- Vegetables: cabbages, carrots, garlic, onions, peppers, radishes (culinary), sweet corn, tomatoes
- Animals: swine, poultry
- Animal Production: feed additives, manure management
- Crop Production: organic fertilizers
- Production Systems: organic agriculture
- Soil Management: composting
- Sustainable Communities: urban agriculture
Since their discovery, antibiotics have been instrumental in treating infectious diseases that were previously known to kill humans and animals. However, their widespread use as a feed additive in food animal production has raised concerns on the possibility of antibiotics appearance in the food supply particularly organically grown foods where animal manure is a major source of plant nutrients. This study characterized the extent of antibiotic uptake by vegetable crops from soils that have been fertilized with antibiotics mixed turkey manure and hog manure, both fresh and after composting. The study involved composting hog and turkey manures in the field, characterizing antibiotic degradation during composting, setting up the field plots at two sites representing clay loam and sandy loam soils, growing 11 different vegetable crops, and characterizing antibiotic presence in various plant tissues and soil at harvest. Nutrient source treatments were fresh and composted hog manure, fresh and composted turkey manure, and inorganic fertilizer. Antibiotics were mixed into the manure as a concentrated feed or in a powder form. Targeted antibiotic concentrations were 10 mg L-1 for liquid hog manure or 10 mg kg-1 for solid turkey manure on dry weight basis. Composting of liquid hog manure was done by mixing wood shavings with manure containing antibiotics and then composting the mixture in cement bins. Vegetable crops tested for antibiotic uptake were spinach, lettuce, cabbage, carrots, radish, onion, garlic, tomatoes, green pepper, sweet corn and potatoes. Antibiotics analyzed in manure, soil, and plant tissues were chlortetracycline, tylosin, monensin, virginiamycin, and sulfamethazine. All analysis was done with ELISA kits.
The results showed that except for chlortetracycline, antibiotic degradation was slower during composting of turkey manure than composting of hog manure. During composting of hog manure, tylosin had the shortest half-life (5 days) followed by chlortetracycline (46 days), sulfamethazine (74 days) and monensin (128 days). Comparatively during composting of turkey manure, tylosin had a half-life of 16 days, followed by chlortetracycline (17 days), virginiamycin (25 days), monensin (257 days) and sulfamethazine (462 days). Except for sulfamethazine and one case of monensin, soil antibiotic concentrations were below the limits of quantification (LOQ) at both planting and at harvest for all antibiotics. Monensin concentration in raw hog manure treatment at planting at both sites and concentration of sulfamethazine at planting and sometimes at harvest for all manure treatments at both sites were above the LOQ. Between planting and harvest, there was greater degradation of sulfamethazine in the Staples plots (49% to 95%) than in the Waseca plots (21% to 99%). Except for some isolated treatments, concentrations of monensin, tylosin, chlortetracycline, and virginamycin in plant tissues of all vegetable crops were below the LOQ at both sites. For some manure treatments, sulfamethazine concentrations were higher than the LOQ for almost all vegetable crops. However, there was no consistent pattern between plant sulfamethazine concentration and its corresponding soil concentrations. In general, sulfamethazine concentrations were higher in radish skin than radish root, carrot skin than carrot root, and potato skin than potato tuber. These differences may be due to direct contact of the skin of these below ground vegetables with soil containing antibiotics and possibly due to lower water content of the skin than inner tissue. Sulfamethazine concentration in most vegetable tissues was less than 10 µg kg-1.
The second objective of this study was to document the presence or absence of antibiotics in certified organic vs. conventional commercial vegetables that have been grown in manure applied soils. For this objective we surveyed a group of five certified organic vegetable growers and two conventional vegetable growers who use either fresh or composted manure on their farm. We analyzed their manure, soil, and vegetable samples for presence of antibiotics. We also analyzed the vegetable produce from two grocery stores who sell certified organic produce. Concentrations of antibiotics found in 5 farms (where samples were available) were much lower than the concentration of antibiotics used in our controlled field study at Waseca and Staples. Soil samples from these farms also did not exceed the LOQ of tetracycline, virginiamycin, and monensin. There were two farms where sulfamethazine concentrations in soil samples were higher than the LOQ. Except for a few cases, concentrations of all antibiotics in vegetables were below the LOQ. Highest sulfamethazine concentration (27.34 µg kg-1) occurred in lettuce harvested in one certified organic farm. In two other certified organic farms, 18% and 54% of vegetables had higher sulfamethazine concentration greater than the LOQ. The conventional turkey manure farm had the highest number (78%) of vegetables containing sulfamethazine. Although some of the concentrations detected are higher than the concentration we detected in vegetables from our experimental plots, they are still relatively low concentrations. There was no specific trend among vegetables in terms of sulfamethazine presence above the LOQ. Except for few cases, antibiotic concentrations in vegetables from two grocery stores were generally below the LOQ. Since vegetables from producers and grocery stores were not analyzed in replicates, we are uncertain whether these concentrations above the LOQ are real or false positives. Overall, various antibiotic concentrations detected in vegetables from our field experiments, farmers’ fields, and from grocery stores are relatively low.
