Chromatic Validation of Herbicides Used in Vegetable Production

Abstract

Herbicides are necessary for successful vegetable production in the Southeastern United States. Along with bare-ground production, low density polyethylene (LDPE) mulches are often utilized to produce multiple crops (2–4) by rotation over the course of a 12-to-24-month period. These include fresh market tomato, pepper, cucurbits, eggplant, and cabbage. For LDPE mulch vegetable production, between each crop growers must apply contact and residual herbicides to mitigate weeds. However, these herbicides can remain on the mulch and injury transplants. Herbicides are often soil applied for bare soil production as well as under the LDPE mulch. Herbicide carryover in soil using these vegetable production methods can also result in management issues. Proper quantification analyzing the dissipation is critical in the decision-making process for growers to prevent unnecessary crop losses. A series of experiments have been conducted to quantify the dissipation of the herbicides flumioxazin, fomesafen, ammonium-glufosinate, glyphosate, halosulfuron-methyl, paraquat, S-metolachlor, and sulfentrazone over time using UPLC/MS and bioassay methods. These methods are presented.

1. Introduction

Weed control in fresh market production of vegetable crops has significantly changed over the past 25 years due to the elimination of the preplant soil fumigant methyl bromide (MBr). Purple (Cyperus rotundus) and yellow nutsedge (C. esculentus) are the some of the most common and troublesome weeds in fresh market vegetable production throughout the southeastern US (Table 1) [1, 2]. The sharp tips of the emerging nutsedge shoots readily pierce low-density polyethylene (LDPE) mulch (Figure 1) and lead to intensive nutsedge infestations [3].

Rank	Vegetables – Cucurbits		Vegetables – Other
	Most common	Most troublesome	Most common	Most troublesome
1	Amaranthus spp.	Amaranthus spp.	Amaranthus spp.	Amaranthus spp.
2	common lambsquarter	Cyperus spp.	common lambsquarter	Cyperus spp.
3	Cyperus spp.	common lambsquarter	common purslane	common lambsquarter
4	Ambrosia spp.	Ipomea spp.	yellow nutsedge	common purslane

Table 1.

Most common and troublesome weeds in vegetables.

Common weeds are defined as weeds most frequently seen while troublesome weeds are those that are the most difficult to control.

Figure 1.

Cyperus spp. that has pierced through LDPE mulch (photo by Kayla Eason).

Along with nutsedges, many other winter and summer weed species proliferate rapidly as vegetables are supplemented with water and nutrients via drip irrigation from tubes inserted at the time a polymer mulch is laid (Figure 2). The use of black polyethylene mulch may alter the environmental characteristics of the cropping system to the benefit of many weed species [4]. As MBr is no longer a weed control option for soil sterilization in plasticulture systems, herbicides are now used to maintain successful fresh market vegetable production over the course of multiple crops [5]. This includes the use of herbicides applied to the soil surface as mulch is laid, or between crops as part of the rotation (Figure 2) [6]. Most LDPE mulch laid for spring vegetable production is followed by a second crop in the autumn and potentially a third crop the following spring [7]. These succeeding vegetable crops can be transplanted directly into the existing mulch covered beds. This allows for multiple crop production using the same beds. This is done in order to minimize expenses associated with mulch and drip tape irrigation by distributing costs over multiple crops. However, the use of herbicides can result in injury and crop losses when sensitive vegetable species are exposed to previous applications [6]. The quantification of herbicide residues from field applied experiments is critical to maintain plant safety, and consistent vegetable production. This is accomplished by using analytical methods that utilize laboratory chromatic validation of herbicides in soil and from the surface of polyethylene mulches used in plasticulture systems.

Figure 2.

Bare soil and low-density polyethylene (LDPE) mulch type beds used for vegetable production (left) (photo by Timothy Grey) and spraying LDPE mulch and weed infestations (right) (photo by Sidney Cromer).

