| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
a U.S. Meat Animal Research Center, P.O. Box 166, Spur 18D, Clay Center, NE 68933-0166
b Agroecosystem Management Research Unit, 120 Keim Hall, Univ. of Nebraska-East Campus, Lincoln, NE 68583-0934. Mention of trade names or commercial products in this article is solely for the purpose of providing specific information and does not imply recommendation or endorsement by the U.S. Department of Agriculture
* Corresponding author (Elaine.Berry{at}ars.usda.gov).
Received for publication February 22, 2007.
 |
ABSTRACT |
|---|
 TOPNOTES ABSTRACT INTRODUCTIONMaterials and MethodsResults and DiscussionConclusionsREFERENCES |
|---|
Abbreviations: VTA, vegetative treatment area • CFU, colony forming units • IMS, immunomagnetic separation • PCR, polymerase chain reaction • PBS, phosphate-buffered saline
|
INTRODUCTION |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMaterials and MethodsResults and DiscussionConclusionsREFERENCES |
|---|
Vegetative treatment systems for management and treatment of feedlot runoff are often proposed as economical and effective alternatives to traditional pond storage and land application systems. While different designs exist, a typical vegetative treatment system consists of a settling basin for solids collection and a vegetative treatment area (VTA) for water and nutrient utilization. Types of VTAs include sloped VTAs, terraces, infiltration basins, and constructed wetlands, and vegetation may include aquatic plants (in the case of wetlands or basins), trees, and grasses, including grass hay (Henry et al., 2006). Various studies have examined the abilities of constructed wetlands to remove fecal indicator bacteria and pathogens from agricultural and domestic wastewater (Hill and Sobsey, 2001; Thurston et al., 2001).
Woodbury et al. (2002, 2003) have designed and investigated a passive beef feedlot runoff control–vegetative treatment system as an effective and sustainable alternative to long-term lagoon storage of runoff. Important system design considerations included elimination of long-term, high volume liquid storage, reduced construction costs, as well as minimization of both the time and the need for specialized equipment for operation and maintenance. With regard to reducing nutrient discharge and reducing the total volume of liquid storage, this system appears to be an improvement over traditional runoff systems (Woodbury et al., 2002, 2003); however, the occurrence and fate of zoonotic pathogens in this system had not been determined. Both Escherichia coli O157 and Campylobacter spp. are regularly isolated from cattle and the production environment, and human foodborne disease caused by these bacteria is commonly associated with the consumption of beef and raw milk (Centers for Disease Control and Prevention, 1993, 2002; Shere et al., 1998; Bae et al., 2005). Furthermore, a large waterborne illness outbreak caused by both E. coli O157 and Campylobacter jejuni was linked to municipal water supply contamination by runoff from land-applied bovine manure (O'Connor, 2002). The protozoan pathogens Giardia and Cryptosporidium are common causes of human waterborne gastrointestinal disease (Leclerc et al., 2002). In addition, both of these parasites are recognized as significant agents of diarrhea in cattle, especially in young calves (Olson et al., 2004). Cattle are frequently infected with these microorganisms and can shed Giardia cysts and Cryptosporidium oocysts in high numbers (Olson et al., 2004). The objectives of this work were to examine the incidence, transport, and survival of the bacterial pathogens E. coli O157 and Campylobacter spp. in the feedlot runoff control-vegetative treatment system. Generic E. coli populations also were determined, as a means of assessing how similar Gram-negative enteric pathogen populations would fare. In addition, the occurrence and levels of Cryptosporidium oocysts and Giardia cysts in selected system samples were assessed.
|
Materials and Methods |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMaterials and MethodsResults and DiscussionConclusionsREFERENCES |
|---|
|
|
Bromegrass hay was harvested from the VTA once annually during the study, in June. Mowing and windrowing were initiated at the north end of the VTA and proceeded to the south end, and were oriented perpendicular (east-west) to the discharge pipes (Fig. 1). Hay samples were collected immediately following cutting and periodically after baling into large round bales, up to 104 d after cutting. Following each cutting, fifteen loose hay samples were collected at evenly spaced intervals across the regions of the VTA that had received basin discharge during the study period (treatment hay), which was approximately the south half of the VTA. Similarly, control hay samples were fifteen loose hay samples collected from across the north half of the VTA, which did not receive basin discharge during the study. Following baling, the bales were segregated according to the VTA region from which the hay was cut. Loose hay was collected and cut into approximately 12-cm-long pieces using clean, ethanol-sanitized scissors before placement into a sterile sample bag. Baled hay samples were taken using a bale corer, which was sanitized with 70% ethanol and wiped dry between uses. Coring was done at the bale ends, with the corer oriented at a 45° angle from the horizontal (up). Two samples were collected from each bale, placed in sterile sample bags, and examined separately.
