| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
a Dep. of Soil & Crop Sciences, Colorado State University, Fort Collins, CO 80523-1170
b USDA-Agricultural Research Service, Southeast Watershed Research Laboratory, Tifton, GA 31793-0946
c Dep. of Civil Engineering, Colorado State University, Fort Collins, CO 80523-1372
d USDA-Agricultural Research Service, Great Plains Systems Research Unit, 2150 Centre Ave., Bldg. D, Ste. 200, Fort Collins, CO 80526
e Dep. of Civil Engineering, Colorado State University, Fort Collins, CO 80523-1372
* Corresponding author (jessica.davis{at}colostate.edu)
Received for publication September 12, 2005.
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
|---|
Abbreviations: TC, tetracycline • CTC, chlortetracycline • STZ, sulfathiazole • SMZ, sulfamethazine • ERY, erythromycin • TYL, tylosin • MNS, monensin • OTC, oxytetracycline • LOQ, limit of quantification • P-PC, pseudo partitioning coefficient • HPLC, high performance liquid chromatography
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
|---|
Antibiotics in the environment may lead to the development of antibiotic resistance, a critical concern as it relates to the efficacy of antibiotics in the treatment of human disease (Boxall et al., 2003). In addition to human health concerns, there are possible toxic effects of antibiotics on aquatic and edaphic organisms. Although most measured concentrations have been well below (an order of magnitude lower) levels that have been shown to be toxic to standard testing organisms, degradation products and interactions among compounds have not been adequately evaluated and could result in synergistic toxic effects (Boxall et al., 2003).
Sulfonamides, including sulfamethazine (SMZ) and sulfathiazole (STZ), have been detected at concentrations as high as 20 mg kg–1 in liquid manure samples (Haller et al., 2002). Macrolides and ionophore polyether groups were measured in liquid manure, and maximum concentrations were 43 µg tiamulin kg–1 and 11 µg salinomycin kg–1 (Schlusener et al., 2003). Campagnolo et al. (2002) measured five groups of antibiotic residuals, including tetracycline (TC), chlortetracycline (CTC), SMZ, and erythromycin (ERY) in animal waste, surface water, and groundwater resources near large-scale swine and poultry feeding operations. The compound with the highest concentration in lagoons was CTC with 1000 µg L–1, followed by 400 µg L–1 of SMZ. Also, 31 and 67% of antibiotics tested were detected in water samples (surface water, tile drainage, and wells) taken near swine and poultry farms, respectively. This study indicates that animal waste stored in lagoons or applied to agricultural fields may be a source of veterinary antibiotics entering the environment.
Yang and Carlson (2003) measured tetracycline and sulfonamide levels in the Cache la Poudre River in northern Colorado; they reported no detectable concentrations in a pristine location in the mountains, but they did detect tetracyclines and sulfonamides in other sampling locations in both urban and agricultural areas. Another study measured residuals of tetracyclines and tylosin (TYL), which are widely used in livestock production, in soil fertilized with animal slurry (Hamscher et al., 2002). Average concentrations of 198.7 µg TC kg–1 and 4.6 to 7.3 µg CTC kg–1 at a soil depth of 10 to 20 cm were found; tetracyclines were not detected below a 30-cm depth, and TYL was not detected in any soil or groundwater samples. This study confirmed that some antibiotics can enter the environment in significant concentrations, and antibiotic residues can accumulate in the soil when liquid manure is applied repeatedly.