2. Importance

Rotation of vegetable production using the same bare soil or LDPE mulch systems reduces cost and promotes environmental stability as there is less soil tillage and LDPE mulch waste produced that must be placed into a landfill [7]. Spring vegetables grown after LDPE mulch fumigation include watermelon [Citrullus lanatus (Thunb.) Matsum and Nak.], pepper (Capsicum annuum L.), tomato (Lycopersicon esculentum Mill.), squash (Cucurbita pepo L.), and eggplant (Solanum melongena L.). A second autumn crop often includes cabbage (Brassica oleracea L.), eggplant, cucumber (Cucumis sativus L.), or squash [3, 4, 5, 6]. There was no injury issue with respect to rotation of vegetables when using MBr as a fumigant. However, using soil applied herbicides initially and between crops as a burndown of the existing crop or weeds, creates a unique concern for potential injury from residues remaining in the soil or on the LDPE mulch, especially to transplants. The adsorption, desorption, mobility, biological degradation, and soil/LDPE mulch properties are important factors that determines the persistence of herbicides used in fresh market vegetable production. Applying residual herbicides to the soil surface at the time LDPE mulch is laid helps to improve weed control, while also maintaining and extending productive use of the LDPE mulch for subsequent crops. Applying herbicides between crops as part of the rotational process improves crop establishment success, and also mitigates weeds that could harbor other insect and disease pests. Therefore, understanding herbicide chemistry will be an integral part of continued successful fresh market vegetable production.

3. Herbicide use in vegetable production

The loss of MBr for weed control led to herbicide alternatives for vegetable production. Since the early 2000s, there have been several herbicide registrations in vegetable crops put in place for bare soil, row middle, soil applied prior to mulch laying, and over-the-top of mulch. These include fomesafen (5-[2-chloro-4-(trifluoromethyl)phenoxy]-N-(methylsulfonyl)-2- nitrobenzamide) [8], flumioxazin (2-[7-fluoro-3,4-dihydro-3-oxo-4-(2-propynyl)- 2H-1,4-benzoxazin-6-yl]- 4,5,6,7-tetrahydro-1H-isoindole-1,3(2H)-dione) [9], sulfentrazone (N-2,4-dichloro-5-[4-difluoromethyl)-4,5-dihydro-3-methyl-5-oxo-1H-1,2,4-triazo-1-yl]phenyl]methanesulfonamide) [10], halosulfuron-methyl (methyl 3-chloro-5-[[[[(4,6-dimethoxy-2-pyrimidinyl)amino]carbonyl]amino]sulfonyl]-1-methyl-1H-pyrazole-4-carboxylate) [11], S-metolachlor (2-chloro-N-(2-ethyl-6-methylphenyl)-N-[(1S)-2-methoxy-1-methylethyl]acetamide) [12], glyphosate (N-phosphonomethyl)glycine) [13], paraquat (1,1′-dimethyl-4,4′-bipyridnium dichloride) [14], and ammonium-glufosinate (2-amino-4- (hydroxymethylphosphinyl)butanoic acid) [15]. These herbicides encompass many different mechanisms of action (MoA) and control a wide variety of monocot and dicot weed species [16]. Table 2 describes several physiochemical properties of the aforementioned herbicides. This variety of herbicides assists growers with ensuring adequate weed control while maintaining crop safety, however proper planning is required to prevent injury to seeded or transplanted crops.

Herbicide	Molecular weight	Kdb	Kocc	Acid Dissociation Constant	Water Solubility	Kowd
	g/mol	(mL/g)		pKa	mg/L	log Kow
Fomesafen	438.76 (acid); 460.74 (Na Salt)	—	60	2.7	50 (acid); 600,000 (Na salt)	2.89 (pH 1)
Flumioxazin	354.34	—	500 to 13,000	Non-ionizable	1.79	2.55
Sulfentrazone	387.19	< 1.0	43	6.56	780	0.99 (pH 7)
Halosulfuron-methyl	434.81	0.36 to 1.6	124	3.5	1650	1.7 (pH 5); −0.02 (pH 7)
S-Metolachlor	283.80	2.16	200	Non-ionizable	488	2.89
Glyphosate	169.07 (acid); 207.16 (K salt)	324 to 600	24,000	2.6 (acid)	15,700 (acid)	0.0006 to 0.0017
Ammonium-glufosinate	181.13 (acid); 198.16 (NH4 salt)	—	100	<2, 2.9, 9.8	12,000 (acid)	0.53 (pH 7)
Paraquat	186.26	—	1,000,000	Non-ionizable	620,000 (salt)	−4.5

Table 2.