Several organizations have established acceptable daily intake (ADI) values for some of the veterinary pharmaceuticals. The ADI values indicate levels that can be ingested daily over a lifetime without any health risks. Most ADI values given for many veterinary pharmaceuticals are less than 10 µg kg-1 body weight per day. Assuming body weights varying from 50-75 kg, total permissible intake will be equivalent to 500-750 µg of pharmaceutical per day. The maximum concentration of antibiotics in vegetable produce in this study is less than 30 µg kg-1 wet weight. This means, one would need to consume 17-25 kg of produce every day to reach the ADI values suggested by various organizations. These simple calculations suggest that antibiotic uptake by various vegetable crops analyzed in this report is not of a major health concern to most adults unless one is allergic to that particular pharmaceutical. In this study, we only characterized the potential uptake of five antibiotics. Because there are other pharmaceuticals that are also administrated to food animals and are excreted in manure, additional studies will be needed to address the potential of those pharmaceuticals to appear in vegetable crops from manure applied soils.
Since our study shows that most antibiotics degrade over time, a practical guideline for organic producers who use conventional manure for plant nutrient source will be to (1) apply composted manure rather than fresh manure, and/or (2) let the manure sit in the soil for as long as possible before planting vegetables in the spring. This extra time will help degrade antibiotics and thus lower the concentration available for plant uptake.
A pdf copy of the full report is available at the following url:
The authors gratefully acknowledge the funding for this project from the North Central Region Sustainable Agriculture Research and Education (NCR-SARE) program. Earlier work on antibiotic uptake by vegetables by Dr. Kuldip Kumar and subsequently by Dr. Holly Dolliver and Dr. Kumar form the basis for this project. The senior PIs are grateful to Drs. Kumar and Dolliver for laying the foundation for this project. There were several individuals who helped the PIs in their field and laboratory work. We are thankful to Josh Worely for his help in vegetable harvest and in laboratory processing of vegetable produce; Dr. Charlie Rohwer, Curt Miller, Scott Coy, Sue Schoenfeld, Kraig Deling, and Gabriel Malima for their help with composting of manure, field set-up and maintenance of vegetable plots at Southern Research and Outreach Center at Waseca; and Bob Schaefer, Michelle Johnson, Matt McNearney, Anina Christensen, Brendan Ward and Anubha Garg for their help with plot set-up and other field work at Central Lakes Irrigation Center at Staples. The senior PI also acknowledges helpful discussions with Mr. Tom Halbach on manure composting. The authors are grateful to both certified organic and conventional vegetable growers for their participation in the survey part of the study.
Since their discovery, antibiotics have been instrumental in treating infectious diseases that were previously known to kill humans and animals. However, their widespread use as a feed additive in food animal production has raised concerns on (1) the development of antibiotic resistance bacteria in the environment and (2) the appearance of antibiotics in food and water supplies. The main pathway for these impacts is when manure containing antibiotics is land applied (Kumar et al., 2005b).
According to the Institute of Medicine about 22.7 million kilograms of antibiotics are used annually in the United States out of which 60% is used in human medicine, and 32% and 8% for non-therapeutic and therapeutic use in livestock agriculture (Shea, 2003). On the other hand, the Union of Concerned Scientists estimate that 15.9 million kilograms of antibiotics are used annually in the United States out of which 13% is used in human medicine, 78% and 6% for non-therapeutic and therapeutic use in livestock agriculture, and 3% in pets (Shea, 2003). Non-therapeutic use of antibiotics in animal agriculture is not only to increase the animal’s ability to absorb nutrients from feed and reach market weight on time, but also to counteract the effects of crowded living conditions and poor hygiene in intensive animal agriculture. Although the antibiotic dose varies from 1 to 100 g per ton of feed depending upon type and size of the animal and the type of antibiotic, as much as 80% of the antibiotics administered orally may pass through the animal unchanged in urine and manure (Levy, 1992).
