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TECHNICAL REPORTS
Surface Water Quality

Herbicide Retention in Soil as Affected by Sugarcane Mulch Residue

Agronomy Dep., Louisiana State Univ. AgCenter, Baton Rouge, LA 70803

^* Corresponding author (mselim{at}agctr.lsu.edu)

Received for publication July 10, 2002.

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Reducing surface and subsurface losses of herbicides in the soil and thus their potential contamination of water resources is a national concern. This study evaluated the effectiveness of sugarcane (Saccharum spp.) residue (mulch cover) in reducing nonpoint-source contamination of applied herbicides from sugarcane fields. Specifically, the effect of mulch residue on herbicide retention was quantified. Two main treatments were investigated: a no-till treatment and a no-mulch treatment. The amounts of extractable atrazine [2-chloro-4-(isopropylamino)-6-ethylamino-s-triazine], metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one], and pendimethalin [N-(ethylpropyl)-3,4-dimethyl-2,6-dinitroaniline] from the mulch residue and the surface soil layer were quantified during the 1999 and 2000 growing seasons. Significant amounts of applied herbicides were intercepted by the mulch residue. Extractable concentrations were at least one order of magnitude higher for the mulch residue compared with that retained by the soil. Moreover, the presence of mulch residue on the sugarcane rows was highly beneficial in minimizing runoff losses of the herbicides applied. When the residue was not removed, a reduction in runoff-effluent concentrations, as much as 50%, for atrazine and pendimethalin was realized. Moreover, the presence of mulch residue resulted in consistently lower estimates for rates of decay or disappearance of atrazine and pendimethalin in the surface soil.

	INTRODUCTION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
ONE OF THE MAJOR OBJECTIVES of the Clean Water Act (Section 319) and the Coastal Zone Management Act (Section 6217) is to evaluate, demonstrate, and implement best management practices (BMPs) to improve water quality. Because applied agricultural chemicals are potential contributors to nonpoint-source pollution, it is essential to quantify each commodity's contribution to water quality problems and evaluate BMPs that have the potential to improve water quality in surface waters. In an effort to reduce soil losses and runoff of applied agricultural chemicals, various forms of soil conservation have been recommended. In fact, the effect of surface crop residues on interception, subsequent wash-off, and movement of herbicide in the soil profile is the primary focus associated with conservation measures in today's agriculture. Several conservation production systems are characterized by the presence of mulch residue left on the soil surface to protect it from water and soil erosion. In fact, several studies on best management practices have shown distinct advantages of minimum or no-till systems (Dao, 1991, 1995; Banks and Robinson, 1982).

The weed control efficacy of herbicides can also be affected by the presence of residue mulch material and is dependent on the mechanism of action of the herbicides (i.e., soil-activated, foliarly absorbed systemic, or contact). The mulch intercepts excess surface chemical spray that would otherwise be sorbed and dispersed in the soil on application. In addition, there is an added benefit of continued slow-release and increased efficiency of these herbicides leading to a potential reduction in postemergence chemical inputs as gradual desorption from the straw mulch may provide extended control of second flushes of weed emergence and growth (Dao, 1991). Dao (1991) also reported that the retention capacity of the near-surface zone could be managed to attenuate metribuzin in the field to achieve optimal herbicidal functions and dissipation. Studies on corn (Zea mays L.) and wheat (Triticum aestivum L.) have shown that crop residue is capable of greater retention of applied chemicals when compared with the soil surface layer (Boyd et al., 1990; Dao, 1995). Moreover, the fate of pesticides and their potential mobility in soils are directly influenced by their retention mechanisms. Clay and Koskinen (1990) and Ma and Selim (1996) reported that herbicide retention by soils was rapid and appeared complete within 24 h. As a result, the validity of the equilibrium approaches is often invoked. Although equilibrium models are often used to describe herbicide retention, the rate at which "apparent" equilibrium is achieved is influenced by limitations to diffusion through the sorbent as well as by specific sorbate–sorbent interactions. Nonequilibrium conditions may also be due to heterogeneity of sorption sites and slow diffusion to sites within the soil matrix, that is, slowly accessible sites (Xue and Selim, 1995; Zhu and Selim, 2000).