Physiochemical propertiesa of various residual (fomesafen, flumioxazin, sulfentrazone, halosulfuron-methyl, and S-metolachlor) and contact (glyphosate, ammonium-glufosinate, and paraquat) herbicides applied in vegetable production for weed control.

^a

Shaner DL. Herbicide Handbook. 10th ed. Lawrence (KS): Weed Science Society of America; 2014.

^b

K_d, Soil sorption coefficient: the ratio of sorbed pesticide to dissolved pesticide at equilibrium in a water/soil mixture.

^c

K_oc, Soil organic carbon sorption coefficient: calculated as K_d divided by the weight fraction of organic carbon present in soil.

^d

K_ow, Distribution coefficient between octanol and water.

Since plastic mulch systems are often used for multiple vegetable crops over several years, weed control is crucial to maintain the integrity of the mulch. Selecting a herbicide that controls weeds while eliminating subsequent crop injury is imperative. The main determining factors when selecting herbicides, with respect to use, are if the MoA has residual or contact efficacy to the target crops and weeds. With vegetable production systems utilizing various types of herbicides, analytical extraction can quantify residue information and determine herbicide behavior in soil and on mulch.

3.1 Residual-type herbicides

Helling [17] describes residual herbicides as those chemicals which provide season-long weed control due to persistence in soil but can have carryover that may injure susceptible rotational crops. This has proven critical in the grower decision making process in terms of vegetable rotations [18, 19]. In the herbicides listed above, fomesafen, flumioxazin, sulfentrazone, halosulfuron-methyl, and S-metolachlor all are residual herbicides used in southeastern US vegetable production.

Fomesafen is a member of the diphenyl ether herbicide family and registered for control of dicot species in agronomic crops, and yellow nutsedge in vegetables [16]. It has soil residual activity [20, 21, 22] with a half-life ranging from 6 to 12 months under aerobic conditions [23]. Fomesafen field half-life (DT₅₀) varied as soil under LDPE mulch was 47 days verses 12 days for bare soil [24], indicating reduced dissipation for this method of use. The behavior of fomesafen in soil is environment dependent [16]. With a typical production soil pH, fomesafen would exist in anionic form, becoming more available as soil pH drops [21]. Multiple studies evaluated preemergence soil residual activity in vegetables, with testing in tomato for control of American black nightshade (Solanum americanum Miller) [25], cucurbits for Amaranthus spp. and other weeds [26], crop tolerance in cantaloupe [27], and pepper [28, 29]. When used in combination with other herbicides in tomato production, fomesafen provided improved purple nutsedge control compared to fomesafen alone [30]. Through multiple public and private research efforts, fomesafen now has state specific vegetable crop registrations as noted via the IR-4 project for cantaloupe, eggplant, pepper, squash, strawberry, tomato, and watermelon [31].

Flumioxazin is a preemergence N-phenylpththalimide herbicide used to control a broad spectrum of weeds in a variety of cropping systems, such as peanut (Arachis hypogaea L.), orchard crops, and plasticulture strawberries [32, 33, 34]. Flumioxazin has negligible photodecomposition but does have variable soil half-lives, which are soil pH dependent [16]. Research on flumioxazin dissipation from LDPE mulch indicated wash-off with water as the main dissipation mechanism, as compared to the same method looking at dry conditions [35]. The DT₅₀ for the wash off experiment was 6 hours, with flumioxazin levels undetectable by 24 hours after treatment. The DT₅₀ of 57 hours for the dry experiment indicated that flumioxazin was persistent and could lead to critical efficacy and injury issues if a grower transplants vegetables onto flumioxazin treated LDPE mulch. This has led to many caveats associated with flumioxazin registrations. For strawberry there is a 30-day PRE-transplant interval when soil applied, and many precautionary requirements for row middle applications, due to potential contact with LDPE mulch [9, 31].

