Pestic. Sci. 1987, 19,79-83

The Determination of the Herbicide Dinoseb in Lentils

Allan J. Cessna

Agriculture Canada, Research Station, Box 440, Regina, Saskatchewan S4P 3A2, Canada

(Received 3 February 1986; accepted 23 April 1986)

A BSTRA CT

Residues of the herbicide dinoseb were determined gas chromato- graphically in lentils which had been treated at two locations in Saskatchewan with post-emergence applications of dinoseb at 1.4 and 1.7 kg ha-'. Herbicide residues, determined at selected times after applica- tion, were not detected at the limit of detection of the analytical method (0.05 mg kg-I) in either the seed and straw at maturity, or in the green foliage six to eight weeks after application. Recoveries of dinoseb were 76% from fortified green foliage at the 0.1 mg kg-I level, and 64% from fortified seed at the 0.05 mg kg-1 level.

1 INTRODUCTION

Lentils (Lens culinaris Medic.) are a recently introduced grain legume with a high potential for becoming an important field crop in western Canada.' One of the major obstacles to the increased production of lentils in this area has been the lack of registered herbicide treatments for the control of broadleaved weeds including charlock (Sinapis arvensis L.) and field pennycress (Thlaspi arvense L.) and volunteer growth of rape (Brassica napus L. and B. campestris L.).2 Although a high level of weed control during the seedling stage is imperative to ensure profitable seed yields with this weakly competitive no registered herbicides for the control of broadleaved weeds were available to the lentil producers in Canada when this study was initiated. Preliminary experiments4p5 indicated that post-emergence applications of dinoseb (2-sec-butyl-4,6-dinitrophenol) effec- tively controlled the above-mentioned broadleaved weeds without undue damage to the lentil seedlings. Thus, this study was designed to determine dinoseb residues in lentils at selected times following post-emergence application and, subsequently, to provide the residue data to the regulatory agencies to support registration of this use.

79 Pestic. Sci. 0031-613X/87/$03.50 0 Society of Chemical Industry, 1987. Printed in Great Britain

80

2 EXPERIMENTAL METHODS

A. 1. Cessna

2.1 Herbicide treatments

Lentil samples for residue analysis were collected from two locations in western Canada. At each location, all treatments were replicated four times.

On 17 May, 1976, at the Crop Development Center, University of Saskatchewan at Saskatoon, Saskatchewan, large-seeded yellow lentils (variety Common Chilean), inoculated with Rhizobium, were planted into summer- fallowed sandy loam (60% sand, 10% silt, 30% clay; 6% organic matter) to 5 cm depth, using a four-row cone seeder fitted with shoe openers. Plots consisted of four 5-m rows of 30 cm spacing. On 6 June, when the crop had six leaves and was 6 cm in height, the amine salt of dinoseb was applied at 1.4 and 1-7 kg ha-l. Both treatments were applied in 146 litre ha-' of water, using a compressed-air plot- sprayer operated at 276 kPa.

On 18 May, at the Agriculture Canada Experimental Farm at Indian Head, Saskatchewan, lentils (variety PI345631) were similarly seeded, using a double-disk drill, into summer-fallowed heavy clay (7% sand, 23% silt, 70% clay; 4.7% organic matter). On 10 June, when the crop had six leaves and was 6 cm in height, duplicate treatments of dinoseb amine salt at 1.7 kg ha-' were similarly applied at 207 kPa and using 112 litre ha-' of water.

2.2 Sampling

At Saskatoon, green foliage samples were randomly collected from the plots at the following times after spraying: 1 day (0.1 kg), 1 week (0.1 kg), 3 weeks (0.3 kg), 5 weeks (0-5 kg) and 8 weeks (1 kg) until the indicated sample size was obtained. Samples were similarly collected at Indian Head on the day of spraying and at 1 , 3 and 6 weeks after spraying. Mature crop samples (1 kg) were collected at both locations. In addition, field samples of mature lentils, which had been sprayed by farmers with dinoseb amine at 1.7 kg a.i. ha-', were collected after swathing from three different fields near Regina. All samples were placed in sealed polyethylene bags immediately after harvest and stored at - 10°C until shipped to Regina on dry ice. The samples were then processed as follows: the green foliage samples were cut into 1-cm lengths whereas the mature crop samples were threshed into seed and straw/chaff fractions, and these fractions were milled through a 1-mm screen. The processed samples were placed in polyethylene bags and stored at -10°C until extraction.

2.3 Moisture content determination

Following completion of all analyses, moisture contents of all samples were determined by heating the samples at 90°C for 24 h in a forced-air oven.

2.4 Analytical method

The extraction, analytical and fortification procedures were carried out as described previously.6 Using electron-capture gas chromatographic analysis, dinoseb was determined quantitatively either as the free phenol (residues >0.3 mg kg-I) or as the methyl ether after methylation with diazomethane

Determination of dinoseb in lentils 81

(residues c0.3 mg kg-'). A single analysis was conducted for each replicate sample.

3 RESULTS AND DISCUSSION

Dinoseb residues were determined as the free phenol only for lentil samples collected 1 and 7 days after application (Tables 1 and 2). Because of the large dinoseb residues present in these samples and the consequent dilution of the sample extract prior to quantitation, there were no background interferences at the retention time for dinoseb. For all subsequent samples, dinoseb was quanti- tated as its methyl ether. Analysis of check samples of straw, seed and green foliage collected after the 7-day sampling showed that interfering peaks at the

TABLE 1 Residues of Dinoseb (mg kg-') Detected in Lentils Treated Post-emergence with Dinoseb

at 1.4 and 1-7 kg ha-' at Saskatoon

Sampling date (days after spraying)

Dinoseb residues (kSE, n=4) (mg kg-I fresh weight)"

1.4 kg ha-' I + 7 kg ha-'

1 7

21 35 56 76 (straw at maturity) 76 (seed at maturity)

21-8 (k1.6) 1.37 (k0.09) 0.43 (kO.10) 0.06 (k0.01)

NDb ND ND

28-6 (f3.5) 1-66 (fO.08) 0.58 (f0-13) 0.07 (f0.02)

ND ND ND

"Residue values are uncorrected for recoveries. bND=not detected at the limit of detection (0.05 mg kg-I).