Over the last five years, the sugarcane industry in Louisiana has moved toward an alternative harvesting system. The traditional whole-stalk harvest system is where the whole stalks of sugarcane plants are piled, cut, burned, picked up, and transported to the mill. The new system involves the use of a combine harvester that cuts the cane stalks into billets, which are directly loaded into wagons for transport to the mill. Extractor fans in the combine separate leaf material from billets. The plant residue falls to the soil surface. It has been reported that mulch produced from the leaf material and plant residue can promote disease and lower yields in the subsequent ratoon crop (Richard, 1999). As a result, burning the leaves off the whole stalks before harvest or burning of the residue on the soil surface following harvest are measures to reduce their effect on disease and/or possible yield reduction.

A literature search revealed that there is no published research that correlates the effectiveness of mulch residue remaining on the soil surface, following sugarcane harvest, with the retention of applied herbicides, leaching losses in the runoff, and their downward movement in the soil profile. Such information is a prerequisite in quantifying the role of mulch residue in minimizing leaching losses of applied agricultural chemicals.

The objective of this study was to evaluate the effectiveness of sugarcane residue (mulch cover) in reducing nonpoint-source contamination of applied chemicals from sugarcane fields. For this purpose, we performed field investigations on sugarcane. Two treatments were investigated: (i) no-till where the mulch was not removed, and (ii) no-mulch, where the residue mulch was maintained in-furrow only. This information is essential for the implementation of control measures or corrective actions needed to reduce herbicide leaching from croplands, thus reducing a watershed's total maximum daily load (TMDL).

	MATERIALS AND METHODS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Field Investigation
In the spring of 1999, several treatments were implemented on a sugarcane (var. LCP85-384) field grown on a Commerce silt loam soil (fine-silty, mixed, superactive, nonacid, thermic Fluvaquentic Endoaquept) located south of Baton Rouge, LA. The field, which is on a private farm, was identified for the purpose of evaluating best management practices, including mulch management, where the effect of mulch on herbicide movement in surface water and weed control was quantified. The sugarcane crop was harvested in December 1998 using a combine harvester, where the mulch residue was not removed or burned. Instead, the mulch was maintained on the soil surface for weed control during the winter months.

In our study, application of chemicals was made according to Louisiana's weed control program for sugarcane, which usually requires two herbicide applications, one during the spring and another before the crop canopy closes. Sugarcane producers in southern Louisiana refer to the latter application as the "layby" treatment, which usually follows the last cultivation during the growing season. We made spring herbicide application on 26 Mar. 1999 where a mixture of atrazine and pendimethalin was applied at the rate of 1.12 and 1.1 kg ha^-1, respectively. In another treatment, rather than atrazine, metribuzin was applied as a spring herbicide (at a rate of 1.1 kg ha^-1) on the same day to test the environmental effectiveness of an alternative herbicide. Our soil management treatments consisted of "no-till," where the mulch residue was not removed and "no-mulch," where the mulch was maintained in-furrow only, that is, the mulch was raked off the row tops and incorporated within the wheel furrows during cultivation or off-barring. Herbicide application at layby was performed on 7 June 1999 with a mixture of atrazine, 2,4-D (dichlorophenoxyacetic acid), and pendimethalin applied (broadcast) at the rate of 2.5, 1.1 and 2.2 kg ha^-1, respectively. This layby application was received by all treatments.

For all treatments investigated, two plots (or replications) were implemented. Each plot consisted of three center rows 25 m in length and outlined with two border rows. In Louisiana, sugarcane is typically planted in rows 1.8 m (6 ft) apart. To monitor the rate of disappearance of applied herbicides, composite samples of mulch residue and surface soil (25-mm depth) were taken along the center row for each plot during the entire growing season. Runoff water samples were collected routinely following rainfall events that caused sufficient runoff to occur. In-row low impact flow event (LIFE) samplers designed by USDA-ARS (Tifton, GA) were installed in-row for runoff sample collection (Sheridan et al., 1996). The LIFE sampler was designed to meet the need for an unattended surface flow for riparian-buffer study areas and provided an excellent way to collect surface water samples.