Sulfentrazone is a soil applied, non-phloem translocated, moderately persistent in soil, aryl triazolinone herbicide [16]. In agronomic crops it has been used to assist with herbicide resistant weed management issues around the world [36]. Given its soil residual activity, the primary uptake mechanism is via plant root absorption followed by xylem translocation [16]. Sulfentrazone DT₅₀ varies as noted with soil under LDPE mulch was 12 to 13 days verses 11 to 20 days for bare soil [24]. Sulfentrazone is a weak acid (pKa of 6.56) and availability increases as soil conditions become more alkaline [37]. Sulfentrazone provides control of both purple and yellow nutsedge. However, growers must consider rotational restrictions to ensure proper use requirements to prevent potential crop injury from carryover [8].

Halosulfuron-methyl is a primidinylsulfonylurea herbicide [16] with preplant incorporated, preemergence, and postemergence Cyperus spp. activity [38, 39], lending to its effective use in numerous vegetable crops [5, 40]. It has multiple vegetable registrations for soil application prior to laying LDPE mulch, and over the top of the mulch between crops [13]. This group of herbicides is typically applied at low use rates, however they still show residual soil activity. Halosulfuron-methyl soil DT₅₀ ranges from 6 to 98 days, depending on soil texture, moisture and temperature regimes [41, 42]. Postemergence halosulfuron-methyl applications over the top of LDPE mulch allows growers to control nutsedge between vegetable crop transplanting [5]. However, persistence on the LDPE mulch can be dependent on multiple abiotic processes including thermal energy from the sun, as noted with a DT₅₀ of 55 MJ m⁻² for halosulfuron-methyl [43]. Halosulfuron-methyl is an important tool for nutsedge control in vegetable production, and dissipation research is critical.

S-metolachlor is a chloroacetamide herbicide that inhibits very long chain fatty acid biosynthesis, that controls yellow nutsedge, annual grasses, several small-seeded broadleaf weeds, and has been agronomically available since the 1970s [16]. It is an important herbicide option for transplant tomato production in the Southeastern US in that it can be preplant incorporated, preemergence, and postemergence applied [3, 30]. S-metolachlor is water soluble at 480 mg L⁻¹ [16], allowing for analytical and bioassay analysis of its dissipation in soil [44, 45] and LDPE mulch production systems [35]. A beneficial aspect of using S-metholachlor in vegetable production is its relatively short DT₅₀ in soil systems allowing for multiple plant back options. There are 60-day rotational restrictions in terms of 2nd and 3rd crop rotations in LDPE mulch systems for many cucurbits, vegetable fruiting, and Brassica groups [11].

3.2 Contact-type herbicides

Contact-type herbicides refer to those which are applied for specific control options to mitigate weeds that could interfere with the succeeding crops. They can also be used to destroy the pervious crop when rotating in plasticulture systems. Registered herbicides for these types of weed control options include glyphosate [14] and paraquat [12] with ammonium-glufosinate having several pending uses in vegetables [10].

Glyphosate inhibits the synthase of the enzyme 5-enolpyruvylshikimate-3-phosphate (EPSP), which disrupts the shikimic acid pathway, therefore leading to a reduction in aromatic amino acids and eventual plant death [46], and utilized around the world in multiple cropping systems. Glyphosate is relied on as an over-the-top herbicide application for several vegetable crops grown on LDPE mulch, with a required 1.25 cm of water applied prior to transplanting, due to its negligible photodegradation losses and high-water solubility (Table 2) allowing for movement off of the plastic mulch [5, 35]. Gray et al. [35] reported glyphosate was still available in efficacious amounts on LDPE mulch out to 120 hours after treatment when no water was applied to wash it off. While glyphosate may be a viable option for control, growers still have to be conscious of potential crop injury [5]. Given its broad-spectrum of weed control for grass, broadleaf, and perennial species, is relatively inexpensive, and available in multiple generic forms, glyphosate will continue to be utilized by vegetable growers using LDPE and bare soil vegetable production.

Ammonium-glufosinate is a glutamine synthetase inhibitor, causing a buildup of ammonium in tissues and subsequently inhibiting photosynthesis and photorespiration, which leads to plant death [47]. Ammonium-glufosinate is water soluble (Table 2) and has pending registrations for use in LDPE mulch systems in the southeastern US [10]. As with glyphosate, the proper amount of irrigation or rainfall before vegetable transplant after ammonium-glufosinate application will be important to prevent injury [48].