Little research has been published that evaluates transport of veterinary antibiotics from agricultural fields to surface water. Addison (1984) developed a model to predict antibiotic transport in runoff and sediments from feedlots; however, the model was not tested with actual field data. A few recent studies have evaluated antibiotic leaching potential (Kay et al., 2005a, 2005b; Kreuzig and Höltge, 2005). Boxall et al. (2002) and Kay et al. (2004) have reported antibiotic leaching to tile drainage systems which then flow to surface water. Kay et al. (2005c) compared three antibiotics in their overland flow concentrations after "irrigation" with a knapsack sprayer. In this unreplicated study, the researchers detected sulfachloropyridazine and oxytetracycline (OTC) in runoff, but tylosin (TYL) was not detected. Burkhardt et al. (2005) reported sulfonamide concentrations in runoff from manured grassland following irrigation applied at the intensity of a heavy rainstorm. Kreuzig et al. (2005) compared sulfonamide transport by runoff from a tilled field to that from grassland and found that tillage reduced runoff volume and, hence, also reduced sulfonamide losses. Kreuzig et al. (2005) also used sprinkler irrigation to initiate runoff and study antibiotic transport, but based on their description, it appears that they used a single-nozzle rainfall simulator calibrated to be similar to rainfall in droplet size and kinetic energy. This is an important distinction because irrigation systems have different drop size distributions, and thus kinetic energies, compared to natural rainfall and rainfall simulations. More importantly, the distribution of water from irrigation systems is quite erratic compared to that of natural rainfall and simulations. Most irrigation systems apply water in "waves" so that after water hits the ground, there is a pause before the next pulse of irrigation water. This difference has a strong influence on runoff and sediment delivery (Römkens et al., 2002). A backpack sprayer does not apply water to the entire plot at the same time and, thus, introduces additional variance from natural rainfall. Our study is unique due to the use of rainfall simulation (not irrigation), the range of antibiotic chemistries evaluated (seven antibiotics from four classes), and the determination of sediment-to-solution partitioning of antibiotic losses.
Rainfall simulation experiments have been conducted to evaluate the transport of pesticides (Truman et al., 1998; Ma et al., 1999; Wesenbeeck et al., 2001; Li et al., 2002; Potter et al., 2003; Muller et al., 2004; Potter et al., 2004; Wauchope et al., 2004). In general, transport behavior of pesticides is mainly influenced by rainfall intensity, soil properties, and pesticide characteristics. Halling-Sørensen et al. (1998) concluded that since they have similar physicochemical properties, antibiotics are expected to have the same fate as pesticides if manure (containing antibiotics) is spread on fields just before a rainfall event. Kay et al. (2004) agreed that processes governing pesticide fate also apply to antibiotics. Therefore, in this study, we utilized simulated rainfall in 6-m2 plots, a methodology common to pesticide transport studies (Potter et al., 2003; Potter et al., 2004; Wauchope et al., 2004), to examine runoff, soil loss, and associated transport of seven selected veterinary antibiotics.
The basic chemical structure of the seven veterinary antibiotics evaluated is shown in Fig. 1 and 2. Tetracyclines are differentiated by various substitutions in the backbone (Lock et al., 1999; Bruno et al., 2002). Selected sulfonamide compounds have two N functional groups, and most sulfonamides are positively charged under acidic conditions and negatively charged under alkaline conditions (Haller et al., 2002). Acid dissociation constants (pKa) for tetracyclines and sulfonamides are 3.3/7.3/9.1 and 2.5/5.5–7, respectively. Macrolides like ERY and TYL are composed of large lactone rings substituted with hydroxyl, alkyl, and ketone groups. Neutral and amino sugars are bound to the nucleus by the substitution of hydroxyl groups (McArdell et al., 2003). Monensin (MNS), an ionophore, consists of a carboxylic polyether backbone that forms pseudo-macro cyclic complexes with cations (Volmer and Lock, 1998).
|
|
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
|---|
) from a Millipore (Billerica, CA) purification system was used when deionized (DI) water was required. Liang et al. (1998) documented the degradation of tetracycline in methanol solutions stored at room temperature for 6 mo. To avoid degradation, standard solutions (100 µg mL–1) of seven antibiotics (TC, CTC, STZ, SMZ, ERY, TYL, and MNS) were prepared in methanol and stored at 4°C for a maximum of 1 mo. Working solutions, 5 mg L–1 and 0.5 mg L–1, were prepared weekly by dilution of the standard solution in methanol.
Experimental Site
The research site was located in north-central Colorado at the Colorado State University Agricultural Research, Development, and Educational Center (ARDEC) north of Fort Collins, CO (Township 8N, Range 68W, and the SW quarter of section 15). The soil was a Fort Collins sandy clay loam (fine-loamy, mixed, superactive, mesic Aridic Haplustalf) with a 2% slope. Surface horizon properties were 550 g sand kg–1, 160 g silt kg–1, 290 g clay kg–1 (Gee and Or, 2002), pH 7.9 (Thomas, 1996), EC of 1.19 dS m–1 (Rhoades, 1996), 18 g OM kg–1 (Nelson and Sommers, 1996), cation exchange capacity (CEC) of 24.2 cmoles kg–1 (Sumner and Miller, 1996), and 44 g CaCO3 kg–1 (Loeppert and Suarez, 1996).