In late September 1999, due to a severe wind storm and excessive rainfall, sugarcane stalks were extensively lodged and continued sampling was not possible. Burning of cane leaves from the entire field was necessary before sugarcane harvest. As a result, our mulch treatments were destroyed and mulch residue was no longer available for subsequent growing seasons. Therefore, an alternative nearby site (8.85 km south of the existing plots) was selected at the LSU AgCenter St. Gabriel Research Station, where a combine harvester was used. The plots and field instrumentation layout is described in detail in Granovsky et al. (1996). This site was also on a Commerce soil, and consisted of six 0.22-ha plots (two replications x three treatments). Each plot consisted of nine rows 150 m in length with 1.8-m spacing, with levees on each treatment. At the lowest corner of each plot a sump made of corrugated iron was dug (1.50 m in diameter, 1.80 m deep) to collect runoff water. Sump pumps were installed in each sump, equipped with flow meters and Isco (Lincoln, NE) samplers. Sugarcane (var. CP70-321) was planted in September 1997 and all sugarcane plots were combine-harvested on 7 Dec. 1999. Four plots were harvested and the mulch was not removed. The other two plots were burned with the sugarcane standing before harvest. Two plots received metribuzin at a rate of 1.0 kg ha^-1 where all other plots received atrazine at a rate of 1.1 kg ha^-1 as spring herbicide application (7 Apr. 2000). Layby application consisting of broadcast atrazine at 2.2 kg ha^-1 was carried on 5 June for all plots. Composite samples of mulch residue and surface soil (25-mm depth) were also collected along the center row for each plot in a similar manner as described above.

Laboratory Analysis
All mulch residue, soil, and runoff water samples were stored at 4°C until laboratory analysis for pesticides. In the laboratory, extraction of pesticides from soil and mulch residue was as follows. Extractions from field (moist) soil (15 g soil and 30 mL solution) was performed by using 0.01 M NaCl methanol and water solution (4:1 v/v), shaking for 24 h, centrifuging, decanting, and removing water with anhydrous sodium sulfate. The extracts were subsequently evaporated and transferred in hexane to 2.0-mL vials (Granovsky et al., 1996). Extractions from mulch-residue samples were similar to that from soil samples except that 1 g of mulch residue with 30 mL of 0.01 M NaCl methanol and water solutions was used. Runoff water samples (250 mL) were extracted using dichloromethane in a separatory funnel. The remaining steps were similar to that for soil extractions.

Extracts from mulch residue, soil samples, and runoff water were analyzed using gas chromatography. The instrument used was a Hewlett-Packard (Palo Alto, CA) 5890 Series II gas chromatograph with a split/splitless inlet, temperature programmed oven, and nitrogen–phosphorus and electron capture detectors. The column used was a Hewlett-Packard PAS-1701 capillary column (25 m in length, 0.32-mm i.d., 0.25-µm film thickness). Operating parameters were: inlet temperature at 250°C; column temperature at 80°C for 1 min, then 30°C per minute to 190°C, then 3.6°C per minute to 260°C; detector temperature at 300°C. Flow of ultra-high purity (UHP) helium through the column was 2 mL min^-1 and makeup gas (5% methane, balance argon) flow rate was 36 mL min^-1. Concentration in solution was back-calculated to a standard solution with known herbicide concentrations. Limits of detection were approximately 0.40 mg L^-1 for atrazine, 0.02 mg L^-1 for metribuzin, and 0.01 mg L^-1 for pendimethalin. The retention times for atrazine, metribuzin, and pendimethalin were 6.139 ± 0.015, 7.014 ± 0.016, and 7.807 ± 0.027 min, respectively.

	RESULTS AND DISCUSSION

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Decay of Sugarcane Mulch
To assess the effect of the presence of a surface mulch residue on the retention of herbicides, the amount of mulch was measured. Eight samples (1 x 1 m) were randomly selected and residue was collected and weighed. In 1999, only one measurement was made on 16 April, and the weight of mulch was 4.23 ± 1.19 Mg ha^-1. The mulch represented 100% coverage of the soil surface and the thickness varying from 50 to 60 mm. For the 2000 growing season, we measured the amount of mulch residue left on the field in each plot following harvest (7 Dec. 1999) when the combine harvester was used. Four plots were harvested and the mulch was not removed. The other two plots were burned with the sugarcane standing before harvest. Four additional measurements of mulch residue were taken during January, April, May, and August 2000. Due to the disappearance (decay) of the residue, no additional sampling was made thereafter. The average amount of mulch on the soil surface decreased continuously from a high of 8.04 ± 2.12 Mg ha^-1 at harvest to a low of 1.66 ± 0.32 Mg ha^-1 on 18 Aug. 2000. The mulch results are given in Fig. 1 along with one standard deviation. With the exception of the 18 August sampling date, the mulch residue maintained 100% coverage of the soil surface. It is of interest to point out that the measured amount of mulch during April 1999 and was well within that measured during 2000, as shown in Fig. 1.