There is very little literature on antibiotic uptake by plants from manure applied soils. Most of the earlier research was directed towards identifying stimulation or toxicity of antibiotics to plants (Norman, 1955; Batchelder, 1981 & 1982). Milgiore et al. (1995) tested the toxicity of sulphadimethoxine and found that 300 mg/L of sulphadimethoxine in agar plate altered the normal development and growth of roots, hypocotyls, and leaves of millet, peas, and maize. Final bioaccumulation amounts in root and stalk/leaves were: 2070 µg/g and 110 µg/g for millet over an 8 day period, 178 µg/g and 60 µg/g for peas over an 18 day period, and 268 and 12.5 µg/g for maize over an 8 day period. Later, Milgiore et al. (1996) showed that sulphadimethoxine bioaccumulation varied in barley roots and foliage from 48 to 79 µg g-1 and 12 to 19 µg g-1. Jjemba (2002) summarized the literature on potential impact of veterinary and human drugs in manure and biosolids on plants and concluded that phytotoxicity of these drugs varies between different plant species and depended upon soil and biosolids characteristics. However, many of the studies in the literature like those above were done in cultures, agar, or soils that were artificially spiked with antibiotics.
The earlier research on antibiotic uptake by vegetable crops was reported by Bewick (1979) who used tylosin and terramycin (oxytetracycline) fermentation wastes as sources of fertilizer for tomatoes. The authors reported that there was no antibiotic detected in any of the tomatoes when these wastes were mixed with compost containing peat. However, two recent greenhouse studies by the associates of the investigators showed that onions, cabbage, and corn take up chlortetracycline and corn, lettuce, and potato take up sulfamethazine from soils mixed with antibiotic containing fresh (non-composted) hog manure (Kumar et al., 2005a; Dolliver et al., 2007). The chlortetracycline treatments included manure where antibiotics had passed through the animal gut as well as manure that was artificially spiked. The chlortetracycline uptake amount increased from 0 to 0.017 µg/g of fresh weight with an increase in antibiotic concentration in soil from 0 to 1600 ?g/pot. However, there was no uptake of tylosin by corn, cabbage, and lettuce possibly due to the larger size of its molecule. The soil used in this experiment was Hubbard loamy sand.
Dolliver et al. (2007) showed a significant difference in sulfamethazine uptake between the control (soil amended with manure with no antibiotics) and the sulfamethazine treatments (soil amended with manure containing sulfamethazine). However, there was no difference in sulfamethazine uptake between 50 mg/L (5 mg pot-1) and 100 mg/L (10 mg pot-1) treatments. The soil used in this experiment was a Waukegan silt loam mixed with sand, peat and compost. Both studies were done for six weeks and antibiotics were only measured in the leaf tissue. From these studies it is not apparent whether or not antibiotic concentrations remains the same in leaves if allowed to grow for full maturity (growing season) and also if antibiotic concentrations in fruits (edible part) are at the same level as in leaves.
A recent study in UK reported the uptake of 10 veterinary medicines by lettuce and carrots from soils that had been artificially spiked with these drugs (Boxall et al., 2006). Seven antibacterial drugs included amoxicillin, enrofloxacin, florfenicol, oxytetracycline, sulfadiazine, trimethoprim, and tylosin. The results showed that after 5 weeks of plant growth, florfenicol, and trimethoprim were present in lettuce at concentrations of 15 µg/kg and 6 ?g/kg whereas enrofloxacin, florfenicol, trimethoprim were present in carrot roots at concentrations of 2.8 µg/kg, 5 µg/kg, and 5.3 µg/kg. The nominal antibiotic concentration was 1.5 mg of a given antibiotic in each container of light loamy sand with low organic matter content. The authors also reported that enrofloxacin (8.5 µg/kg) and florfenicol (38 µg/kg) were higher in carrot peel than the carrot root. However, oxytetracycline, tylosin and sulfadiazine concentrations in lettuce and carrot were below the detection limits. Using the acceptable daily intake (ADI) values reported in the literature for antibiotics in meat, both Boxall et al. (2006) and Dolliver et al. (2007) concluded that antibiotic concentrations in plants did not pose any appreciable risk to a majority of the population. However, both authors concluded that the prolonged effect of continued exposure may be important in some populations which have a weak immune system.
