Pestic Sci. 1977, 8, 359-365

Persistence of the Herbicide AC 92,553, N-( l-ethylpropyl)-2,6-dinitro-3,4-xylidine, in Soils

Ailan Walker and William Bond

National Vegetable Research Station, Wellesbourne, Warwick C V35 9EF

(Manuscript received 13 January 1977)

The effects of soil temperature and soil moisture content on the rate of loss of N- (lethylpropyl)-2,6-dinitro-3,4-xylidine (I, A C 92,553) were measured under controlled conditions. The time for 50 % disappearance in a sandy loam soil at 75 % of field capacity was inversely related to temperature (98 days at 30°C; 409 days at 1O'C). At 25°C. the half-life increased with decreasing soil moisture content (122 days at 75% of field capacity; 563 days at 12.5 %). In seven soils with different properties there was a trend towards a slower rate of loss as the organic matter content of the soils increased and the half-life varied from 72 to 172 days, first-order kinetics being obeyed. The herbicide was lost rapidly from an inert surface and 97% loss was recorded after 28 days at 25°C. Losses from soil surfaces occurred more slowly and were greater from wet compared with dry soil. In the field, it was more persistent when incorporated than when applied to the soil surface. More than 60% of I incorporated in April 1975 could be detected the following September, but when applied to the soil surface, only about 20% of the applied dose remained by this time. Residues measured by gas- liquid chromatography using a thermionic nitrogen detector closely paralleled those measured by a bioassay based on the root growth of buckwheat.

1. Introduction

The herbicidal properties of N-( I-ethylpropyl)-2,6-dinitro-3,4-xylidinea (I, A C 92,553) were des- cribed by Sprankle.' The compound was reported to show potential for selective weed control in a number of crops including cotton, soybeans, maize, wheat and rice. Preliminary field trials also suggested some possible uses in vegetable crops2 Since all of these possible uses involve soil applica- tion, tests were made to determine the persistence of I in the soil.

( I ) AC92,553

2. Experimental

2.1. Materials Soils were collected from the surface lOcm of seven fields at the National Vegetable Research Station and their properties are shown in Table 1. The herbicides used were a commercial emulsi- fiable concentrate formulation of I (33 % a.i.) and samples of analytical grade I and trifluralin.

a B.S.I. common name: pendiniethalin.

359

360 A. Walker and W. Bond

2.2. Analytical methods 2.2.1. Gas-liquid chromatography I was extracted by shaking duplicate 50 g amounts of soil with water (5 ml) and methanol (50 ml) on a wrist-action shaker for 1 h. The extracts were filtered and water (25 ml) and hexane (10 ml) were added to 25 ml of the clear supernatant. The mixture was shaken vigorously for 2 min, anhy- drous sodium sulphate (5 g) was added to aid separation of the layers and the mixture shaken for a further 30 s. The concentration of I in the hexane layer was determined directly using a Pye Unicam Series 104 gas-liquid chromatograph fitted with a thermionic nitrogen detector (rubidium chloride tip). A glass column (4 mm i.d. x 1.5 m) packed with 5 % “SE 30” on 80-100 mesh “Chromosorb W High Performance” was used. The operating temperatures of the injection port, column and detector were 225,215 and 300°C respectively. Gas flow rates were: carrier gas (nitrogen) 60 ml/min, hydrogen 28 ml/min and air 450-500 ml/min. Duplicate 5 p1 injections of the unknown solutions were made and the peak heights compared with those obtained from similar injections of standard solutions of analytical grade I in hexane. The mean recovery from soil fortified with an emulsifiable concentrate formulation of I at concentrations in the range 0.2 to 4.0 mg/kg was 98+_4.4%.

2.2.2. Bioassay The bioassay used was similar to that described for determination of chlorpropham residues by Roberts and W i l ~ o n . ~ Treated soil was serially diluted with untreated soil to give a range of dilutions from 1.0 to 0.016 of original soil. Two 6 cm pots from each dilution, including untreated controls, were sown with 10 seeds of buckwheat (Fagopyrum esculentum Moench) which had been pre- germinated at 25°C for 24 h. The pots were watered from below, randomised and maintained in the dark for 72 h in an incubator at 25°C. With each assay, a standard series with known concentrations of I in the soil was run. After incubation, the roots were washed free of soil and the length of the main root of each seedling recorded. The dilution in which the mean root length was reduced by 50% from untreated controls was derived graphically, and this was compared with the concentration required to produce the same response in order to determine the concentration of herbicide in the original soil. In Sheep Pens soil (Table l), the mean ED50 for 16 standard series was 0.34k0.01 mg/kg air-dry soil.