Paraquat is a bipyridylium that inhibits photosystem I, which creates reactive and toxic radicals within the plant [49]. The actual cause of tissue damage in a plant is from a mixture of the rapid cycling between the paraquat ion and paraquat radical and the large number of electrons flowing through photosystem I. Symptoms progress to necrosis within 1 or 2 days from application. Paraquat is currently registered for bare soil and LDPE mulch production for its control of annual grass and broadleaf weeds [12] but must be properly managed to prevent crop injury [5]. Gray et al. [35] reported paraquat was still available in efficacious amounts on LDPE mulch out to 120 hours after treatment when no water was applied to wash it off, but photodegradation is a major dissipation mechanism. The rapid efficacy and excellent control of many weed species, along with multiple dissipation mechanisms from LDPE mulch with rainfall and sunlight, allows growers an alternative to glyphosate and ammonium-glufosinate for contact herbicide selections.

4. Research

The use of soil applied and over-the-top of mulch herbicides in vegetable plasticulture systems presents a unique opportunity in understanding the dynamics of herbicide dissipation and persistence when evaluated using a variety of field, bioassay, and laboratory techniques. Quantification of herbicides can be achieved using various instrumentation, with high performance liquid chromatography (HPLC) coupled with mass spectrometry (MS) being one of the most common and powerful tools available, given its ability to separate, identify, and quantitate various compounds present in a wide range of sample types. Studies conducted to quantify vegetable herbicide soil dissipation or movement from the mulch surface had similar objectives and sample preparation methods, while chromatographic and mass spectrometer instrument parameters were adapted to best fit individual compounds.

4.1 Field experimental procedures

Both residual and contact herbicide field studies were conducted on Tifton loamy sand, utilized raised beds (1.8 m x 6.1 m) that were left bare or covered with LDPE mulch or totally impermeable film (TIF), and all herbicides applied at their respective field rates. Soil applied herbicides under mulch were sampled by cutting an ‘L’ shape in the mulch using a box cutting knife and pushing an aluminum cylinder (7.62×7.62 cm) into the soil until flush with the surface (Figure 3). Soil cores were wrapped in aluminum foil and stored individually at −10°C until analysis. Sampling of contact herbicides applied over-the-top of mulch was done by cutting a square (0.1 m²) out of the mulch using an open-faced frame and box cutting knife (Figure 3). Mulch samples were placed and stored individually in plastic bags at −10°C until analysis.

Figure 3.

Procedure for sampling soil under mulch using an aluminum soil core (left) and an example of mulch sampling (right), which depicts a 0.1 m² square sample taken from LDPE mulch (photos by Kayla Eason).

4.2 Herbicide extraction

Solid phase extraction methods are dependent on the sample type, physiochemical properties of the target herbicide, and equipment available. Residual herbicides can be extracted from soil by combining a sub-sample of the soil core with an organic solvent mixture, shaking on a reciprocating shaker for several hours, and then transferring the supernatant into another vial for analysis or further cleanup if needed. While methods following that outline are widely used, specialized equipment can expedite the extraction process. One such example is microwave-assisted extraction (MAE), which can greatly reduce the volume of solvent and soil consumed while also reducing extraction time [50, 51, 52]. Contact herbicide extraction from the surface of mulch is accomplished by placing mulch samples into flasks filled with solution, shaking the flasks for several hours on a reciprocating shaker, and once complete transferring a sub-sample to a smaller vial for analysis or further cleanup.

4.3 Herbicide quantification

Once herbicide extraction is completed, vials (typically 2 mL in size) are placed into designated slots within the specialized chromatographic system. The analytical analysis methods vary by herbicide due to differing solubility, structure, and other physiochemical properties. Specific methods for the validation of residual and contact herbicides used in vegetable production is described in the subsequent subsections (4.3.1–4.3.7). For the purpose of this chapter, all herbicides were analyzed by a Waters™ Acquity HPLC system coupled with a Waters™ 2998 PDA UV detector and Waters™ QDa MS detector (Figure 4). Unless otherwise noted, LC separation was performed on a Symmetry C18 reverse-phase column (4.6×75 mm, 3.5 μm; Waters Corporation) for fomesafen, flumioxazin, S-metolachlor, and sulfentrazone, a Cortecs® C18 reverse-phase column (4.6×50 mm, 2.7 μm; Waters Corporation) for halosulfuron-methyl and paraquat, and an Anionic Polar Pesticide column (2.1×100 mm, 5 μm; Waters Corporation) for glyphosate and ammonium-glufosinate. The various herbicide amounts were quantified by correlating peak area detected to those of analytical grade standard solutions of various known concentrations. Selectivity was tested by using blank samples with no interfering peaks detected.