Following the annual removal of basin solids in September 2004, nine soil cores were taken from the basin. Taking care to avoid cross-contamination with the outside portion of the cores, subsamples from the inside of the soil cores were recovered at depths of up to approximately 0.91 m for use in microbial analyses.
Adequate rainfall rate and volume accompanied by high runoff flow rates are required to trip the sample collectors located at the Parshall flumes (Woodbury et al., 2002). When available, runoff entering and/or exiting the basin (pen runoff and basin discharge, respectively) that was collected at the flumes was subjected to microbial analyses.
Microbiological Analyses
For isolation of E. coli O157, samples were subjected to nonselective enrichment and immunomagnetic separation (IMS) procedures as described by Berry and Miller (2005). Briefly, 10-g samples of soils, basin solids, manure, hay, basin liquid (10 mL), or runoff (10 mL) and 90 mL of tryptic soy broth (TSB; Becton, Dickinson and Company, Sparks, MD) were measured into a sterile filtered sample bag and the bag contents were blended with a Stomacher 400 Circulator (200 rpm, 30 sec; Seward Limited, London, UK). A 1.2-mL volume was removed to a sterile tube for use in generic E. coli enumeration as described below. The remaining sample mixture was incubated at 37°C for 7 h. Following this incubation, a 1-mL volume of the sample enrichment was subjected to IMS using Dynabeads anti-E. coli O157 (Dynal Biotech ASA, Oslo, Norway) and plating onto sorbitol MacConkey agar plates containing 0.05 mg L–1 cefixime and 2.5 mg L–1 potassium tellurite. The plates were incubated at 37°C for 22 to 24 h and examined for suspect colorless, sorbitol negative colonies of 1 to 2 mm in diameter, which were tested with E. coli O157 latex agglutination reagents (Oxoid Limited, Basingstoke, England). Identities of presumptive isolates were confirmed using standard biochemical procedures (Hitchins et al., 1998) and by PCR screening for genes specific for enterohemorrhagic E. coli and E. coli O157 (Paton and Paton, 1998).
Procedures similar to those described by Berry et al. (2006) were used for isolation of Campylobacter spp. from runoff system samples. One-g samples of soils, basin solids, or manure, or 1-mL samples of basin liquid or runoff were added to 13.5 mL of Bolton selective enrichment broth (BEB; Oxoid) in a sterile 15-mL conical tube. For hay samples, 1 mL of the initial 10–1 TSB dilutions of the hay (see above; before incubation) were placed into the BEB tubes. The BEB tubes were incubated at 37°C for 4 h, followed by incubation at 42°C for 20 h, after which 20 µL were streaked onto Campy-Cefex agar plates (Stern et al., 1992) and incubated microaerobically at 42°C for 48 h in anaerobic jars containing CampyGen (Oxoid). Suspect colonies were tested using PANBIO-Campy (jcl) latex agglutination reagents (PANBIO, Inc., Columbia, MD) and further confirmed by PCR screening for genes specific for the thermophilic Campylobacter spp. (Eyers et al., 1993; Fermér and Olsson Engvall, 1999).
For determination of generic E. coli concentrations, the retained volumes of the initial 10–1 TSB dilutions of soils, basin solids and liquid, and hay were serially diluted further if necessary in 2% buffered peptone water and 1 mL volumes were plated onto Petrifilm E. coli/coliform count plates (3M Microbiology Products, St. Paul, MN), which were incubated at 37°C for 24 h before counting characteristic E. coli colonies (Berry and Miller, 2005).