2.3. Laboratory experiments 2.3.1. Incubation studies Separate quantities of air-dried soil (1 kg) were treated with I by adding the required amount in water to give a final herbicide concentration of 4 mg/kg dry soil. Mixing was achieved by passing the soil several times through a 2 mm mesh sieve. After mixing, the soils were stored in polythene bags for 24 h at room temperature during which time duplicate 20 g subsamples from each treat- ment were dried at 110°C. After this 24 h period, separate 600 g amounts from each treatment were weighed into I -litre wide-mouthed polythene bottles and appropriate amounts of water added to give soil moisture contents of 75% of field capacity (Table 1). The remaining soil was stored at - 10°C for use as recovery checks throughout the experimental period. The bottles were stoppered with cotton wool plugs and incubated at 25°C. Sufficient water was added to the soils each week to maintain the required soil moisture content. At intervals during the subsequent 140 days, the amounts of herbicide remaining were determined by the gas-liquid chromatographic method described above.

Further samples of Sheep Pens soil, treated with I at 4.0 mg/kg, were incubated at temperatures of 10,15,20 and 30°C at a soil moisture content of 75 % of field capacity, and at 25°C with soil mois- ture contents of 12.5, 25, 37.5, 50, 62.5 and 75% of field capacity. Four samples (50 g) for each treatment were incubated in 100-ml conical flasks with aluminium foil caps. All the samples were extracted and their content of I determined after 140 days.

Persistence of AC 92.553 herbicide in soils 361

2.3.2. Losses from inert and soil surfaces A solution of analytical grade I (200 mg/litre) in acetone was prepared, and subsamples (1 ml) were pipetted on to 28 aluminium planchets (50.8 mm diameter). This gave a deposition of 10 pg/ cm2, equivalent to 1.0 kg/ha. When the acetone had evaporated, the herbicide was redissolved from four of the planchets and made up to 10 ml. The remaining planchets were placed in an incubator, in the dark, at 25°C. At intervals during the subsequent 28 days, the herbicide from four planchets was redissolved in acetone (10 ml). On each occasion, the herbicide concentration in acetone was determined by gas-liquid chromatography.

To two further series of 28 planchets, air-dried Sheep Pens soil (1 g) was added together with water (1 ml) and the soil was evenly dispersed over the surface with a spatula. The samples were dried for 24 h at room temperature and this produced a thin film of soil particles over the surface of the planchet. I in acetone was added to the planchets, as above, and when the solvent had evaporated, four initial recovery checks were made and the remaining planchets were incubated in the dark at 25°C. One series of planchets was maintained dry and to the other, water (0.2 ml) was pipetted evenly over the surface once each day. The herbicide which remained was determined at intervals over 28 days as above. The experiment was repeated with trifluralin under the same conditions. The gas-liquid chromatographic method for analysis of trifluralin was the same as that for I except that the injection port and column temperatures were 215 and 195°C respectively.

2.4. Field experiments Field plots were prepared on 22 April and 25 July 1975, and on 6 April and 11 May 1976. In 1975 the experiments were in Sheep Pens field and in 1976 in Big Ground. Separate plots (6 x 1.5 m) were sprayed with I at a rate of 2 kg a.i./ha. On esch occasion, four plots were prepared and on two of these the herbicide was incorporated to a depth of 3 4 cm with a rotary power harrow. On the other two plots the herbicide remained on the surface. Immediately after application, 30 cores (2.5 cm diameter to a depth of 7.5 cm) were taken from each plot at random positions. The cores from each plot were bulked, thoroughly mixed by passing several times through a 2-mm mesh sieve and the total weight of sieved soil recorded. The samples were stored at - 10°C until analysis. Further soil samples were taken at intervals during the subsequent 10-23 weeks. The herbicide concentration in the soil was determined by the gas-liquid chromatographic method described above. In 1975, the samples were also analysed by bioassay in order to compare residue measurements made by both chemical and biological estimation techniques.

