

AICRP on Weed Management, PJTSAU

Rajendranagar, Hyderabad-30

Biennial conference of ISWSOn

Doubling the Farmers Income by 2020 : The Role of Weed Science

Background

• Weed competition in early crop growth stages of beetroot results in yield reduction upto 42-78 % (Kumar et al, 2012)

• No recommendation of pre-emergence herbicides to beetroot

• Data on persistence of herbicides applied to beetroot as PE in soil and beetroot tubers is not available

• To assess the persistence of pendimethalin, alachlor, metribuzin, oxyfluorfen and oxadiargyl in soil when applied as pre-emergence herbicides.

• To understand the dissipation pattern pendimethalin, alachlor, metribuzin, oxyfluorfen and oxadiargyl in soil .

• To test the beetroot tuber (at harvest) for herbicide residues

Objectives

Methodology

Treatments

1. Pendimethalin 38 % CS 580g/ha as PE + HW at 30 DAS

2. Oxyflourfen 23.5 % EC 150 g/ha as PE + HW at 30 DAS

3. Alachlor 50 % EC 1000 g/ha as PE + HW at 30 DAS

4. Oxadiargyl 6% EC 75g/ha as PE + HW at 30 DAS

5. Metribuzin 70 % WP 500 g/ha as PE+ HW at 30 DAS

6. Pendimethalin 38 % CS 580g/ha as PE fb Quizalofop p ethyl (Q-P-E) 5 % EC @ 50 g/ha as PoE @

20-25 DAS

7. Oxyflourfen 23. 5 EC 150 g/ha as PE fb Q-P-E 5 % EC 50 g/ha as PoE @ 20-25 DAS

8. Alachlor 50 % EC 1000 g/ha PE fb Q-P-E 5 % EC 50 g/ha as PoE @ 20-25 DAS

9. Oxadiargyl 6%EC 75g/ha as PE fb Q-P-E 5 % EC 50 g/ha as PoE @ 20-25 DAS

10. Metribuzin 70 %WP 500 g/ha as PE fb Q-P-E 5 % EC 50 g/ha as PoE @ 20-25 DAS

11. HW at 20, 40 DAS

12. Unweeded check

Agro-climatic zone : Southern Telangana ZoneSite of the experiment : College farm, College of Agriculture,

Rajendranagar, Hyderabad, PJTSAU(780 19’19” N & 78024’35” E)

Climate : Semi-arid

Sample collection

• For residue analysis, soil samples (0-15 cm) were collected at 0,

1, 5, 15, 30, 45, 60, 90 DAA (days after herbicide application)

and at harvest.

• Beetroot tubers were collected at harvest.

• Samples were stored at -200C and brought to room temperature

at the time of analysis.

Residue Estimation

• Alachlor residues were estimated according protocol given by Ramesh

and Maheswari, 2004

• Oxyfluorfen residues were determined according to the protocol given

by Sankaran et al 1993.

• Residues of oxadiargyl were analyzed according the modified

QuEChERS method procedure outlined by Shi et al., 2010.

• Metribuzin residues estimated by procedure given by Bedmer et al 2004

• Pendimethalin residues were quantified using Sankaran et al 1993

Results

Mean recoveries of the herbicides from soil/ beetroot tuber studied through fortification with analytical standards (2.0 mg/kg to 0.001 mg/kg).

Herbicide Recovery in soil (%)

Recovery in Beetroot tuber(%)

LOQ (mg/kg)

Metribuzin 85.0 - 92.8 84.6 - 86.8 0.025

Pendimethalin 96.2 - 102.0 88.9 - 90.2 0.020

Oxyfluorfen 86.6 - 90.4 90.8 - 91.2 0.015

Alachlor 86.4 - 88.9 82.6 - 84.6 0.025

Oxadiargyl 90.2 - 91.8 85.8 - 86.8 0.025

0.025 mg/kg 0.05 mg/kg

0.25 mg/kg 0.50 mg/kg

Alachlor standards

Concentration Area0.5 55112

0.25 214250.1 11526

0.05 48120.025 19120.01 1006

Alachlor standard curve

0.025 mg/kg 0.05 mg/kg

0.25 mg/kg 1.0 mg/kg

Oxyfluorfen Standards

Concentration Area0.025 56120.05 12960

0.1 21822

0.25 523220.5 103626

1 233261

Standard curve of Oxyfluorfen

Metribuzin standard chromatogram 0.10 mg/kg

Metribuzin standard curve

Concentration Area0.01 61562

0.025 1487190.05 275018

0.5 20047921 5403726

5.0 ppm 2.0 ppm

0.5 ppm 0.1 ppm 0.05 ppm

Chromatograms of standard Oxadiargyl

Concentration Area0.025 5821

0.05 14650

0.1 18526

0.25 565670.5 100285

1 242425

Standard curve of Oxadiargyl

Phyto-toxicity of herbicides on crop

• Phytotoxicity was observed with pre emergence application of

herbicides.

