Abstract
Six plant protection products (esfenvalerate, diflufenican, isoproturon, terbutylazine, chlorsulphuron, and metsulphuron-methyl) were added to pig and bovine liquid manure, and the degradation of the compounds was followed over a period of 57 &lparlsulphonylureas)(sulphonylureas) or 77 (other compounds) weeks. After 57 weeks more than 50% of the sulphonylureas remained in the bovine liquid manure, while 25% remained in the pig liquid manure. After 77 weeks >90% terbutylazine, 50–60% enfenvalerate, and 10–40% diflufenican and isoproturon had been degraded in both bovine and pig liquid manure. With exception of terbutylazine, the examined plant protection products were neither quickly nor extensively degraded. The practice to add unused spraying liquid or rinse-water containing plant protection products to liquid manure constitutes a risk that unwanted compunds are inadvertently added to the fields when the liquid manure is used as fertilizer, and this may cause crop damage.
Introduction
Pollution from point sources, e.g., spillage while filling and rinsing of spraying equipment, is a major cause of contamination of water resources with plant protection products (PPPs) (Frede et al., Citation1998; Helweg, Citation1994; Helweg et al., Citation2002; Müller et al., Citation2002). To reduce environmental problems caused by disposal of excess spraying liquid and the rinsings of water after rinsing spraying equipment, the Swedish Board of Agriculture (Citation1997) recommends that filling of spraying equipment should be performed over a biobed. A biobed essentially consists of a clay-sealed trough filled with a mixture of soil, peat, and straw with a highly active microbial composition optimized for degradation of PPPs (Torstensson & Castillo, Citation1997; Torstensson, Citation2000). Any spillage is retained in the biobed and degraded. If a biobed is not available, it is recommended that filling and external rinsing is carried out on a concrete platform with, e.g., a manure pile, and excess spraying liquid and rinse water are disposed of into liquid manure tanks (Swedish Board of Agriculture, Citation1997; Swedish Environmental Protection Agency, Citation1997) This recommendation is based on the assumption that the microbial activity in the liquid manure tanks will facilitate the degradation of the PPPs. However, our knowledge of PPP degradation in manure and liquid manure is rudimentary and few relevant studies have been published. Caution is called for, as it is well known that many PPPs degrade slowly under anaerobic conditions, and liquid manure tanks are usually anaerobic environments. If degradation is too slow, liquid manure amended with PPPs will be spread onto fields. This is probably of little environmental concern as the levels will be small compared to normal spraying. However, sulphonylurea herbicides are phytotoxic at very low levels, and cases of crop damage have been reported in Sweden and Denmark in crops sown after the spreading of liquid manure from tanks that have received rinse water and excess spraying liquid containing sulphonylureas. The damage has occurred mainly in oilseed crops and sugar beet and has been characteristic of sulphonylurea herbicide damage.
The purpose of this study was to obtain preliminary data on PPP degradation in pig and bovine liquid manure. The main focus was on two sulphonylurea herbicides suspected to have caused crop damage after disposal in liquid manure. Four additional compounds belonging to different classes of PPPs and with different physico-chemical properties were included for comparison.
Materials and method
Manure
Pig liquid manure was obtained from a 300-m3 tank at Stora Bärsta Farm, Uppsala, Sweden. Bovine liquid manure from dairy cows was obtained from a 500-m3 tank at Gälla School Farm, Uppsala, Sweden. The inorganic characteristics of the liquid manures are given in .
Table I. Characterization of the pig and bovine liquid manures.
Instruments and chemicals
Pure standards of diflufenican, esfenvalerate, isoproturon, and terbutylazine (Larodan Fine Chemicals, Ulricehamn, Sweden) were used. Commercial formulations of metsulphuron-methyl (Ally 20 DF, Du Pont), and chlorsulphuron (Glean 20 DF, Du Pont) were obtained from Odal (Uppsala, Sweden). The added application levels were calculated from the declared content of the formulations. All solvents were pesticide grade (Labscan, Stillorgan, Co. Dublin, Ireland) and used without further purification.
For the determination of sulfphonylureas, liquid chromatography-mass spectrometry (LC-MS) was performed on a Waters Alliance 2690 liquid chromatograph with a Micromass Platform LCZ single quadruple mass selective detector (Waters, Millford, MA) using atmospheric pressure electrospray (APES) ionization (positive ions). The column was a Hypersil Hy Purity C18 (5 µm, 150×4.6 mm, Agilent, Stockholm, Sweden) held at 35°C. Injection volume was 100 µl. The eluent was a linear gradient consisting of acetonitrile and water with 0.2% formic acid, starting with 20% acetonitrile and reaching 60% acetonitrile after 12 min. The flow rate was 0.9 ml/min through the column, reduced to 0.1 ml/min into the ion source. The electrospray cone voltage was 35 V. Detection was carried out in the selected ion monitoring mode. For chlorsulphuron, fragments at m/z 141, 167 and 358 were monitored and for metsulphuron-methyl, m/z 141, 167 and 282. Carbaryl (Larodan Fine Chemicals) was used as the internal standard.
