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fechnical Bulletin 147 ISSN 0070-2315

EFFECT OF TEMPERATURE AND SOIL MOISTURE ON DEGRADATION OF ALACHLOR, PENDIMETHALIN AND PROMETRYN

N.A. Vouzounis and P.G. Americanos

(Accepted November 1992)

TECHNICAL BULLETIN

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ACIZICULTURAL RESEARCP I . INSTlTUTE

AGRICULTURAL RESEARCH INSTITUTE

MINISTRY OF AGRICULTURE AND NATURAL RESOURCES

NICOSIA CYPRUS

Technical Bulletin 147 ISSN 0070-2315

EFFECT OF TEMPERATURE AND SOIL MOISTURE ON DEGRADATION OF ALACHLOR, PENDIMETHALIN AND PROMETRYN

N.A. Vouzounis and P.G. Americanos

(Accepted November 1992)

SUMMARY

The effect of temperature and soil moisture on the degradation of the soil-acting herbicides alachlor, pendimethalin and prometryn was examined in 1988. Results indi­cated that the degradation rate of all three herbicides examined increased as temperature increased from 10 to 30°C in soils with 15% water content (close to field capacity). When soil moisture content (smc) was reduced to 2% the rate of degradation was retard­ed even though temperature was maintained at 30°C. Alachlor degraded more rapidly than either pendimethalin or prometryn under all temperatures when smc was 15%. Pro­metryn degraded more rapidly than pendimethalin when initial concentration was 10 ppm but not when it was 20 ppm. At 30°C and 2% smc pendimethalin had the slowest degradation rate.

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INTRODUCTION degradation rates so that marked differences in persistence may occur following applica­

In Cyprus, where the mediterranean cli­ tion in spring, summer or autumn. Studies mate favours multiple cropping, crop phyto­ on the persistence of eight dinitroanilines toxicity resulting from residual activity of showed that these herbicides dissipated soil-acting herbicides applied to preceeding more rapidly in wet than in dry soil (Jacques crops has been observed. and Harvey, 1979). Similar observations

Rates of pesticide degradation in soil are were made on metribuzin persistence (Web­influenced by various soil and climatic fac­ ster et aI., 1978). Since soil temperature and tors including clay and organic matter con­ soil moisture content are significant factors tent, soil pH, temperature and moisture. Va­ in herbicide degradation, it was considered riation in soil temperatures greatly influence that the study of dissipation of three widely

used herbicides in Cyprus at various temper­atures could yield useful information on their degradation rates.

MATERIALS AND METHODS

The degradation and persistence of the herbicides alachlor, pendimethalin and pro­metryn applied to a sandy-loam soil at two concentrations (10 and 20 ppm) was investi­gated under various controlled conditions. For each herbicide and concentration the four treatments examined were storage tem­peratures of 10, 20 and 30 0C all at 15% soil moisture content (smc) and of 30 °C at 2% smc. Soil was air-dried (2% smc) and passed through a 3 mm sieve. Twenty five kg of sieved soil was sprayed with each of the her­bicides in a volume of 50 ml water, using a hand sprayer to give a final herbicide con­centration of 20 ppm w/w. This was fol­lowed by thorough mixing of the soil, part of which was then diluted (1: 1) with untreated soil to give an initial concentration of 10 ppm. Duplicate 2-kg soil samples were made up for each treatment and stored in closed polythene bags (one for three weeks and one for six weeks) at 10, 20 and 30 °C constant temperature. An additional sample was pre­pared for the initial bioasay test and two more were kept in the freezer (0 OC) for the second and third bioassays, three and six weeks after application (WAA). The initial sample was used to prepare the initial dose response curve, by diluting it with untreated sieved soil in order to achieve concentrations of 20, 10, 5, 2.5, 1.25, 0.6 and 0.3 ppm. The same procedure was followed for the frozen samples three and six WAA. The bioassays were carried out in pots (7 cm2 and 7.6 cm deep) in a glasshouse during autumn 1988 at an ambient temperature of 18 oc. Ten seeds of each bioassay species were sown in each treated pot and in an untreated control. The crop species used were barley for alachlor and prometryn and wheat for pendimethalin and treatments were replicated four times. Germinated seeds were allowed to grow for :hree weeks and plants were harvested at 70und leve. Their fresh weight was deter­1ined immediately.