Figure 4.

Waters™ Acquity high performance liquid chromatography system (top and bottom left) equipped with a column heater (middle) and coupled with a Waters™ 2998 PDA ultraviolet detector (top right) and Waters™ QDa mass spectrometer detector (bottom right).

4.3.1 Fomesafen

Chromatographic conditions consisted of a gradient mobile phase, water (H2O) + 0.1% formic acid (A) and acetonitrile (ACN) + 0.1% formic acid (B), that started at 90% A, decreased to 10% A at 4.0 min, held isocratic for 2.0 min, then ramped up to 90% A at 6.1 min. The flow rate was 0.6 mL/min with a run time of 8 min. The MS was run in electron spray ionization (ESI) negative mode utilizing both multiple reaction monitoring (MRM) from 50 to 600 Da and single ion recording (SIR) at 437 Da (Figure 5).

Figure 5.

Chromatogram depicting fomesafen.

4.3.2 Flumioxazin

LC separation was performed at an ambient column temperature for each sample. The mobile phase consisted of ACN + 0.1% formic acid (A) and H2O+ 0.1% formic acid (B) and held isocratic at 70% A for 2.7 min per injection. Flow rate was set to 0.75 mL/min and the injection volume was 10 μL. The MS was run in ESI positive mode using MRM from 5 to 600 Da and SIR at 355 Da (Figure 6). Methods were adapted from previous literature [52, 53].

Figure 6.

Chromatogram depicting flumioxazin (left) and sulfentrazone (right).

4.3.3 Sulfentrazone

The LC mobile phase consisted of ACN + 0.1% formic acid (A) and H2O + 0.1% formic acid (B) and held isocratic at 60:40 (%A:%B) for 2.5 min. The injection volume was 2.5 μL with a flow rate of 1.0 mL/min. The MS was run in ESI negative mode using MRM from 5 to 600 Da and SIR at 385 Da (Figure 6).

4.3.4 Halosulfuron-methyl

The mobile phase, H2O+ 0.1% formic acid (A) and ACN + 0.1% formic acid (B), followed a gradient at 70% A for the initiation, at 0.8 min was 10% A, held at 10% A for 1.2 min, increased to 70% A at 2.3 min, and then held at 70% A for 1.0 min. Flow rate was 1.0 mL/min with an injection volume of 8.0 μL. The MS was run in ESI positive mode using MRM from 50 to 600 Da and SIR at 435 Da (Figure 7). The column was maintained at ambient temperature for each injection.

Figure 7.

Chromatogram depicting halosulfuron-methyl (left) and S-metolachlor (right).

4.3.5 S-metolachlor

LC conditions consisted of a mobile phase, H2O + 0.1% formic acid (A) and ACN + 0.1% formic acid (B), which followed a gradient of 90% A at initiation, 10% A at 2.1 min, held isocratic for 3.0 min, ramping up to 90% A at 5.1 min, and then held isocratic for 2.0 min. The flow rate was 1.37 mL/min with a run time of 7.0 min and an injection volume of 200 μL. The column was heated and maintained at 25°C for each injection. The MS operated in ESI positive mode using MRM from 50 to 600 Da and SIR at 284 Da (Figure 7).