From September 2003 to January 2004, the collected manure samples were examined for the presence of Cryptosporidium oocysts and Giardia cysts. Analyses of the runoff system samples for these protozoan pathogens were conducted from May 2004 through April 2005. For these analyses, 10 g of each of the five basin solids samples were pooled and mixed. In addition, 200 mL of each of the five basin liquid samples were combined into a 1-L pooled sample. Equivalent amounts of five each of the 10 VTA soil samples were combined and mixed to make two separate pooled VTA soil samples. A modification of USEPA Method 1623, utilizing concentration, IMS, and immunofluorescent staining, was used for determining Cryptosporidium oocyst and Giardia cyst concentrations in basin liquid, pen runoff, and basin discharge samples (USEPA, 2001). Filtration was not performed due to the turbid nature of these samples. Instead, 1 L of basin wastewater was concentrated by centrifugation (1500 x g for 10 min) and the pellet resuspended 1:1 in formalin. For manure samples, 5 g was resuspended in 40 mL phosphate-buffered saline (PBS) and shaken for 10 min. The sample was concentrated by centrifugation (1500 x g for 15 min) and the pellet resuspended 1:1 in formalin. For recovery of protozoa in soil, a modification of the method described by Kuczynska and Shelton (1999) was performed. After extraction and sieving soil samples, the resulting solution was centrifuged at 1500 x g for 10 min. The supernatant was discarded and the pellet resuspended 1:1 in formalin. All pellets (basin, manure, and soil) were stored at 4°C until IMS and immunofluorescent antibody staining (IFA) could be conducted. Immunomagnetic separation (Aureon Biosystems, Vienna, Austria) and IFA (Waterborne, New Orleans, LA) were performed for all pellets according to manufacturers' instructions.
Statistical Analysis
Generic E. coli populations were converted to log10 CFU per kg or L values for statistical analyses. Analysis of variance and the Tukey-Kramer multiple comparisons test were performed on the converted bacterial population data using InStat (version 3.00, GraphPad Software, San Diego, CA); the model included the fixed effect of day of year. Differences in the isolation of generic E. coli, E. coli O157, and Campylobacter spp. from basin discharge hay and control hay were assessed by using the two-tailed Fisher exact test (Uitenbroek, 1997). This test was also used to compare differences in the numbers of pathogen-positive runoff system samples during different seasons. For all analyses, differences were considered significant when P values were less than 0.05.
|
Results and Discussion |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMaterials and MethodsResults and DiscussionConclusionsREFERENCES |
|---|
During the 26-mo study, there were four separate runoff events, occurring in September 2003, March 2004, May/June 2004, and June/July 2004, where rainfall volume in combination with snowmelt and/or liquid volume in the basin were adequate to result in measurable basin discharge to the VTA (Table 1). In addition to these four events, rains occurring in late March/early April 2005 and mid-August/early September 2005 also resulted in minor basin discharges before sampling on 28 March, 12 April, 22 August, and 12 September of that year. Basin discharge introduced generic E. coli, E. coli O157, and Campylobacter spp. to the VTA, as indicated by their detection in collected basin discharge liquid (data not shown) and in VTA soils following discharge (Table 1). Before the first basin discharge in September 2003, there had been no discharge into the VTA for over a year. This was reflected in the low concentrations of generic E. coli recovered from the VTA soils before this event. Generic E. coli levels in the control soil, that does not receive basin discharge, were typically below detectable levels (the detection limit was 1.00 log10 CFU g–1; data not shown).
Following discharge to the VTA, generic E. coli levels in the VTA soils increased (P < 0.05), and then steadily decreased (P < 0.05) over time (Table 1). However, lower residual populations persisted for long periods. This was especially evident for generic E. coli populations following the first and fourth basin discharge events (occurring in September 2003 and June/July 2004, respectively). Following the first basin discharge, average generic E. coli populations in VTA soils were 9.15 log10 colony forming units (CFU) kg–1 of soil. Over a period of 53 d (from 12 Sept. 2003 to 4 Nov. 2003), the average populations declined (P < 0.001) to 6.15 log10 CFU kg–1 soil, and then remained approximately constant (P > 0.05) at about 6.00 log10 CFU kg–1 soil for an additional 118 d through the winter until the second discharge event which occurred in March 2004. Initial generic E. coli populations in VTA soils following the second and third basin discharges in March 2004 and May/June 2004 were not as high (P < 0.05) as those occurring in September 2003, at 6.49 and 6.56 log10 CFU kg–1 of soil, respectively, and the lengths of time monitored before the next discharge events were shorter. However, the VTA soil concentrations of generic E. coli that remained before the subsequent discharges were also lower (P < 0.05), at 4.61 and 4.96 log10 CFU kg–1 soil. Average generic E. coli populations in VTA soils following the fourth basin discharge occurring in late June/July 2004 were 7.51 log10 CFU kg–1 soil. Similar to VTA soil populations observed following the first basin discharge event, the initial generic E. coli concentrations declined (P < 0.001) about 3 log10 CFU kg–1 soil (from 7.51 to 4.74 log10 CFU kg–1 soil from 6 July 2004 to 7 Dec. 2004), then remained constant (P > 0.05) at a lower level of about 4.50 to 4.70 log10 CFU kg–1 soil through the winter until 7 Mar. 2005, the sampling date before the next, minor basin discharge.