For the determination of diflufenican, esfenvalerate, isoproturon, and terbutylazine, gas chromatography (GC) was performed on a Hewlett-Packard 5890 gas chromatograph (Agilent, Wilmington, Delaware, USA) with dual electron capture detectors (ECDs) or a Varian 3400 (Varian, Walnut Creek, California, USA) with dual thermoionic detectors (NPDs). Two columns with different polarities (CP-Sil 5CB and CP-Sil 19 CB, 30 m×0.32 mm×25 µm, Chrompak, Middelburg, The Netherlands) were fitted to the same split-splitless injector, but with individual detectors (injector temperature 250°C, splitless injection, split open after 1 min, detector temperature 300°C). The carrier gas was nitrogen. The temperature programme was 90°C for 1 min, 30°C/min to 180°C, 4°C/min to 260°C, isothermal 12 min. Ethion (Larodan Fine Chemicals) was used as the internal standard.
Procedures
Sulphonylurea
Chlorsulphuron and metsulphuron-methyl (nominally 25 mg of each) were added to pig and bovine liquid manure (1000 g), with three replicates of each. The liquid manure was gently shaken and left overnight, after which it was again shaken and a first sample (50 g) was taken from each replicate to check the concentration at the start of the experiment, and for recovery testing. The samples were incubated at 20°C in the dark. Additional samples were taken after 22 and 57 weeks. At each sampling, manure (50 g) from each of the replicates was weighed into a centrifuge bottle and vigorously shaken with carbonate buffer (0.1 M, pH 10, 100 ml). The samples were centrifuged (10 min, 6200 rpm), and the supernatant was decanted into a 200-ml E-flask. The pH was adjusted to between 3 and 3.5 with hydrochloric acid (1 M) and the solution transferred to a centrifuge bottle and centrifuged (10 min, 6200 rpm) and the supernatant decanted to a fresh E-flask where carbaryl was added as surrogate standard. Solid phase extraction cartridges (ENV+, 200 mg, 6 ml cartridge, International Sorbent Technology, Hengoed, Mid Glamorgan, UK) were activated with acetone:ethyl acetate (1:1, v:v, 10 ml) followed by ultra-pure water (10 ml) (Young, Citation1998). The aqueous phase from the liquid manure was passed through the ENV+ cartridges without previous filtration. The cartridges were then rinsed with water (10 ml) and eluted with acetone:ethyl acetate (1:1, v:v, 2×3 ml). The solvent was evaporated to dryness under a gentle stream of air, and the residue redissolved in acetonitrile (2.5 ml) and filtered (Acrodisc CR PTFE filter, 0.45 µm, VWR, Spånga, Sweden). Before quantification, an aliquot of the filtrate was diluted to a volume corresponding to a nominal concentration of 1 µg/ml of pesticides using the LC start eluent.
Non-sulphonylureas
Esfenvalerate, terbutylazine, isoproturon and diflufenican (50 mg of each) were added to pig and bovine liquid manure (1000 g), with three replicates of each. The liquid manure was gently shaken and left overnight, after which it was again shaken and a first sample (50 g) was taken out from each replicate to check the concentration at the start of the experiment, and for recovery testing. The samples were incubated at 20°C in the dark. Additional samples were taken after 17 and 77 weeks. Samples (50 g) were weighed into aluminium trays and air-dried for 7 h at 25°C. Each sample was then carefully ground in a polished porcelain mortar, weighed, and Soxhlet-extracted overnight using acetone:dichloromethane (6:1, v:v). Prior to extraction, ethion was added as a surrogate standard. After extraction, cyclohexane (10 ml) was added and the solvent mixture was evaporated on a rotary evaporator to a volume of less than 3 ml that was transferred to a 50-ml volumetric flask and made up to volume with cyclohexane:acetone (9:1, v:v). Before quantification, an aliquot was diluted to a volume corresponding to a nominal concentration of 1 µg/ml of each pesticide at the start of the experiment.
Degradation was recorded solely as the disappearance of the active mother substances. No effort was made to account for degradation products or degree of mineralization.