• Germination failure (>90 %) was observed with metribuzin PE

application.

• Pendimethalin, oxyfluorfen plots recorded reduced number of cluster

of seedlings per seed.

• Alachlor, oxadiargyl showed no phytotoxocity

Metribuzin toxicity resulted in poor germination of beetroot.

Bio-efficacy & Economics

Highest WCE, Higher gross returns, net returns and B.C ratio was obtained with hand weeding twice at 20 and 40 DAS which was at par with pre-emergence application of alachlor @ 1000 g/ha fb hand weeding at 30 DAS.

Persistence of metribuzin, pendimethalin, oxyfluorfen, alachlor and

oxadiargyl in soil samples at different days after application

DAA Metribuzin Pendimethalin Oxyfluorfen Alachlor Oxadiargyl

0 0.633 0.577 0.181 0.552 0.093

5 0.521 0.526 0.155 0.299 0.053

15 0.389 0.367 0.113 0.147 0.033

30 0.207 0.156 0.081 0.024 0.027

45 0.103 0.053 0.059 BDL BDL

60 0.026 0.021 0.031 BDL BDL

90 BDL BDL BDL BDL BDL

Harvest

(120) BDL BDL BDL BDL BDL

Dissipation of metribuzin, pendimethalin and alachlor in soil

Dissipation of oxyfluorfen and oxadiargyl in soil

Herbicides dissipation in soil followed a first-order decay process:

C=Coexp –kd .t------------------(1)

Where Co is the initial concentration in soil, C is the concentration in the soil aftert days, and Kd is the dissipation rate coefficient.

The linearized Equation 1 is:

log (C /C0) = —0.4343 Kd t--------------(2)

Herbicide DT50 DT90

Metribuzin 17.75 49.94

Pendimethalin 18.06 50.25

Alachlor 8.15 40.34

Oxyfluorfen 34.38 114.82

Oxadiargyl 12.28 52.52

DT50 and DT90 of herbicide used in Beetroot trial

Alachlor residues in soil samples at harvest

Alachlor residues in Beetroot tuber samples

Oxyfluorfen residues in beetroot tuber samples

Pendimethalin residues in beetroot tubers at harvest

Oxadiargyl residues in beetroot tuber samples

Oxadiargyl residues in Soil samples at harvest

Herbicide MRL(mg/kg)

Metribuzin 0.10

Pendimethalin 0.05

Alachlor 0.10

Oxyfluorfen 0.05

Oxadiargyl 0.10

MRLs and ADIs for herbicides under study

Sources1. http://www.ecfr.gov/cgi-bin/text idx?SID=a3b649316ccb17c31211db2edd81f789

&mc=true&node= pt40.24.180&rgn=div5#se40.26.180_1220. Accessed on 27-07-2016

2. http://ec.europa.eu/food/plant/pesticides/eu-pesticides database/public/?event=pesticide residue. selection&language=EN. Accessed on 27-07-2016

Conclusions

• Residues of metribuzin, oxyfluorfen and pendimehalinpersisted upto 60DAA in soil. Alachlor and oxadiargyl residues were detected up to 30 DAA.

• Dissipation of the herbicides was biphasic.

• Residues of all the herbicides (except metribuzin) in beetroot tubers at were below MRL.

Thank you

III. LEACHING POTENTIAL (L)The GUS index (Gustavson, 1989) Groundwater ubiqity score is used to calculate the potential for groundwater pollution and for leaching of pesticides into surface water via the drainage systems.

This index is based on the chemical’s adsorption (Koc) and persistence (DT50) in soil. Mean values of Koc and DT50 soil are used for this parameter as follows:

GUS = log(DT50soil) x (4-log(Koc))

GUS is usually interpreted as follows:

GUS > 2.8 high leaching potentialGUS 1.8-2.8 medium leaching potentialGUS < 1.8 low leaching potential

However, Roberts (1996) used a classification based on the mean half-life of the pesticide in the soil: I) Impersistent [or "nonpersistent"], DT50 <5 d; II) 2)Slightly persistent, DT50 = 5-21 d; III) 3) Moderately persistent, DT5() = 22-60 d; and IV) 4)Very persistent, DT50 >60 d. For the purpose of identifying a set of

"residualherbicides" for this review, however, this author has set DT50 ^ 40 d as indicatingmoderate to long persistence.