These data suggest that temperature or other environmental or biological factors associated with the fall and/or winter seasons may enhance persistence of E. coli in soil. Log reductions per day were calculated from generic E. coli population data from the first samples collected following a basin discharge and the final samples collected before the next discharge event. Log10 CFU reductions per day in VTA soils for the first and fourth basin discharges, which encompassed the fall and winter months, were lower (0.019 and 0.013 log10 CFU d–1, respectively) than for the second and third basin discharge events (0.034 and 0.022 log10 CFU d–1, respectively), which were restricted to the spring and summer months. Within the first basin discharge, log10 reduction of generic E. coli in VTA soil from 12 Sept. 2003 to 4 Nov. 2003 was 0.057 log10 CFU d–1, and from 4 Nov. 2003 to 1 Mar. 2004 was much lower at 0.002 log10 CFU d–1. Similarly, within the fourth basin discharge event, log10 reduction of generic E. coli in VTA soil from 6 July 2004 to 8 Nov. 2004 was 0.021 log10 CFU d–1, and from 8 Nov. 2004 to 7 Mar. 2005 was 0.004 log10 CFU d–1.
This apparent seasonal effect was also evident in the incidence of E. coli O157 in VTA soils following the first basin discharge event. At the initial sampling and on 3 Oct. and 20 Oct. 2003, 50, 50, and 67% of VTA soil samples were positive for E. coli O157, respectively. E. coli O157 was recovered from VTA soil samples on all sample dates from 1 Dec. 2003 to 1 Mar. 2004, with percentages of positive samples ranging from 10 to 30%. In contrast, E. coli O157 introduced to the VTA by subsequent basin discharges in the late spring and summer, including the minor discharges occurring in 2005, appeared to decrease more rapidly to levels below the detection limit (Table 1). For example, on the first sampling following the fourth basin discharge in late June/July 2004, 4 of 10 samples (40%) were E. coli O157-positive. Upon continued sampling on 19 July 2004 and 2, 16, and 30 Aug. 2004, 0, 20, 0, and 10% of VTA soil samples were positive for E. coli O157, respectively. Thereafter, from 14 Sept. 2004 until the minor discharge occurring in late March 2005, all VTA soil samples were negative for this pathogen.
The association of cooler soil temperatures and the longer environmental survival of E. coli in soils has been described previously. Environmental E. coli isolates survived longer in soil when held at 4, 15, and 25°C, than at 30 or 37°C (Ishii et al., 2006a). Topp et al. (2003) reported that initial E. coli soil populations of 6.00 log10 CFU g–1 soil persisted at levels greater than 5.00 log10 CFU g–1 soil at 4°C after 30 d, while declining to about 3.00 log10 CFU g–1 soil at 30°C during the same timeframe. Greater persistence of E. coli and/or E. coli O157:H7 at cooler temperatures in stream water (Terzieva and McFeters, 1991) and in livestock manure (Wang et al., 1996; Kudva et al., 1998) has also been reported. In contrast, Mukherjee et al. (2006) reported a more rapid reduction rate of E. coli O157:H7 populations in manure-amended soil at 4°C than at ambient temperatures of about 12 to 15°C. Natvig et al. (2002) found that a 1 March manure application in Wisconsin reduced soil survival of E. coli and Salmonella as compared to 1 June manure application. They concluded that these bacteria may have survived better in the manure-fertilized soil at warmer temperatures and/or that freeze-thaw cycling of the soil in the early spring may have been unfavorable to bacterial survival. In vitro experiments demonstrated that while freeze-thaw cycling did not eliminate E. coli or Salmonella in manure-amended soils, it may cause reductions in viable cells (Natvig et al., 2002).