The initial dose response curves, derived om the bioassay of the initial and frozen mples, were used as recovery checks for

the analytical method throughout the exp\ mental period.

RESULTS AND DISCUSSION

The test of homogeneity of the regressic lines of the dose response curves (0, 3 and WAA) revealed that all regression lines wen parallel. In addition, there were no signifi· cant differences in the mean (slope of the re­gression line) response to time or rate of de­cline of fresh weight (regression coefficient). The relationship between the herbicide con­

centration and the fresh weight of indicator plants (% control) for the three herbicides is described by the exponential curves (initial dose response curves) appearing in Figs 1,2 and 3.

The general formula used was:

Y=AXb

where Y =predicted herbicide concentration, A =constant, b =regression coefficient,

and X =percent fresh weight of control. The fresh weight of the respective indica­

tor plants, expressed as percentage of the control, was highly correlated with the con­centration of alachlor, pendimethalin and prometryn, the correlation coefficients being 0.89,0.97 and 0.97, respectively.

AJachlor The concentrations (ppm) of alachlor

three and six weeks after application were estimated from the plant weight expressed as a percentage of the untreated control using the equation of the initial dose response curve (Table 1). In practice, concentrations of 10 and 20 ppm of these herbicides corre­spond to 8 and 16 kg a.i./ha, respectively, at a 10 cm soil depth and at a bulk density of 1.25 g/cm3, which are several times higher than concentrations applied under cropping conditions.

Analysis of variance on the degradation of alachlor revealed that there were signifi­cant differences among treatment means (re­maining concentrations, ppm), three and six weeks after application of 10 and 20 ppm (Table 1). It was further evident that temper­ature greatly accelerated degradation of alachlor in the presence of soil moisture. At a relatively low temperature (10 OC) and 15% smc, degradation of alachlor was slow

and it was even slower at 30 °C in the drier soil with only 2% smc. Results of studies carried out by many researchers have shown that breakdown of soil-applied herbicides is more rapid at higher temperatures and higher moisture content (Walker, 1974; Walker and Bond, 1978; Smith and Walker, 1977).

The effect of temperature and moisture on alachlor breakdown was similar regard­less of initial concentration, but the rate of degradation was mostly greater at the initial concentration of 20 ppm than at 10 ppm. These results are similar to those observed by Zimdahl and Clark (1982), who in labora­tory studies found that the degradation rate of alachlor at 20 °C was greater at 50 and 80% than at 20% field capacity and that at higher temperatures degradation of alachlor was greater at 50% field capacity.

Pendimethalin Degradation of pendimethalin was en­

hanced in the presence of soil moisture as soil temperature increased from 10 to 30 °C

Table 2. Remaining concentration of pcndimcthalin (ppm) three and six weeks after application

Initial (10 ppm) Initial (20 ppm)

Temp. Moisture <:>C) content 3 WAA 6 WAA 3 WAA 6 WAA

10 15 9.25 3.42 17.10 16.22 20 15 2.32 1.90 4.20 4.00 30 15 2.27 1.30 3.27 2.75 30 2 5.05 4.50 10.50 9.00

LSD (0.05) 5.31 0.70 4.13 3.88

(Table 1). Unlike alachlor the degradation of pendimethalin was greater at 30 °C under dry conditions than at 10 °C under moist conditions, suggesting that the degradation rate of this herbicide is influenced by tem­perature to a greater extend than is the case of alachlor. Table 2 shows the remaining concentrations of pendimethalin 3 and 6 WAA estimated from the initial dose re­sponse curve (Fig. 2). The rate of degrada­tion at 20 and 30 °C with 15% smc was sim­ilar three weeks after application of 10 ppm, but at 10 °C breakdown of pendimethalin was slow for both initial concentrations as it was also at 30 °C in dry soil (2% smc). Zim­dahl et al. (1984), working with pendimeth­alin in three soils found that degradation in­creased as soil temperature rose from 10 to 30 oc. The rate was the same at 75 and 100% field capacity, but slower at 50%.