Recently, Herklotz et al. (2010) used cabbage (Brassica rapa var. pekinensis) and Wisconsin Fast Plants (Brassica rapa) to test potential uptake and accumulation of carbamazepine, salbutamol, sulfamethoxazole, and trimethoprim at 232.5 µg L-1 concentrations under hydroponic conditions. All four chemicals were detected in the root and leaves of cabbage. Concentration in roots were 98.9 ?g kg-1 carbamazepine, 114.7 µg kg-1 salbutamol, 138.3 µg kg-1 sulfamethoxazole, and 91.3 µg kg-1 trimethoprim. While all four pharmaceuticals were detected in leaf, stem, and root of Wisconsin Fast Plant, only carbamazepine and salbutamol were detected in the seedpods. In an ecological risk assessment study of tricolsan effects on cucumber, corn, ryegrass, soybean, tomato, and wheat in quartz sand, Reiss et al. (2009) found significant reduction in mean shoot length at the two highest concentrations tested (280 and 930 µg kg-1). In this study, shoot and root dry weight also decreased significantly at the highest concentration relative to the control.
With greater emphasis on sustainable farming, there is even greater demand for organic foods, where manure is the primary source of nutrients. At this time, there are no guidelines on the presence of antibiotics in manure used for certified organic farming.
Furthermore, most of the studies in the literature have been done on a few vegetable crops and only for a short time. Also, the previous research does not indicate the extent of antibiotic uptake under natural soil conditions. Furthermore, there are hardly any data on antibiotic uptake by vegetable plants grown in soils mixed with composted manures.
Approved Antibiotic for use in Agriculture
Agricultural producers use antibiotics in feed and water not only to treat, prevent, and control disease, but also to encourage growth and feed efficiency. McEweon and Fedorka-Cray (2002) observed that about 83% of food-producing animals are administered at least one antibiotic for disease prevention or growth promotion. According to Shea (2003), roughly 90% of the antibiotics used in agriculture are administered for non-therapeutic uses such as growth promotion. Table 1 lists FDA approved antibiotics for growth promotion as well as for treatment of food animals. In this study, we selected five antibiotics for plant uptake from manure applied soils. The antibiotics were: chlortetracycline, monensin, tylosin, virginiamycin, and sulfamethazine. The following text briefly describes the properties of these antibiotics.
Chlortetracycline: Tetracyclines are widely used in livestock production for therapeutic treatment of intestinal and respiratory infections and also for growth promotion. Chlortetracycline, the first member of tetracycline group, was discovered in the late 1940s by Dr. Benjamin Duggar. Tetracycline inhibits cell growth by inhibiting protein synthesis. Tetracyclines are excreted as active compounds and enter the soil environment when manure is land applied as a source of nutrients for crops. De Liguoro et al. (2003) applied manure to soil from calves treated with oxytetracycline (820 ?g/kg) and found that oxytetracycline was present in soil even after 5 months but it could not be detected in water. Hamscher et al. (2002) reported that tetracycline and chlortetracycline could be measured in a soil treated with liquid manure at soil depths down to 30 cm and are known to persist and accumulate in soil. However, persistence of chlortetracycline depends upon the soil temperature. Gavalchin and Katz (1994) showed that there was no degradation of chlortetracycline applied at 5.6 mg kg-1 of soil at 4°C after 30 days. However, only about 44% and 88% of the applied chlortetracycline remained in the soil at 30°C and 20°C, respectively. Recommended dosage for turkeys is 25 mg of tetracycline per pound body weight (FDA Center for Veterinary Medicine). Figure 1 shows the molecular structure and Table 2 lists the molecular properties of tetracycline.
Monensin: Monensin belongs to the ionophore (ion-carrier) group of antimicrobials mainly used in veterinary medicine (Haney and Hoehn, 1967). Among several identified types, monensin A is derived from the fermentation of the fungus Streptomyces cinnemonensis and is a sodium-selective carboxylic ionophore. Monensin is approved as a growth promoter for cattle and poultry but not for swine. Monensin is the only FDA permitted feed additive for lactating cows to increase milk production. Monensin enhances growth and feed efficiency in cattle by altering ruminal fermentation and in poultry by preventing disease (Tedeschi et al., 2003).
Approximately 1.5 million kilograms of monensin is used subtherapeutically in the cattle and poultry industry in the USA (Mellon et al., 2001). Monensin prevents and controls coccidiosis in cattle and calves (Mellon et al., 2001). However, more than 90% of monensin is excreted and concentrations greater than 1 mg L-1 have been detected in manure (Kumar et al., 2005b). Unlike many antibiotics used in veterinary medicine, monensin is not used in human medicine because it is highly toxic (Dolliver et al., 2008a). Monensin has also been shown to adversely affect plant growth at concentrations >1 mg kg?1 (Brain et al., 2004). Although there is potential for monensin to persist in the environment due to lack of hydrolysis and slow photolysis, the biodegradation rates suggest a rapid biological attenuation (Tedeschi et al., 2003). Monensin half-life has been reported from less than 2 days (Sassman and Lee, 2007) to 13.5 days (Carlson and Mabury, 2006). Sassman and Lee (2007) reported the partition coefficient of monensin varies from 0.92 to 78.6 (L kg-1) for various soils at an aqueous phase concentration of 0.05 ?mol L-1. Figure 2 and Table 3 show the molecular structure of monensin and its properties.