View larger version (25K): [in this window] [in a new window]   Fig. 1. Amount of mulch residue remaining on the soil surface versus time during the 2000 growing season.

View larger version (32K): [in this window] [in a new window]   Fig. 2. Measured and simulated atrazine concentration versus time in the top 25 mm of Commerce soil for the no-mulch treatment (top figure) and the no-till (mulch not removed) treatment (bottom figure). Two atrazine applications were made during 1999: spring application on Day 0 (26 March) and layby (preemergence) on Day 73 (7 June). Simulations are based on best fit using the first-order decay model (Eq. [1]).

View larger version (30K): [in this window] [in a new window]   Fig. 3. Measured and predicted atrazine retained by sugarcane mulch versus time from the no-till treatment. Two applications were made during 1999: spring application on Day 0 (26 March) and layby (preemergence) on Day 73 (7 June). Simulations are based on best fit using the first-order decay model (Eq. [1]).

The large values of atrazine retained by the mulch residue over time illustrate its high retention compared with that for the soil. We are not aware of earlier studies where herbicide retention by sugarcane mulch was measured. Nevertheless, Shelton et al. (1995) reported a laboratory-measured sorption capacity for dried and ground cornstalk of 860 mg kg^-1. Our maximum extractable atrazine from sugarcane mulch did not exceed 80 mg kg^-1. To illustrate the strong sorption of atrazine by mulch residue, Abdelhafid et al. (2000) measured high K_d values for wheat straw compared with soil (15.01 versus 0.77 L kg^-1, respectively). Their results were based on 24 h of equilibration.

For the 2000 growing season, results of extractable atrazine from the surface soil and mulch residue versus time are given in Fig. 4 . These results illustrate the extensive scattering of the measured values from our field experiments. In addition, measured concentrations following each atrazine application were much lower than those observed during 1999. For the surface soil, except for times immediately following applications, a comparable concentration range for atrazine was observed (up 2.0 mg kg^-1). However, this was not the case for the mulch residue, where the concentrations were consistently lower and did not exceed 20 mg kg^-1 compared with more than 50 mg kg^-1 during 1999. Reasons for such lower concentrations are not clear and cannot be contributed to precipitation. This is because during April and May 2000, the amounts of precipitation received were 25 and 4 mm, respectively. Overall, year 2000 received 71% of normal precipitation and was the third driest year on record. Reasons for the extensive variability are perhaps due to wind drift during application, interception by the crop at layby, and soil heterogeneity, among others.

View larger version (31K): [in this window] [in a new window]   Fig. 4. Measured and predicted extractable atrazine concentration versus time from the top 25 mm of surface soil (top figure) and the mulch residue (bottom figure). Two applications were made during 2000: spring and layby applications on Day 0 and Day 59 (7 April and 5 June, respectively). Simulations are based on best fit using the first-order decay model (Eq. [1]).

[1]

where W (mg kg^-1) is herbicide concentration at time t (days) and is a dummy variable (integer) indicating the time of application. Here, = 0 for times t less than t₁ (t < t₁), and = 1 for t > t₁, where t₁ is time for the layby (or second) herbicide application. Therefore, S₁ and S₂ represent initial concentrations associated with the first and the second application, respectively. The role of the rate coefficient (d^-1) in the model is incorporated in the same way. For convenience, we set ₂ = 0 in Eq. [1], since tests for ₂ indicated that it was not significantly different from zero. Thus, one can implicitly assume that only one "overall" rate coefficient for herbicide disappearance is applicable for the growing season, as was the case in this study. As a result, in our analysis only three parameters, ₁, S₁, and S₂, need to be estimated.