The survival in soil of fecal indicator bacteria and pathogens has been a current research topic because of the risks for food crop and water contamination when livestock manure is used as a soil amendment. These studies have also reported that these microorganisms can often survive for long periods following manure application to land, and have also shown they can multiply in these soils. When bovine manure was incorporated into soil under temperatures typical of late spring or early summer in Wisconsin, initial E. coli levels of about 5.00 log10 g–1 soil at first increased by 1 to 2 logs, and then persisted for up to 19 wks (Lau and Ingham, 2001). Nicholson et al. (2005) applied beef manure slurry by both incorporation into arable soil and by surface application to grassland soil, and found that E. coli O157, Campylobacter, and Salmonella at initial levels of about 2 to 3.5 log10 g–1 soil survived in the soils for up to a month. E. coli survived up to 162 d in pasture soils following natural deposition by cattle or swine (Avery et al., 2004). Factors affecting E. coli (and E. coli O157) persistence in manure-amended soils may include, but are not limited to: soil type, amount of manure, application (incorporation vs. surface-applied), temperature, moisture, competition for limited nutrients, and protozoan predators (England et al., 1993; Fenlon et al., 2000; Lau and Ingham, 2001; Gagliardi and Karns, 2002; Avery et al., 2004; Byappanahalli and Fujioka, 2004; Hutchison et al., 2004; Berry and Miller, 2005). Several studies have observed that the presence of clay in soils may improve the survival of E. coli (Fenlon et al., 2000; Lau and Ingham, 2001; Gagliardi and Karns, 2002). The Hastings silt-loam soil of the VTA has an appreciable clay content (fine, smectitic, mesic Udic Argiustolls) which may have played a role in E. coli persistence in the VTA soil following basin discharge.
Most studies of E. coli survival in soil have focused in manure-amended soils. Because of the high nutrient levels present in the liquid basin discharge (Woodbury et al., 2003), the VTA soils receiving this discharge may in many ways resemble these manure-fertilized soils. However, several recent works have demonstrated that E. coli can adapt to life outside of the mammalian gastrointestinal tract, becoming "naturalized" to (Ishii et al., 2006a), and existing and growing in a variety of natural soil and sediment environments (Byappanahalli and Fujioka, 1998, 2004; Desmarais et al., 2002; Byappanahalli et al., 2006; Ishii et al., 2006a). Furthermore, these E. coli are not confined to soils in tropical or subtropical environments, but have also been found to persist and grow in soils in temperate environments that experience a wider range of extreme temperatures and weather conditions (Byappanahalli et al., 2006; Ishii et al., 2006a). The existence of natural E. coli in soils and sediments, and its potential introduction to waterways, confounds its use as an indicator of fecal contamination.
Campylobacter were not recovered from VTA soils following the first basin discharge event occurring in September 2003 (Table 1), although cattle in the pens were shedding Campylobacter spp. during this time. Following discharge occurring in March 2004, 60, 40, and 20% of VTA soil samples were positive on 9 Mar., 15 Mar., and 29 Mar. 2004, respectively, and then were negative again until the next discharge. Immediately following the third basin discharge in May/June 2004, 3 of 10 VTA soils on 17 May 2004 were positive for Campylobacter spp. However, subsequent VTA samples were again negative until after the fourth basin discharge. On 6 July 2004, Campylobacter was recovered from 1 of 10 VTA soil samples. All VTA soil samples collected from this time until after the minor basin discharge events occurring in March and April 2005 were negative for this pathogen. On 28 Mar. 2005 and 12 Apr. 2005, 10% and then 70% of VTA soil samples were Campylobacter-positive. Thereafter, VTA soils were negative for the pathogen until after the next minor discharge events that occurred in mid-August/early September 2005. Thus, while basin discharge clearly introduced Campylobacter spp. into the VTA, the results indicate that Campylobacter do not survive in soils as well as E. coli O157. When present, the higher numbers of Campylobacter-positive VTA soils following discharge corresponded to greater numbers of positive samples in basin solids and liquids. The apparent greater prevalence of Campylobacter spp. in runoff system samples collected during the spring is discussed below.