4.3.6 Glyphosate and ammonium-glufosinate

Mobile phase was H2O + 0.9% formic acid (A) and ACN + 0.9% formic acid (B), with the gradient starting at 10% A, ramping to 60% A at 2.0 min, and increasing to 90% A at 4.0 min. The flow rate was maintained at 0.75 mL/min for 3.5 min. The injection volume was 7.5 μL and column temperature set to 40°C for the entire run. There was a strong (10:90 ACN:H2O) and weak seal wash utilized (90:10 ACN:H2O). Two injections were run per sample at a run time of 4.0 and 2.0 min per injection for glyphosate and ammonium-glufosinate, respectively. The MS operated in ESI negative mode using MRM from 50 to 600 Da and simultaneously running SIR at 168 and 180 Da for glyphosate and ammonium-glufosinate, respectively. Figure 8 shows a chromatogram of glyphosate at 168 Da and ammonium-glufosinate at 180 Da using these chromatographic and MS conditions. This method was developed specifically for the novel anionic, polar pesticide column [54].

Figure 8.

Chromatogram depicting glyphosate (left) and ammonium-glufosinate (right).

4.3.7 Paraquat

LC conditions consisted of a 200 mM ammonium formate buffer (pH 3.7) (A) and ACN (B) mobile phase, which was held isocratic at 50:50 (%A:%B) for 3.0 min at a flow rate of 0.5 mL/min. The injection volume was 20 μL and column temperature set to 30°C for the entire run. A H2O:ACN seal and needle wash solvent (50:50 v/v) was used. The MS operated in ESI positive mode using MRM from 50 to 600 Da and simultaneously running SIR at 186 Da.

5. Data analysis

Chromatographic data requires further analysis before validation. Most herbicide dissipation data are subjected to either a linear or non-linear regression method [55]. The kinetic model used should best describe individual data since herbicide dissipation can vary greatly between compounds. Most residual herbicide dissipation data can be described using a single first-order kinetic model, specifically the exponential decay equation:

$$\text{y}=B_0{e}^{-k\left(t\right)}$$E1

where y is the measured herbicide concentration, B₀ is the initial herbicide concentration when time (t) is 0, k is the first-order dissipation constant, and t is the amount of time elapsed after herbicide application. The amount of time taken for 50% of a herbicide to dissipate (half-life) can be determined when following single first-order kinetics:

$$D{T}_{50}=\ln \left(2\right)/k$$E2

where DT₅₀ is the half-life and k is the dissipation rate determined when herbicide concentration is regressed against time. These equations (Eqs. (1) and (2)) can be used to describe herbicide dissipation in soil and from the surface of mulch.

Temperature, moisture, sunlight, and microbial populations can all influence the rate of herbicide dissipation, with factors being intertwined and influential on each other. Given the complexity of herbicide behavior, dissipation cannot always be described using the exponential decay equation (Eq. (1)). When herbicide concentrations follow both a slow and fast decay pattern, a bi-phasic kinetic model (Eq. (3)) can be used to describe dissipation:

$$y=\left(\left(B_0\right)\left(\%Fast\right)\left(0.01\right)\right)e^{-k_{fast}\left(t\right)}+\left(\left(B_0\right)\left(100-\%Fast\right)\left(0.01\right)\right)e^{-k_{slow}\left(t\right)}$$E3

where y is the measured herbicide concentration, B₀ is the initial herbicide concentration when time (t) is 0, %Fast is the fraction of the model accounted for by the faster of the two phases, k_fast is the first-order dissipation rate constant for the fast phase of the model, k_slow is the first-order dissipation rate constant for the slow phase of the model, and t is the amount of time elapsed after herbicide application. The k_fast and k_slow rate constants can be applied to the half-life equation (Eq. (2)) to determine the half-life for both phases of herbicide dissipation.

5.1 Validation of herbicide concentrations over time

When quantified using chromatography, herbicide dissipation over time coupled with edaphic and environmental information creates a complete picture of how the specific compound will behave. This information directly impacts decision making when selecting the appropriate herbicide to use. The following results are interpretations of quantified herbicide concentrations over time using the methods previously discussed.

When comparing fomesafen dissipation under LDPE mulch and in bare soil, application under mulch greatly extended the half-life (Figure 9). In bare soil beds fomesafen dissipation was rapid when compared to dissipation under LDPE mulch (Figure 6), with a half-life of 2 days for bare soil and 58 days under mulch. Figure 9 also highlights the difference between a single first-order kinetic model and bi-phasic kinetic model, with samples under LDPE mulch best fitted to the exponential decay equation (Eq. (1)) and the bare soil samples fitted to a double exponential decay equation (Eq. (3)). The half-life of flumioxazin (30 days) is shorter than fomesafen and sulfentrazone (71 days).