The analysis of variance (ANOVA) table of the regression analysis (not shown) suggests that a first-order approach provided a good description of atrazine dissipation in surface soil as well as mulch residue based on our model. Values of the coefficient for correlation (r²) of the model to atrazine results from surface soil ranged from 0.63 to 0.82 (see Table 1) . Low r² values are indicative of the extent of scattering of atrazine results versus time. However, as indicated in Table 2 , higher r² values of 0.77 and 0.87 were obtained for atrazine in the mulch residue, which indicate that the model provides adequate description of atrazine dissipation results. Based on model estimates, S₂ was significantly different from zero for all cases considered (see Tables 1 and 2). Moreover, for the 1999 growing season, overall dissipation rates ₁ (or simply ) of 0.028 and 0.022 d^-1 were estimated based on Eq. [1] for the surface soil and mulch residue, respectively (see Tables 1 and 2). These values correspond to half-lives (t_1/2) of 26.4 and 31.8 d, respectively. For the 2000 growing season, estimates for were 0.024 and 0.018 d^-1 for the surface soil and mulch residue, respectively, and correspond to t_1/2 values of 29 and 38.3 d, respectively. Such results indicate that the rate of dissipation is consistent among the two growing seasons. Moreover, the rate of dissipation of atrazine retained by the mulch was slightly lower than that for the surface soil. Although Ghadiri et al. (1984) did not estimate retained by flat and standing wheat stubble, based on their results, we estimate half-lives of 5 to 7 and 12 to 15 d, respectively. Such values are somewhat smaller than our half-life values. We are not aware of atrazine dissipation () values for sugarcane mulch residue.

Pendimethalin
This herbicide was applied with atrazine in the same tank mixture at spring and layby (preemergence) application. Pendimethalin is effective for most annual grasses and broadleaf weeds. According to Wauchope et al. (1992), pendimethalin, which is commonly known as Prowl, has an average half-life of 90 d with a low water solubility of 0.275 mg L^-1. Solubility for atrazine and metribuzin in water are 33 and 1200 mg L^-1, respectively. Wauchope et al. (1992) also reported an average retention coefficient (K_oc) for pendimethalin as 5000 m³ kg^-1, whereas a value for atrazine was given as 100 m³ kg^-1. Such a high retention capacity is well manifested by the extremely high concentrations of extractable pendimethalin retained by the surface soil layer and mulch residue as illustrated in Fig. 5 and 6 . These results are for the no-mulch and the no-till treatments during the 1999 growing season.

View larger version (34K): [in this window] [in a new window]   Fig. 5. Measured and predicted pendimethalin concentration versus time in the top 25 mm of surface soil for the no-mulch (top figure) and no-till (bottom figure) treatments. Two applications were made during 1999: spring application on Day 0 (26 March) and layby (preemergence) on Day 73 (7 June). Simulations are based on best fit using the first-order decay model (Eq. [1]).

View larger version (29K): [in this window] [in a new window]   Fig. 6. Measured and predicted pendimethalin retained by sugarcane mulch versus time from the no-till treatment. Two applications were made during 1999: spring application on Day 0 (26 March) and layby (preemergence) on Day 73 (7 June).

Nonlinear regression was used to provide estimates for the rates of disappearance of applied pendimethalin during 1999 (see solid curves in Fig. 5 and 6) in a similar fashion to atrazine (Eq. [1]). Despite the extensive scattering, the first-order decay model was considered adequate in describing pendimethalin results versus time. Model goodness of fit to the data as indicated by the values of the coefficient of correlation (r²) are given in Tables 1 and 2. Based on model estimates, overall pendimethalin dissipation rates of 0.019 and 0.021 d^-1 were estimated for the surface soil from the no-mulch and no-till plots, respectively (see Table 1). These values correspond to half-lives (t_1/2) of 37.2 and 33.5 d, respectively. Similarity in the rate of dissipation, regardless of the presence of mulch, is perhaps indicative of the strong affinity of pendimethalin by the soil as reported in Wauchope et al. (1992). Estimates for rate of dissipation of pendimethalin retained by the mulch residue was 0.035 d^-1, which corresponds to t_1/2 of 19.6 d (see Table 2). This is a higher rate than that for the surface soil and is perhaps due to increased volatilization and/or photodegradation. Schleicher et al. (1995) reported an estimated time of 23 d to account for 50% of detectable residue (DT₅₀) of pendimethalin in the entire sampling zone including verdure, thatch, mat, and surface soil layers. Based on their results of concentration versus time, we estimate an approximate dissipation or t_1/2 of 12 to 15 d for pendimethalin retained by the thatch layer. Our estimated half-life of 19.6 d for the sugarcane mulch is somewhat longer and is possibly due to lower rate of photodegradation and volatilization. In another study, Zimdahl et al. (1994) reported half-lives for pendimethalin for two surface soils in the range of 5 to 36 d, which is consistent with our estimates.