In the 4 yr before this study, sprinkler-irrigated corn (Zea mays L.) had been grown for grain under conventional tillage. The site was prepared to simulate typical soil preparation for corn planting. On 24 June 2004 glyphosate was sprayed at 3.45 L ha–1, on 6 July the land was disked twice, on 7 July fertilizer was applied at a rate of 88 kg P2O5 ha–1 (0–45–0) and incorporated with a springtooth cultivator followed by 1.3 cm irrigation, on 13 July the area was harrowed, and then a planter (John Deere 7300 Maxemerge) was run through the plots empty to simulate planting conditions.
Antibiotic Application Method
To evaluate a worst-case scenario, antibiotics were applied 1 h before rainfall simulation using a backpack sprayer. This brief period between application and rainfall simulation minimized photo-degradation losses. Based on similar studies of pesticide transport (Leonard, 1990; Potter et al., 2003), we expected that the 1-h delay would maximize antibiotic concentrations in runoff. Kay et al. (2004) found that antibiotic concentrations in tile drainage declined with time after application. Kreuzig and Höltge (2005) also reported a high leaching risk for sulfadiazine (a sulfonamide antibiotic) only when antibiotic application was followed immediately with heavy precipitation. On the other hand, Burkhardt et al. (2005) found higher sulfonamide concentrations in runoff from manured grassland when irrigation occurred 3 d after application than when irrigation was applied 1 d after application.
An antibiotic solution was prepared by dissolving seven antibiotics (TC, CTC, STZ, SMT, ERY, TYL, and MNS) in methanol (2.5% volume/volume of final volume) and then bringing the solution to volume with DI H2O to achieve a 1.0 mg L–1 concentration for each of the antibiotics. These antibiotics were chosen to represent a variety of classes: tetracyclines (TC, CTC), sulfonamides (STZ, SMT), macrolides (ERY, TYL), and ionophores (MNS) with a range of properties (Table 1). A backpack sprayer with 4 nozzles (#8005 with screen) on a 1.5-m boom was operated at 207 kPa (30 psi), the operator's pace was calibrated using a metronome, and two passes were made to achieve an application rate of 209 mg per plot. This application rate was based on measured levels of antibiotics in 18 hog and dairy lagoons (Carlson et al., 2004) with the goal of applying the approximate amount of antibiotics that would normally be applied when utilizing liquid manure at 220 kg N ha–1, a typical N application rate for corn, assuming 0.6 g N L–1 effluent (Davis et al., 2002). We assumed an average concentration of 1 mg L–1 for all antibiotics in manure effluent; this value is approximately 90% of the average measured CTC concentration in liquid manure, three times the average measured STZ concentration, and 100 to 1000 times the average concentrations for the other antibiotics (Carlson et al., 2004).
|
Rainfall Simulations
Three rainfall simulation plots (6 m2; 2 m wide by 3 m long) were established within a 0.5-ha area at the ARDEC complex (19 July 2004). The plots were established parallel to the slope (
2%) and row direction, and consisted of two half-beds containing one row each (76-cm row spacing) and a wheel-track middle. A 3-m-wide by 4-m-long area surrounding each 6-m2 simulator plot was tilled and received antibiotic application identical to the test area to allow soil material to be splashed in all directions. Immediately before simulating rainfall, soil water content was determined gravimetrically (Gardner, 1986) from samples taken from three areas outside of each 6-m2 rainfall simulation plot. Gravimetric soil water samples averaged 31 (CV = 13%) and 189 (CV = 3%) g water per kg dry soil in 0- to 2- and 2- to 15-cm depth increments, respectively.
Simulated rainfall was applied to each 6-m2 plot with an oscillating nozzle rainfall simulator (Frauenfeld and Truman, 2004) that used 80150 Veejet nozzles (median drop size = 2.3 mm). The simulator was placed 3 m above each plot. Well water was used in all simulations (pH = 8.0, EC = 0.10 dS m–1). Simulated rainfall was applied in a variable rainfall intensity pattern (Fig. 3) obtained from 15-min natural rainfall data collected at Fort Collins (1975 to 2002) and Byers (1971 to 2002), Colorado (EarthInfo, 2004). These two sites represent all northeastern Colorado climate stations with at least 10 yr of data available. Rainfall during the months of March through August were analyzed to determine the most extreme event occurring during the row-crop growing season over a >30 yr period, and the simulated worst-case rainfall pattern was based on this most extreme rainfall event. Rainfall duration for each simulation was 60 min.
|
Sample Pretreatment
Runoff samples were transferred into 50-mL centrifuge tubes for separation of the aqueous and solid phases before antibiotic analysis. Samples were centrifuged at 1917 g for 30 min. The aqueous phase was filtered through a 0.2-µm pore size glass fiber filter and decanted to another 1-L sampling bottle. The solid phase was collected in plastic bags. Both aqueous and sediment samples were kept in a cooler during the experiment and transferred to a refrigerator the same day. Solid phase samples were air-dried in a dark room to prevent photodegradation.