Tylosin: Tylosin was identified as the top ten most frequently detected antibiotics in surface water in the United States from 1999 to 2000 (Kolpin et al., 2002). Tylosin is one of many macrolide antibiotics that is used exclusively in veterinary applications. When used as a therapeutic drug, tylosin inhibited the ability of bacteria to synthesize its proteins. Tylosin is used as a growth promoter in swine, cattle, and poultry production (Rabølle and Spliid, 2000). For growth promotion, tylosin concentrations in swine feed ranges from 10 to 100 g tylosin/ton feed (Elanco Animal Health Tylan® Premix product label). Figure 3 shows the molecular structure of tylosin and Table 4 lists it molecular properties.
Sulfamethazine: Sulfamethazine belongs to sulfonamides class of antibacterial. Sulfonamides were discovered in the 1930s and have been widely used in aquaculture, agriculture, and in veterinary and human medicine. Sulfamethazine is used for treatment of respiratory disease and for growth promotion in cattle, sheep, swine, and poultry. Daily recommended dose of sulfamethazine sodium in drinking water per pound body weight varies from 56 to 113 mg for swine and 53 to 130 mg for turkey (FDA Center for Veterinary Medicine). Sorption of sulfamethazine decreases with an increase in pH (Boxall et al., 2002). Sukul and Spiteller (2006) reported sulfamethazine concentration as high as 40 mg kg-1 in liquid manure from swine and calves. Figure 4 and Table 5 show the molecular structure of sulfamethazine as well as its properties, respectively.
Virginiamycin: Virginiamycin belongs to the streptogramin class of antimicrobials and is known to promote livestock growth by increasing ileal amino acid digestibility (Stewart et al., 2010). Virginiamycin has also been used to prevent and control diseases in poultry, swine, and cattle. It consists of two principal components designated as macrolactone virginiamycin M1 and the hexadepsipeptide virginiamycin S1. The commercial product contains about 75% of the M1 factor, 20% of the S1 factor and 5% of other minor analogues. The presence of the S component markedly enhances the antimicrobial activity of the M component. Virginiamycin is also used in the ethanol (biofuel) industry to prevent microbial contamination. Figure 5 shows the molecular structure of virginiamycin M1 and S1 factors and Table 6 lists their molecular properties.
Except for monensin, all other antibiotics discussed above can be used in swine and poultry production. Monensin cannot be used in swine as subtherapeutic agent. Table 7 lists the residue limits in various meat tissues for all five antibiotics used in our study. Antibiotic residues in animal products are regulated by the United States Department of Agriculture, Food Safety and Inspection Service. According to Van Dresser and Wilcke (1989), penicillin, oxytetracycline, and sulfamethazine are the most frequently detected residues in food products.
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The goal of this study was to determine the extent of antibiotic uptake by vegetable crops from soils that have been fertilized with antibiotic laden turkey manure and hog manure, both fresh and after composting. In this paper, we refer to fresh manure as manure that is just taken out of the turkey barn or from a hog lagoon and has not gone through the composting or extensive curing process.
Specific objectives of the study were: (1) to test the extent of antibiotic (chlortetracycline, tylosin, monensin, virginiamycin, and sulfamethazine) uptake by eleven vegetable crops (spinach, lettuce, cabbage, carrots, radish, onion, garlic, tomatoes, green pepper, sweet corn and potatoes) from two different textured soils that have been fertilized with antibiotic-containing turkey manure and hog manure, (2) to document the differences in antibiotic uptake by vegetables that have been fertilized with fresh vs. composted turkey and hog manures, and (3) to document the presence or absence of antibiotics in certified organic vs. conventional commercial vegetables that have been grown in manure applied soils.
Since sulfamethazine has been shown to be taken up by vegetable plants and since it is also slow in degrading during composting (Dolliver et al., 2007), we were further interested in studying the adsorption behavior of sulfamethazine in our two soils. Recently, Kahle and Stamm (2007) suggested that presence of phosphorus may have some effect on sulfamethazine adsorption on soils. Since manure contains significant amount of phosphorus, an additional objective of this study was to test the adsorption potential of sulfamethazine in our two soils both with and without the presence of phosphorus.