TABLE 2 Residues of Dinoseb (mg kg-') Detected in Lentils Treated Post-emergence with Dinoseb

at 1.7 kg ha-' at Indian Head

Sampling date (days after spraying)

Dinoseb residues ( M E , n=4) (mg kg-lfresh weight)"

1.7 kg ha-' 1.7 kg ha-l - b 76.8 (f3.1) 0

7 0.35 (k0.02) 0.56 (f 0.06) 21 0.04 (kO.01) ND' 42 ND ND 80 (straw at maturity) ND ND 80 (seed at maturity) ND ND

"Residue values are uncorrected for recoveries. bThese samples were not collected due to a very heavy rain (101-7 mm) which continued for 2 days after spraying. cND=not detected at the limit of detection (0.05 mg kg-').

82 A . J . Cessna

retention time for dinoseb methyl ether were consistently less than the equivalent of 0.005 mg kg-I. These background interferences readily permitted a limit of detection of 0.05 mg kg-'.

The moisture contents of the green foliage samples from Saskatoon fell within the range of 80.3+1.6% (1 day after spraying) to 75.5+_1.1% (56 days), whereas those for the straw and seed were 7.2+0-4% and 6.7+_0-3%, respectively. The moisture contents of the samples collected at Indian Head were similar to those of the Saskatoon samples.

Recoveries of dinoseb were determined from fortified samples of untreated green foliage and mature seed. Six replicates were analyzed at each fortification level in each substrate. The recovery from the green foliage was76.4+4.0% at the 0.1 mg kg-l level, and 64.6+3.4% and 63.0+7-5% from the seed at the 0.1 and 0.05 mg kg-' levels, respectively.

Dinoseb residues in the lentils at Saskatoon decreased rapidly with time (Table 1) and reflected the ratio of the two application rates at each sampling date until 35 days after treatment. Residues resulting from the 1.7 kg ha-' application rate were significantly higher (t-test, R 0 . 0 5 ) than those observed for the 1.4 kg ha-' rate, but only for the 1- and 7-day samples. After 56 days, residues in the green tissue had decreased below the limit of detection of the analytical method (0.05 mg kg-l) and were not significantly different from the 56-day checks. Rapid decreases in dinoseb residues have also been observed in lucerne, peas and horse bean^.^ The rapid decrease in dinoseb residues in lentils in the present study would be due in part to growth dilution. Metabolism studies suggest that reduc- tion of the nitro groups and oxidation of the sec-butyl side chain8v9 could also account for significant losses of dinoseb within a few days after application. In addition, photolysis of dinoseb in droplets deposited on leaf surfaces8S1O and volatility losses1° occurring after hydrolysis of the amine salt to the free phenol may also have contributed to the rapid decline of dinoseb residues in the crop canopy. Chromatograms of the treated straw samples showed peaks at the reten- tion time for dinoseb methyl ether, which were greater than the background peaks in the corresponding checks. However, the dinoseb residues represented by these peaks (0.014fO-007 and 0.019+0.008 mg kg-l for the 1-4 and 1.7 kg ha-' rates, respectively) were less than the limit of detection. Chromatograms for the treated seed were not significantly different from the corresponding checks.

Dinoseb residues in the Indian Head samples (Table 2) decreased more rapidly than those in the Saskatoon samples, no doubt due to wash-off resulting from a heavy rainfall, which prevented the collection of the second set of samples on the day of spraying. The rain began shortly after application (10 June, 12-2 mm) and continued for the next 2 days (11 June, 35.1 mm; 12 June, 54.4 mm). Conse- quently, dinoseb residues in the green foliage had decreased to less than 0.05 mg kg-' by 21 days following application, and residues in both the grain and the straw were not significantly different from the corresponding checks.

Analysis of the lentil seed and straw samples collected from treated fields in the Regina vicinity indicated that dinoseb residues resulting from post-emergence applications by farmers of dinoseb amine salt at 1.7 kg ha-l were not significantly different from those observed in the corresponding samples collected at Saska-

Determination of dinoseb in lentils 83

toon and Indian Head. Residue data from the present study indicated that dinoseb residues in lentil foliage 42-56 days after treatment, and in the mature seed, were much less than 0.1 mg kg-l, the residue tolerance (Huston, B., private communication) on which the registration of the post-emergence use of dinoseb on lentils was subsequently based.

ACKNOWLEDGEMENTS

The author would like to thank M. Aldred and M. Bishop for their technical assistance in the residue analyses; N. W. Holt, Agriculture Canada Experimental Farm, Indian Head, Saskatchewan, and B. N. Drew, Crop Development Center, University of Saskatchewan, Saskatoon, for conducting the field treatments and collecting and forwarding the lentil samples to Regina; and J. Buchan, Special Crops Specialist, Saskatchewan Agriculture, Regina, Saskatchewan, for collect- ing the dinoseb-treated samples from commercial fields for analysis. The analyti- cal grade dinoseb and dinoseb methyl ether were supplied by Dow Chemical of Canada Ltd, Sarnia, Ont.

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