Sample Extraction
Aqueous samples were cleaned and enriched using solid phase extraction (SPE) as described by Lindsey et al. (2001) and Yang and Carlson (2003). A 3 mL/60 mg capacity HLB cartridge was used to extract the applied compounds. The HLB cartridge was known to have a durable capacity for both hydrophilic and hydrophobic compounds and was also suitable for TC to avoid the irreversible binding of this compound to silanol groups. One hundred twenty mL of sample was carefully measured into a flask, and the pH was adjusted to 2.3 to 2.5 for TC, CTC, STZ, and SMZ using 40% (volume/volume) H2SO4 solution. For ERY and TYL, the pH was adjusted to 5.0. This was followed by the addition of 0.5 mL of 0.02 M Na2EDTA to chelate any metals in the sample. No pH adjustment or addition of Na2EDTA was used for analysis of MNS.
Before loading the sample, the HLB cartridge was preconditioned with 3 mL of methanol, then 3 mL of 0.5 M HCl, followed by 3 mL of DI water. Methanol and 0.5 M HCl were used to activate the cartridge and to remove any metal residuals in the cartridge, respectively, and DI water was used to remove impurities in the cartridge. The sample was loaded under a 276 kPa (40 psi) vacuum with a flow rate of approximately 2 mL min–1. After loading the sample, the SPE cartridge was rinsed with 9 mL of water to remove any impurities and excess Na2EDTA. For quantification of the analytes in the samples, 50 µL of the internal standard, simatone (0.24 mg L–1 stock solution), was added to a 15-mL graduated cylinder before adding the extract sample to the cartridge. Simatone was chosen as the internal standard based on a previous study and has shown good response to our experimental conditions (Lindsey et al., 2001). Five mL of methanol was used to extract the loaded sample into the cylinder, and then the sample was concentrated with evaporation in a 50°C water bath under a gentle N gas flow to prevent oxidation. Samples were concentrated to a volume of 50 µL, and 70 µL of mobile-phase solution (0.01 M formic acid) were added. Samples were transferred into amber vials equipped with 150-µL glass inserts for high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) analysis.
For sediment samples, two stages of analytical preparation were conducted. The first stage was to extract compounds from the solid phase into the liquid phase. McIlvain buffer solution (pH 4.0) was used for TC, CTC, STZ, and SMZ, and ammonium hydroxide buffer solution (1 M, pH 10.0) was used for ERY, TYL, and MNS to extract each of the compounds from the solid phase into the liquid phase. McIlvain buffer solution was prepared according to USDA (United States Department of Agriculture) guidelines (USDA, 2003). Ammonium hydroxide buffer solution (1 M) was titrated with formic acid to adjust the pH to 10.0.
One g of sediment sample was weighed with a precision of 0.001 g and transferred into 40-mL vials. Twenty mL of McIlvain buffer solution or ammonium hydroxide buffer solution were added followed by 200 µL of 0.02 M Na2EDTA. The sample was vigorously mixed in the parallel shaker (Model No-4626, Lab-line instrument) for 20 min at 400 excursions min–1. The sample was then centrifuged at 1434 g (IEC Clinical Centrifuge, International Equipment Co., Needham Heights, MA) for 15 min followed by filtration using 0.2-µm glass fiber filters. The filtered sample was decanted into another 40-mL vial and kept in the refrigerator. The extraction was repeated once in the same manner as described above, and supernatants were combined for the SPE cleanup procedure.
The second stage of the analytical preparation was cleanup and concentration of the sample. The SPE condition for this cleanup and concentration step was the same as the aqueous sample except the addition of 0.5 M HCl was omitted.