Metribuzin
Contrary to atrazine and pendimethalin, metribuzin was applied as an alternative herbicide only once as a spring application during 1999 and 2000. As shown in Fig. 7 , considerable soil-extractable amounts of metribuzin were measured from surface soil regardless of the presence of mulch residue. Measured amounts ranged from 3.0 to 4.0 mg kg^-1 initially and continued to decrease over time reaching 0.3 to 0.4 mg kg^-1 some 60 d following application. Moreover, metribuzin results showed consistently lower extractable amounts retained by the surface soil in the presence of mulch residue compared with the treatment where the mulch was removed from the top of the sugarcane rows. Such differences were due to the interception of applied metribuzin by the mulch residue and are consistent with earlier results of Sorenson et al. (1991). In their experiment, metribuzin dissipation was measured following preemergence and split application on no-till soybean [Glycine max (L.) Merr.] in rotation with corn or wheat. Moreover, our metribuzin concentrations in the surface soil continued to decrease and reached levels below 0.1 mg kg^-1 some 100 d following application regardless whether or not the sugarcane mulch residue was removed from the top of the rows (see Fig. 7).

View larger version (29K): [in this window] [in a new window]   Fig. 7. Metribuzin concentration versus time in the top 25 mm of surface soil from the mulch and off-barred treatment (top figure) and the no-mulch treatment (bottom figure) during the 1999 growing season.

View larger version (33K): [in this window] [in a new window]   Fig. 8. Extractable metribuzin concentration versus time from the mulch residue (top figure) and the 25-mm surface soil (bottom figure) in the no-mulch and off-barred plot treatment during 2000.

[2]

which is similar to that of Eq. [1]. In addition, = 0 for the mulch treatment and = 1 for the treatment where the mulch was removed. The terms S₁ and S₁ + S₂ represent initial concentrations for mulch and no-mulch treatments, respectively. Therefore, an estimate of S₂ that is significantly different from zero signifies significant difference between the treatments. The role of the rate coefficient in the model is incorporated in the same way. Estimates for the rates of metribuzin disappearance during 1999 and 2000 are given in Tables 1 and 2. In addition, predictions based on first-order decay (Eq. [2]) are shown by the solid curves shown in Fig. 7 and 8. Based on estimates, overall metribuzin dissipation rates of 0.044 and 0.055 d^-1 were estimated for the surface soil in the presence and absence of mulch residue, respectively (see Table 1). These values correspond to half-lives (t_1/2) of 15.7 and 12.5 d, respectively. Such values indicate a slightly higher rate of metribuzin dissipation of surface soil when the mulch residue was removed. Our results are in agreement with Banks and Robinson (1982), who reported that the presence of wheat straw mulch affects the soil reception of applied metribuzin but not subsequent degradation in the soil. From our 2000 results, a half-life of 15.6 d was estimated for the surface soil. This t_1/2 value is similar to those estimated from the 1999 growing season and in the range of half-lives based on other studies. Southwick et al. (1995) reported a half-life of 22.3 d for metribuzin in a Sharkey soil. Based on reported data for several soils, Wauchope et al. (1992) reported half-life values of 40 d. Our estimated t_1/2 for metribuzin retained by the mulch residue was 24 d, which is significantly larger than that for surface soil.