Results and discussion
The mean recoveries at the start of the degradation experiments are given in and . Generally the recoveries were good, except for the sulphonylureas and, to a certain extent, terbutylazine. The low recovery of terbutylazine may be due to the relatively rapid degradation that was seen subsequently. In contrast, the relatively low initial recovery of the sulphonylureas was followed by a fairly low degradation rate and may indicate a rapid binding of these compounds to the organic material in the liquid manures. As indicated by the high standard deviation, the low recovery of esfenvalerate in pig liquid manure and isoproturon in bovine liquid manure was due to one replicate deviating substantially from the others.
Table II. Recoveries (%, mean±SD, n = 3) on day 1, and concentrations (mg/kg dr.w., mean±SD, n = 3) at the different sampling weeks, and estimated half-lives (D50) of sulphonylurea herbicides added to pig and bovine liquid manure and incubated at 20°C in the dark.
Table III. Recoveries (%, mean±SD, n = 3) on day 1, and concentrations (mg/kg dr.w., mean±SD, n = 3) at the different sampling weeks, and estimated half-lives (D50) of non-sulphonylurea plant protection products added to pig and bovine liquid manure and incubated at 20°C in the dark.
Initially, during the first 21 weeks, there was a loss of both of the tested sulphonylurea herbicides (). However, in the bovine liquid manure the concentrations stabilized and were the same after 57 weeks as after 21 weeks. In the pig liquid manure the concentrations continued to decrease, but there were still detectable levels of both chlorsulphuron and metsulphuron-methyl after 57 weeks.
The most readily degradable compound was terbutylazine (). After 77 weeks, >90% of the terbutylazine had been degraded in both pig and bovine liquid manure. About 60% of esfenvalerate was degraded in pig, and about 50% in bovine, liquid manure while about 30% of diflufenican had been degraded in pig, and 15% in bovine, liquid manure. Of isoproturon about 15% had been degraded in pig liquid manure and 35% in bovine liquid manure. In most cases where degradation was indicated, it seemed to be more effective in pig than in bovine liquid manure.
As the intention of the experiment was to test if a practice that is recommended actually performs its intended function, we first started using commercial formulations of all six compounds. However, except for the sulphonylureas, the solubility of the formulations in the liquid manures was poor. To circumvent this problem we instead used pure standards of the four non-sulphonylureas.
The aim of this preliminary experiment was not an in-depth investigation of the degradation rate in the respective liquid manures. Nevertheless, this study indicates that there is a problem with disposing of PPPs in manure, and it is motivated to make at least a rough calculation of the half-lives of the individual compounds in the liquid manures to obtain a picture of how long it may take before liquid manure that has received PPPs can be used. The half-life estimates are presented in and . Using the recovery on the first day as the baseline, it seems that for many of the combinations of PPP and pig or bovine liquid manure, the half-life of the PPP is more than six months or even one year. This is also the case for the sulphonylureas. For highly potent pesticides, such as the sulphonylureas, there is a risk that the concentrations in the liquid manure may be phytotoxic even after a long storage time. To obtain a better estimate of the half-lives we would have needed more replicates and more frequent sampling. In a parallel study (Torstensson et al., Citation2001) it was found that the half-life of glyphosate was more than two years in both pig and bovine liquid manure.
The recommendation to dispose of excess pesticides in liquid manure is intended as an alternative to biobeds. However, in a biobed the microbial consorts are specially adapted to degrade PPPs. The carbon source in a biobed is straw, which is fairly difficult for the microorganisms to utilize. The degradation of the PPPs is via cometabolism with the straw (Castillo, Citation1997). In liquid manure, on the other hand, although the total microbial activity may be higher than in the biobed, the microorganisms are not specifically adapted to degrade PPPs and have an ample source of easily available carbon. Therefore there is no reason to suppose that the microorganisms will metabolize the pesticides to any large extent, especially as it is well known that anaerobic degradation, which is dominant in liquid manure tanks, of many PPPs is slow. In effect, although the recommendation to dispose of the PPPs in liquid manure will probably solve an environmental problem, it may also introduce a plant production problem. This problem will be larger with sulphonylurea herbicides than other compounds because of their high efficacy.
Acknowledgements
This study was financed by the Swedish Board of Agriculture and by a scholarship to L.Volkova from the Swedish Institute.
Related Research Data
References
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- CAS numbers of the studied compounds :
- Chlorsulphuron [64902-72-3], metsulphuron-methyl [74223-64-6], diflufenican [83164-33-4], esfenvalerate [66230-04-4], isoproturon [34123-59-6], terbutylazine [5915-41-3] .