High Performance Liquid Chromatography-Tandem Mass Spectrometry
The HPLC analyses were performed using an HP 1100 Series Liquid Chromatograph (Agilent, Palo Alto, CA) equipped with an Agilent 1100 Series Thermostatted Auto Sampler and a variable-wavelength UV detector. An XTerra MS C18 (Waters, Milliford, MA) 2.1 x 50 mm (2.5-µm pore size, end-capped) reversed-phase column was used to analyze the standards and samples. A C18 guard column (Phenomenex, Torrence, CA, USA) was used to filter any particulates from the sample.
A combination of three mobile phases was used depending on the antibiotic group being measured. Mobile phase A was composed of 0.01 M formic acid (pH 2.74), mobile phase B was 9.5 M acetonitrile mixed with 0.01 M formic acid, and mobile phase C was pure methanol. The column temperature was set to 15°C for tetracycline and sulfonamide measurement, and the gradient for tetracycline was ramped from 96% mobile phase A and 4% mobile phase B to 70% mobile phase A and 30% mobile phase B for 19 min and then 96% mobile phase A and 4% mobile phase B for 1 min. The gradient for measurement of sulfonamides was programmed for 21 min with the same conditions as for tetracyclines. For macrolides, the column temperature was set to 45°C, and the gradient was programmed to ramp from 80% mobile phase A and 20% mobile phase B to 65% mobile phase A and 35% mobile phase B for 14 min and then to the original condition for 1 min. Ten min of post-run time was used to equilibrate the column. The flow rate was set at 0.32 mL min–1, and the injection volume was 20 µL for tetracyclines, sulfonamides, and macrolides.
For ionophore polyethers, the column was maintained at 15°C with a flow rate of 0.25 mL min–1. The gradient was ramped from 50% mobile phase A and 50% mobile phase C to 10% mobile phase A and 90% mobile phase C in the first minute and held isocratic for 19 min. The injection volume was 20 µL, and a 10-min post-run period was allowed between each analysis to re-equilibrate the column.
A ThermoFinnigan LCQ Duo ion-trap mass spectrometer (ThermoQuest, Woburn, MA) equipped with a heated capillary interface and electrospray ionization was used to perform the mass spectrometric analysis. Ten-µM standard solutions of tetracyclines, sulfonamides, macrolides, and ionophore polyethers were made in DI water and injected using the LCQ Duo syringe pump at a flow rate of 5 µL min–1 to optimize the mass spectrometry parameters as needed. Nitrogen gas was used for drying and nebulizing. The spray voltage was set to 4.5 kV, and the capillary voltage autotuned to 21 V. The capillary temperature was set to 165°C, and the instrument was operated in the positive ion mode. The sheath gas flow rate was optimized at 40 units, and the auxiliary gas was turned off. The precursor mass and product ion optimized tandem mass spectrometry parameters are summarized in Table 2, and the recoveries and limits of quantification (LOQ) for these analyses are shown in Table 3.
|
|
A mixed model (SAS, 2002) was utilized to complete an analysis of variance evaluating the main effects of antibiotic, replicate, and sampling time on antibiotic concentrations and P-PCs (repeated measures). If the main effect was significant (p < 0.05), then least significant differences (LSDs) were used to compare the least-square means of the antibiotic concentrations and P-PCs. In addition, a general linear model was run within each interval to evaluate differences among antibiotic concentrations and P-PCs.
The absolute antibiotic losses and the relative losses (% of applied and % associated with sediment) were analyzed in a similar manner to concentrations and P-PCs using a mixed model; however, for these parameters, there were no repeated measures, just one data point per plot.
|
RESULTS AND DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
|---|
10%.
|
Runoff and sediment concentrations averaged over time were significantly different among antibiotics (Table 5). Aqueous concentrations were greatest for MNS and least for the tetracyclines and TYL. On the other hand, sediment concentrations were highest for ERY, followed by MNS and then TYL. The average sediment concentration for SMZ was below the LOQ and, therefore, cannot be distinguished from zero.
|
|
There were also significant differences among antibiotics in the absolute losses measured in runoff and erosion (Table 6). The MNS was lost in the greatest amounts, followed by ERY, and TC and CTC were lost in the least amounts. The same differences were found when relative loss was calculated as a percentage of the measured amounts applied. Note that all of the antibiotics lost <0.1% of the total applied in this 1-h extreme rainfall event only 1 h after antibiotic application. Kay et al. (2005c) also reported losses <0.1% for OTC and sulfachloropyridazine and undetectable losses of TYL from their plot without a tramline (comparable to our plots). Kreuzig et al. (2005) and Burkhardt et al. (2005) measured slightly higher runoff losses (2.5%) for sulfonamides applied to cropland.