Figure 9.

Fomesafen dissipation in soil left bare or under LDPE mulch. Each line represents the first-order change in herbicide concentration as a percentage of the applied amount. Data points indicate the means of replications with error bars representing the standard error of each mean.

Glyphosate and ammonium-glufosinate dissipation from the surface of LDPE mulch can be described by the exponential decay equation (Figure 10). DT₅₀ was determined to be 3.3 and 1.4 days for glyphosate and ammonium-glufosinate, respectively. Both herbicides were below the respective limit of detection by 18 days after application. Using similar sampling, processing, and analytical methods Hand et al. [56] reported glyphosate to have a half-life of 2.2 days when applied to the surface of plastic mulch. These findings correlate to the amount of rainfall received, which given the water solubility of both glyphosate and ammonium-glufosinate was the most influential factor in dissipation (Figure 11).

Figure 10.

Herbicide dissipation from the surface of LDPE mulch over time using the exponential decay equation (Eq. (1)) with nonlinear regression applied, for glyphosate (A) and ammonium-glufosinate (B). Each line represents the first-order change in herbicide concentration as a percentage of the applied amount. Data points indicate the means of replications with error bars representing the standard error of each mean. Parameter estimates for glyphosate (A): y = 110.922e^(−0.2092x), R² of 0.8996, and a k-value of 0.2092; ammonium-glufosinate (B): y = 110.077e^(−0.5069x), R² of 0.9114, and a k-value of 0.5069.

Figure 11.

Glyphosate and ammonium-glufosinate dissipation from the surface of LDPE mulch over time as influenced by rainfall. Each line represents the first-order change in herbicide concentration as a percentage of the applied amount. Data points indicate the means of replications with error bars representing the standard error of each mean. Parameter estimates for glyphosate (A): y = 110.922e^(−0.2092x), R² of 0.8996, and a k-value of 0.2092; ammonium-glufosinate (B): y = 110.077e^(−0.5069x), R² of 0.9114, and a k-value of 0.5069. Bars represent cumulative rainfall over time.

Further investigation on the removal of herbicides from the surface of plastic mulch found that after at least 0.63-cm of irrigation glyphosate and ammonium-glufosinate both had less than 0.1% (of the applied concentration) remaining on the mulch. S-metolachlor wash-off from the surface of LDPE mulch required at least 1.27-cm of irrigation to have less than 5% remaining. Halosulfuron-methyl concentrations on the surface of mulch after 1.27-cm of irrigation were less than 2%.

6. Conclusion

Many vegetables are produced using a plasticulture system, which generally consists of raised beds of soil covered with plastic mulch. Herbicides are applied to the soil once beds are made but before they are covered with mulch. These herbicides need to provide the longest residual weed control possible while still maintaining crop safety. Producers often utilize the same plastic mulch for two to five crop production cycles over the course of multiple growing seasons, allowing growers to spread the cost of production over multiple crops and mitigating the cost of reapplying mulches for each crop. The time between the termination of one crop and the planting of another allows for the germination, emergence, and establishment of troublesome weeds in old plant holes. Often, one of the greatest challenges in plastic mulch vegetable systems is eliminating the first crop and any weeds growing under and through holes in the mulch prior to planting the subsequent crop. Herbicides that can be applied over-the-top of plastic mulch prior to transplanting a crop without damage are crucial for maintaining vegetable weed management systems. Herbicide use in vegetable production can raise concerns if information is not known about carryover effects or how the individual herbicide will behave in soil or on the surface of the mulch bed. This can be mitigated by quantifying herbicide dissipation in various vegetable production scenarios by utilizing field and analytical techniques, such as chromatographic systems.

Chromatography is a multi-faceted analytical tool that can separate, identify, and quantitate various compounds present in a wide range of sample types. These analytical systems can be used to quantitate the persistence of various herbicides on soil and plastic mulch. Coupling chromatographic outputs with environmental data, such as rainfall and cumulative solar radiation, we can predict half-life information and dissipation rates for individual herbicides. Growers can directly use this information when making crucial weed management decisions.