Runoff Losses
Year 2000 was the third driest year on record in southern Louisiana. As a result, no runoff was collected during the 2000 growing season. Thus, only runoff results from the 1999 growing season are presented. As can be seen in Fig. 9 and 10 , most runoff events occurred following the layby application. For atrazine, concentrations in the runoff from the no-till and no-mulch treatments are shown in Fig. 9. The difference between the two treatments was that in the no-mulch treatment, the mulch was removed from the row tops and the rows were off-barred before band applications of herbicides. As illustrated in Fig. 9, the lowest average concentration of atrazine was consistently measured in the no-till treatment. Maximum concentration did not exceed 80 µg L^-1 for the no-till, whereas it reached 160 µg L^-1 when the mulch was removed, which illustrates the influence of mulch in reducing runoff losses of applied herbicides.

View larger version (27K): [in this window] [in a new window]   Fig. 9. Rainfall distribution and atrazine concentration in runoff water during 1999.

View larger version (29K): [in this window] [in a new window]   Fig. 10. Pendimethalin concentration in runoff water during 1999.

Metribuzin was only detected at extremely low concentrations in the runoff and at no time did it exceeded 10 µg L^-1 regardless of whether the mulch residue was removed from the sugarcane rows (figure not shown). Selim et al. (2000) published data showing concentrations of 200 to 300 µg L^-1 metribuzin in runoff waters 6 d following spring application in 1994 and 1995. In this study, we did not encounter runoff events until some 47 d (12 May 1999). Following such a long duration, low runoff concentrations are expected due to decreased amounts of metribuzin concentrations retained by the surface soil as well as mulch residue.

Several studies measured high initial concentrations of herbicides in runoff waters from soils under various management practices (Southwick et al., 1992; Isensee and Sadeghi, 1999). For example, Isensee and Sadeghi (1993) quantified concentrations and runoff amounts of atrazine, alachlor, and cyanzine herbicides from no-till (NT) and conventional tillage (CT) 0.25- to 0.50-ha plots. All herbicides were tank-mixed and applied (broadcast) uniformly across the plots one day after planting. They found that atrazine concentrations were 2 to 10 µg L^-1 higher in runoff water from the NT compared with the CT plots. The highest atrazine concentrations occurred 3 d following applications and ranged from 2061 to 2733 µg L^-1 for NT and 773 to 989 µg L^-1 for CT. Such concentrations are considerably higher than that measured in our experiment. Isensee and Sadeghi (1993) also reported that the values decreased to 36 µg L^-1 for NT and 3 to 4 µg L^-1 for CT some 34 d following applications. Our runoff results shown in Fig. 9 are consistent with those reported for broadcast applications by Isensee and Sadeghi (1993).

	CONCLUSIONS

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
In this study we evaluated the effectiveness of sugarcane residue (mulch cover) in reducing nonpoint-source contamination of applied herbicides from sugarcane fields. For this purpose, two main treatments were investigated: no-till (100% mulch) and no-mulch treatments. The effect of mulch residue on herbicide retention was quantified following spring as well as postemergence (layby) applications. The amounts of extractable atrazine, metribuzin, and pendimethalin from the mulch residue and the surface soil layer were quantified during 1999 and 2000 growing seasons. The mulch residue intercepted significant amounts of applied herbicides. Extractable concentrations were at least one order of magnitude higher for the mulch residue compared with that retained by the soil. Nonlinear regression was used to provide estimates for rates of decay or disappearance of the herbicides retained by the surface soil as well as the mulch residue. The presence of mulch residue resulted in consistently lower rates of disappearance of atrazine and pendimethalin in the surface soil. Moreover, the presence of mulch residue on the sugarcane rows was highly beneficial in minimizing runoff losses of the herbicides applied. A minimum of 50% reduction in runoff effluent concentrations for atrazine and pendimethalin was realized when the mulch residue was not removed.

	ACKNOWLEDGMENTS

 
The authors wish to thank J.L. Griffin, R.L. Bengtson, Cornelia Loeser, M. Sandeep, and the staff at the St. Gabriel Research Station, LSU Agricultural Center, for their assistance in this study. This study was funded in part by the Nonpoint Source Program, Louisiana Department of Environmental Quality (Contract no. 24400-93-32), Andrew Barron, project officer.

	NOTES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 
Approved by the Director of the Louisiana Agricultural Experiment Station as Manuscript no. 02-14-0886.

	REFERENCES

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS AND DISCUSSION CONCLUSIONS REFERENCES

 

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