|
In general, the values reported here for relative losses associated with sediment tend to be higher than those in the literature, probably due to the field condition being recently tilled with no plant cover, a worst-case scenario. Kreuzig et al. (2005) stated that the sulfonamides lost in runoff occurred predominantly in the aqueous phase, and losses via suspended matter were of low relevance (<1% of absolute loss). Kay et al. (2005c) also reported particulate losses of a sulfonamide to be only 1.1% of absolute loss, but the field was in wheat stubble, thus reducing erosion potential. We evaluated different sulfonamides than Kreuzig et al. (2005) or Kay et al. (2005c) in our study on bare, tilled soil and found the relative losses associated with sediment to be 5% and 23% for SMT and STZ, respectively (Table 6).
Few comparable data exist for tetracyclines. Kay et al. (2005c) measured a relative loss for OTC associated with sediment to be 7.4% on a field in wheat stubble. We evaluated TC and CTC on bare ground and found much higher relative losses, 65% and 34%, respectively. The ground cover and kind of soil have critical impacts on runoff and soil loss. In addition, antibiotics from the same class can vary considerably in their chemical properties.
Although concentration maxima tended to occur within the first 5 to 10 min of the rainfall simulations (Fig. 4), the maximum rates of mass loss occurred later (Fig. 5) due to the occurrence of peak runoff and soil loss values at 20 to 25 min (Fig. 3). The aqueous loss rates peaked at 20 min for TC, ERY, TYL, and MNS, and at 25 min for CTC, STZ, and SMT (Fig. 5). The maximum sediment loss rates occurred at 25 min for all of the antibiotics measured except for ERY, which peaked at 20 min.
|
|
|
This study evaluated the transport of antibiotics applied directly to soil in an aqueous matrix. Burkhardt et al. (2005) demonstrated that pig slurry application to soil increased runoff volume, apparently due to surface sealing and reduced infiltration that resulted in increased sulfonamide losses compared to a control that did not receive slurry. Davis-Carter and Burgoa (1993) revealed a similar impact on atrazine runoff from soil-applied dairy effluent. In addition, Burkhardt et al. (2005) found that manure application also affected the relative mobility of the sulfonamides. Hence, the manure matrix has both physical and chemical effects on environmental fate. Kreuzig and Höltge (2005) also reported that the manure matrix effect may have an important impact on sulfonamide mobility in soils. On the other hand, Kay et al. (2005b) concluded that pig slurry had no impact on OTC leaching. Therefore, the importance of the matrix effect may vary depending on antibiotic chemistry. In the future, antibiotic transport should be monitored from fields receiving manure applications to evaluate the impact of the manure matrix on transport characteristics.
|
CONCLUSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
|---|
If agricultural antibiotic runoff is proven to result in the development of antibiotic resistant genes or toxicity to aquatic organisms, then management practices will be needed to reduce antibiotic runoff. Erosion control practices are predicted to result in reduced loss of TC, ERY, and TYL to surface water bodies, but other tested antibiotics probably will not respond to soil conservation practices alone. Other methods will need to be developed in addition to erosion control practices to reduce the off-site transport of the other tested antibiotics to the environment.
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONCONCLUSIONREFERENCES |
|---|
This article has been cited by other articles:
|
|
 |
|
  H. Dolliver, K. Kumar, S. Gupta, and A. Singh Application of Enzyme-Linked Immunosorbent Assay Analysis for Determination of Monensin in Environmental Samples J. Environ. Qual., May 1, 2008; 37(3): 1220 - 1226. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
H. Dolliver and S. Gupta Antibiotic Losses in Leaching and Surface Runoff from Manure-Amended Agricultural Land J. Environ. Qual., May 1, 2008; 37(3): 1227 - 1237. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
H. A.S. Dolliver and S. C. Gupta Antibiotic Losses from Unprotected Manure Stockpiles J. Environ. Qual., May 1, 2008; 37(3): 1238 - 1244. [Abstract] [Full Text] [PDF]
|
|
|
|
|
|
H. Dolliver, S. Gupta, and S. Noll Antibiotic Degradation during Manure Composting J. Environ. Qual., May 1, 2008; 37(3): 1245 - 1253. [Abstract] [Full Text] [PDF]
|
|
| |||||||||||||||||||||||||||||||||||||||||||||||
) and erosion rates (
) for a variable-intensity simulated rainstorm.