3. Results and discussion 3.1. Laboratory experiments The rate of degradation of I in the seven soils examined conformed to first-order kinetics. When the logarithms of the concentrations remaining were plotted against the time of incubation, good straight line relationships were obtained. The half-lives calculated from the slopes of the lines together with the correlation coefficients for the lines of best fit are shown in Table 1. The data show that under

Table 1. Soil properties and half-lives for I at 25°C and 75% of field capacity

Field Carbon Clay capacity Correlation

Soil ( %) ( %) PH (%) cofficient"

Soakwaters 0 . 8 7 27.6 7 . 1 17.9 -0 980 Gravel Pits 1.08 20 .4 6 . 7 17.6 -0 998 Gallas Leys 1.12 37.8 7 . 5 25 .1 - 0 969 Sheep Pens I .20 18.3 6 2 14.3 -0.993 Big Cherry 1.33 20.0 7 . 0 17.0 -0 995 Pump Ground 1.75 20.5 6 8 18.7 - 0 995 Water Meadows 6 .81 60 4 6 . 3 44 .2 -0 .987

n For first-order kinetics, a plot of log (residual I) against time.

Half-life" (days)

72 87

132 129 139 127 172

362 A. Walker and W. Bond

these warm, moist conditions, I is a relatively persistent herbicide with half-lives varying from 72 days in Soakwaters soil to 172 days in soil from Water Meadows. Although only a limited range of soils was examined, there is a trend towards slower rates of loss as the organic matter content of the soil increases.

The effects of soil temperature and moisture on the rate of loss from Sheep Pens soil are shown in Table 2. The results have been calculated as equivalent half-lives following single point determina-

Table 2. Half-lives for I in Sheep Pens soil at different temperatures and soil moisture levels

.. Temperature Half-life" (days) at soil moisture (% of field capacity) ("C) 75 62.5 50 37.5 25 12.5

.. ~ . -~

30 9 8 k 3 . 2 25 122 k 3 . 8 1 6 6 k 3 . 8 2 3 9 k 1 0 . 1 2 2 5 k 8 . 6 261k10 .5 5 6 3 k 8 8 . 2 20 I68 k 17.1 15 265+10.5 10 409k27 .9

First-order kinetics assumed to hold.

tions of residue levels after 140 days. With a decrease in temperature from 30 to lOT, the half-life increased from 98 to 409 days, and when the data from all of the temperatures were plotted accord- ing to the Arrhenius equation (i.e. the logarithm of the half-life against the reciprocal of theabsolute temperature) a good straight line fit was obtained (r=0.993). The calculated activation energy is 51.8k 3.54 kJ/mol (12.39k0.837 kcal/mol) which compares with the values of 64.9 and 72.0 kJ/mol calculated for the dinitroaniline herbicides isopropalin and oryzalin respectively by Gingerich and Zimdahl.4

At 25T, the half-life increased from 122 days at 75% of field capacity to 563 days at 12.5% of field capacity (Table 2) and this suggests that I could have potential for long soil persistence under dry conditions.

Volatilisation can be a major route for loss of some dinitroaniline herbicides from soil5 and the vapour pressure of I (3 x mmHg at 25°C) suggests that this herbicide may also be significantly volatile. The results in Table 3 show that this is so. When incubated on a metal surface at 25T , 70% was lost in 14 days and over 95% was lost in 28 days. Losses of I, however, were much lower than those of trifluralin (vapour pressure, 2 x 10-4 mmHg at 295°C) which is the most volatile of the dinitroaniline herbicides currently in use.5 Both trifluralin and I were less volatile when the alu- minium planchets were coated with a film of soil particles prior to addition of the herbicide (Table 3). When the soil surface was dry only 30% loss of both compounds occurred in 28 days, most of which took place during the first 1-7 days. When the soil was wetted daily, however, losses were increased and trifluralin (95% loss in 28 days) was more volatile than I(60% loss in 28 days). This result is similar to that reported by Parochetti et al.6 for the volatilisation of trifluralin and I from soil. Reduced losses by volatilisation from dry compared with wet soils have been recorded in many studies of herbicide volatilisation.7-~

Table 3. Loss of I and trifluralin from inert and soil surfaces

% of initial application remaining . ~ ~ . _ _ _ _ . _ _ . . _~. ~ _ _ _ _ _ _

I Trifluralin Time ____ ~ _ _ _ _ _ _ ~ ~ _ _ _ _ . _ _ ~

(days) Metal surface Dry soil Wet soil Metal surface Dry soil Wet soil

4 hours 1 80 84 74 1 76 59 2 54 81 64 0 73 51 4 36 81 60 0 79 21 7 34 72 56 0 74 19

14 27 73 52 0 66 11 28 3 68 40 71 5

-~ .~ - ~ .___

- - - - - 48

-

Persistence of AC 92,553 herbicide in soils

120 1 363

- - - ----A

I 1 I I I 1 I 0 40 00 I20 160

Davs

Figure 1. Persistence of I in the field in 1975 (Time 0, April 22). Surface (0 , A); incorporated (0, A).

cm

E 00 E P

- 0 ‘ 40

20

I I I I 1 I I 0 40 00 120 160

Dovs

Figure 2. Persistence of 1 in the field in 1976 (Time 0, April 6). Surface (0, A); incorporated (0, A).