Prometryn There was no significant effect of tem­

perature or moisture content on prometryn degradation at an initial concentration of 20 ppm three weeks after application (Table 3). Significant differences, however, were ob­served after six weeks for both concentra­tions and for 10 ppm three weeks following application, degradation increasing with temperature and moisture content. At 30 °C with 2% smc and at 10 °C with 15% smc degradation of prometryn was slow.

Walker (1976) studying the effect of soil temperature and soil moisture content on the degradation rates of prometryn under con­trolled conditions found that the time for 50% disappearance at 25 °C was increased from 30 to 590 days with a reduction in soil moisture content from 14 to 5%. Moreover, the rate of degradation increased as the ini­tial herbicide concentration increased. La­boratory experiments carried out by Walker and Zimdahl (1981) showed marked effects of both temperature and soil moisture con­tent on the rates of herbicide loss with re­duced rates of degradation in cooler and dri­er soils. The results of the present study agree with those observations and demon­strate the very marked effect that environ­mental conditions can have on the rate of herbicide degradation. The concentration of prometryn remaining, estimated from the fresh weight of barley using the equation of

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Firure 2. Initial dose response curve of barley to penLimethalin in sandy loam soil

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Firure 3. Initial dose response curve of barley to prometryn in sandy loam soil

4

Table 3. Remaining concentration of prometryn (ppm) three and six weeks after application

fuitia! (10 ppm) fuitial (20 ppm)

Temp. Moisture fC) content 3WAA 6WAA 3WAA 6WAA

10 15 2.37 1.85 4.65 2.95 20 15 1.75 1.30 5.72 1.62 30 15 1.40 0.95 5.72 1.25 30 2 3.15 1.87 4.72 2.35

LSD (0.05) 0.85 0.32 2.36 0.60

the initial dose response curve of prometryn (Fig. 3), is shown in Table 3.

CONCLUSIONS

Under controlled environment, persis­tence of alachlor, pendimethalin and promet­ryn was more protracted under cold or dry than under wann and relatively moist condi­tions. In Cyprus, where rainfall is uncertain, herbicide persistence under rainfed condi­tions could be protracted. However, in areas where irrigated crops are grown, herbicide dissipation can be expected to be more rapid. In winter, when soil temperatures are com­paratively low (8 to 12 °C), herbicide persis­tence is prolonged even though there may be adequate soil moisture. Occasionally cold winters with limited rainfall may occur and in such cases very slow degradation of soil­applied herbicides can be expected. Howev­er, although these results provide useful indi­cations of herbicide persistence in relation to the controlled environment, caution needs to be exercised in ascribing generalizations to the field where environmental factors remain variable. In the field, enhanced rates of dissi­pation might be expected as a result of in­creased microbial activity, leaching and in­creased absorption.

Investigations into the rate of dissipation of residual herbicides applied in different seasons should yield useful data on the possi­ble effects on rotational cropping.

ACKNOWLEDGEMENTS

The authors thank Dr. R.J. Froud - Wil­liams for reviewing the manuscript.

REFERENCES

Jacques G.L., and RG. Harvey. 1979. Persistence of dinitroaniline herbicides in soil. Weed Science 27:660-665.

Smith AE., and A Walker. 1977. A quanbtabve study of Asulam persistence in soil. Pesticide Science 8:449-456.

Walker A 1974. A simulation model for prediction of herbicide persistence. Journal of Environ­mental Quality 3:396-401.

Walker A 1976. Simulation of herbicide persistence in soil I. Simazine and prometryn. Pesticide Science 7:41-49.

Walker A, and W. Bond. 1978. Simulation of the persistence of metamitron activity in soil. Proceedings British Crop Protection Confer­ence. Weeds 2:565-572.

Walker A, and RL. Zimdahl. 1981. Simulation of . the persistence of atrazine, linuron and meto­lachlor in soil at different sites in the USA Weed Research 21:255-265.

Webster G.R.B., L.P. Sarna, and S.R Macdonald. 1978. Nonbiological degradation of the her­bicide metribuzin in Manitoba soils. Bulletin of Environmental Contamination and Toxi­cology 20:401-408.

Zimdahl RL., and S.K. Clark. 1982. Degradation of three acetanilides in soil. Weed Science 30:545-548.

Zimdahl RL., P. Catizone, and AC. Butcher. 1984. Degradation of pendimethalin in soil. Weed Science 32:408-412.

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