Because of long periods of little rain, loss of liquid to evaporation, and/or frozen conditions, basin solids and liquids often were not available for sample analyses (Table 1). In general, any precipitation that was adequate to cause runoff from the pens served to replenish the basin with both generic E. coli and the target pathogens. This was indicated by changes in population levels of E. coli in basin solids, basin liquids, and approach soil samples between basin discharge events, as well as by the sporadic isolations of both E. coli O157 and Campylobacter from these samples between discharges (approach soil data not shown). However, peak numbers of generic E. coli levels and pathogen isolation frequencies in basin samples tended to be highest following large rains that were adequate to fill the basin and result in discharge. The abilities of E. coli O157 or Campylobacter spp. to persist in basin liquids or solids were difficult to ascertain because of the continual addition of runoff to the basin. In addition, further work would be needed to compare survival of these fecal bacteria in shallow basins to their survival in deep anaerobic lagoons which are typically used for feedlot runoff collection and storage. All soil cores taken from below the basin in September of 2004 were negative for both E. coli O157 and Campylobacter spp. at all depths examined (Table 2 ). Enumerable generic E. coli were found in 5 of 9 soil samples at depths between 0 and 0.15 m, and in 1 of 9 samples at depths between 0.31 to 0.61 m. These results indicate that there was minimal leaching and transport of the target bacteria through the soils below the basin.
|
Utilization of basin discharge liquid for hay production raises the question of the risks for contaminating the hay and potentially reintroducing pathogens into the herd via feeding the hay. Rainfall and subsequent basin discharge events occurring on 13, 18, and 22 May 2004, before cutting the hay on 1 June 2004, provided an opportunity to assess these risks. On 17 May, 50 and 30% of VTA soil samples were positive for E. coli O157 and Campylobacter spp., respectively. On 1 June, the same day that the hay was cut, 40% of VTA soil samples were positive for E. coli O157. Among a total of 30 samples of loose hay collected from VTA regions that had received basin discharge (15 on 1 June and 15 on 3 June right before baling), one sample was positive for E. coli O157 and no samples were positive for Campylobacter spp. (Table 3 ). Neither pathogen was isolated from baled hay samples at 20, 41, or 69 d after cutting. The differences in generic E. coli isolation were not significant (P > 0.05) between hay from VTA regions that had received runoff as compared to control hay. Similar results were seen in the second season for hay cut on 16 June 2005. Neither pathogen was recovered from either loose hay or baled hay from basin discharge hay or control hay. Generic E. coli was recovered more frequently from basin discharge hay than control hay immediately following cutting; however, generic E. coli recovery from the basin discharge hay declined during storage of the bales. While these data indicate that there is some risk for hay contamination, the risk appears to be low.
|
|
|
Conclusions |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMaterials and MethodsResults and DiscussionConclusionsREFERENCES |
|---|
|
ACKNOWLEDGMENTS |
|---|
|
NOTES |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMaterials and MethodsResults and DiscussionConclusionsREFERENCES |
|---|
|
REFERENCES |
|---|
|
TOPNOTESABSTRACTINTRODUCTIONMaterials and MethodsResults and DiscussionConclusionsREFERENCES |
|---|
This article has been cited by other articles:
|
|
 |
|
  L. Xiao Overview of Cryptosporidium Presentations at the 10th International Workshops on Opportunistic Protists Eukaryot. Cell, April 1, 2009; 8(4): 429 - 436. [Full Text] [PDF]
|
|
|
|
 |
|
 S. P. Oliver, D. A. Patel, T. R. Callaway, and M. E. Torrence ASAS Centennial Paper: Developments and future outlook for preharvest food safety J Anim Sci, January 1, 2009; 87(1): 419 - 437. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 D. M. Crohn and M. L. Bianchi Research Priorities for Coordinating Management of Food Safety and Water Quality J. Environ. Qual., June 23, 2008; 37(4): 1411 - 1418. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| The SCI Journals | Agronomy Journal | Crop Science | |||
| Journal of Natural Resources and Life Sciences Education |
Vadose Zone Journal | ||||
| Soil Science Society of America Journal | Journal of Plant Registrations | The Plant Genome | |||