364 A. Walker and W. Bond

3.2. Field persistence The results from the field experiments are shown for 1975 in Figure 1 and for 1976 in Figure 2. Following all four spraying dates, I was initially lost more rapidly from surface compared with incorporated applications. After the second sprayings in 1975 and 1976, up to 50% of the amount applied was lost from the soil surface in the first 12-14 days. Rates of loss from surface treatments made earlier in each of these years were somewhat smaller, with 50% loss in 30-40 days, probably reflecting the lower temperatures at the soil surface and the resultant decrease in volatilisation. When incorporated, 1 was a very persistent herbicide, and following incorporation in April 1975, about 80% of the amount applied could be detected after 20 weeks. After application in April 1976, 65-70% remained 23 weeks later. This very long persistence was probably, in part, a reflection of the dry summer weather in 1975 and 1976. Total rainfall at Wellesbourne from April to August inclusive in 1975 was 160 mm, and in 1976 was 140 mm (of which 53 mm was recorded from 28-30 August). These values compare with the 25-year average for this period of 260 mm. The surface soil was very dry for long periods in both of these years and therefore rates of loss by degradation would be reduced (Table 2) and volatilisation losses would also be minimi~ed.~#7 It is probable that persistence would be somewhat less in a wetter summer.

A A

A A A

/ 0 o A A

0

I I I I I 0.5 I .O 1.5 2 .o 2.5

Restdue by c, I c (mq/kq)

Figure 3. Comparison between soil residues of I in 1975 measured by bioassay and by gas-liquid chromatography Surface (0 , A); incorporated (0, A).

Persistence of AC 92,553 herbicide In soils 365

In 1975, the samples from the persistence experiments were analysed by bioassay as well as by gas-liquid chromatography with the results shown in Figure 3. The close correspondence between the two sets of data suggests that degradation and loss of I from the soil are accompanied by an equivalent decrease in biological activity. There are few reports in the literature in which biological and chemical residue measurements have been compared, although Geissbuhlerlo presented evi- dence that biologically determined residues of fluometuron in soil closely paralleled the amounts of parent herbicide extracted from the soil.

4. Conclusions

The rate of loss of I from soil is affected by soil type, soil temperature and soil moisture content. In the soils examined and under the range of conditions tested, the herbicide was relatively persistent. Significant losses from surface applications may occur through volatilisation although these are greatly reduced when the soil surface is dry. In field experiments, up to SO% of the amount applied could be recovered 20 weeks after application following incorporation into the soil but only approxi- mately 20 % remained following application to the soil surface.

Acknowledgements Thanks are expressed to Cyanamid of Great Britain Ltd for providing samples of commercial and analytical grade AC 92,553 and to Elanco Products Ltd for the sample of pure tritluralin. The technical assistance of Miss P. A. Brown is also gratefully acknowledged.

References 1. 2. 3. 4. 5. 6. 7. 8. 9.

10.

Sprankle, P. L. Proc. I2th Br. Weed Control Conf. 1974, p. 825. Roberts, H. A,; Bond, W.; Ricketts, M. E. Proc. I2lh Br. Weed Control Conf. 1974, p. 427. Roberts, H. A.; Wilson, B. J. Weed Res. 1962, 2, 60. Gingerich, L. L.; Zimdahl, R. L. Weed Sci. 1976, 2. 431. Helling, C. S. J. Envir. Quality 1976, 5, 1. Parochetti, J. V.; Dec, G . W.; Burt, G. W. Weed Sci. 1976, 24, 529. Talbert, R. E.; Smith, D. R.; Frans, R. E. Weed Sci. 1971, 19, 6. Spencer, W. F.; Farmer, W. J.; Cliath, M. M . Residue Rev. 1973, 49, 1. Spencer, W. F. ; Cliath, M. M. J. agrir. Fd Chem. 1974, 22, 987. Geissbiihler, H. In Degradation of Herbicides (Kearney, P. C.; Kaufman, D. D. eds), Marcel Dekker Inc., New York, 1969, p. 94.