Polymeric Systems for the Delivery of Herbicides to Improve Weed Control Efficiency

Abstract

Weeds are unwanted plants, which interfere with the crop production. Weeds compete with crops for resources, causing severe yield loss. Chemical weed control through herbicides is a quite effective and reliable strategy to manage weeds. Herbicides constitute a major share of the global pesticide market. However, the applied herbicides undergo losses in the agroecosystem in different ways (chemical degradation, microbial decomposition, photo-degradation, leaching, run-off, and volatilization), thus lowering the herbicidal action coupled with contaminating ecosystem and groundwater. Encapsulation of herbicides is an innovative approach that addresses issues associated with the application of herbicides for controlling weeds. Encapsulation represents the embedding of an active ingredient in shell of polymeric material to achieve the controlled release of the active ingredient at the desired rate. The encapsulation of herbicides enhances stability, solubility, and bioactivity and alters the release pattern of herbicide resulting in improved weed control efficiency. Further, encapsulation lowers the application rate of herbicides, which in turn reduces the residue carryover of herbicide in soil and minimizes the environmental hazards. Therefore, encapsulated herbicide formulation has greater significance in the future weed management and will become ground-breaking technology in the chemical era of weed control.

1. Introduction

Weeds are as old as agriculture that influence crop growth and yield. Weeds compete with crops for resources such as space, water, nutrients, and light, which indirectly affect crop growth. Weeds inflicted tremendous yield loss besides deteriorating crop and grain quality, chocking water flow in an irrigation channel, sheltering crop pests, and causing ailments in living beings. The estimate on yield reduction due to weeds was one-third among agricultural pests [1]. The average loss in agricultural production due to weed infestation were 5, 10, and 25% in most developed, developing, and least developing nations, respectively [2]. Weeds cause a yield reduction of 10–80% depending on crops, type of weed flora infested, and magnitude of crop-weed competition [3]. Generally, yield loss due to weeds is 37% in developing countries, where either 90–95% or complete crop failure is common in certain locations [4]. There are 30,000 weed species infesting various crops on a global scale, out of which, 18,000 weed species are problematic causing severe yield losses in crop production. Estimates of 826 weed species were recorded in India, of which, 198 weed species were reported as serious weeds, while 80 weed species were classified as very serious weeds [5].

The degree of weed infestation is at an increasing rate in agricultural and non-cropped lands. The cost incurred for the adoption of weed control measures increases the cost of cultivation and reduces profit for the farmers. Manual weeding and mechanical weed management practices were the major options to manage weeds in agricultural fields. However, due to demand for human labor and increase in labor wages force the farmers to use herbicides to manage weeds. An annual average of 2 million tonnes of pesticides is consumed worldwide, where consumption of herbicides positioned first (47.5%) [6]. Herbicides are now widely used to manage weeds in modern agriculture. However, herbicides undergo various losses in soils after application viz. photo-degradation, volatilization, leaching, microbial degradation, run-off, etc., which in turn lowers the weed control potential of herbicides. Moreover, off-site transport of applied herbicides causes groundwater contamination. Nowadays, low volume herbicides are available, which show higher herbicidal activity and weed control efficiency over conventional herbicide formulations. Nevertheless, low volume herbicides are also reported with faster degradation potential in soil and increase the risk of environmental pollution. Therefore, encapsulation is an innovative and promising approach for developing controlled release formulations. Encapsulated formulation minimizes the herbicide loss in agroecosystem and improves weed control efficiency at a lower dosage. Encapsulation regulates the release and availability of active ingredients in the targeted site. Encapsulation involves the loading of active ingredients in the secondary material. The secondary materials are generally polymeric systems that regulate the release of active ingredients into the environment through diffusion-mediated process. Thus, encapsulation enhances the herbicidal activity and achieves higher weed control efficiency at a lower application rate. With the background, the chapter discusses the encapsulation of herbicides in the polymeric system and the characteristics of formulations and their scope in weed management.

2. Weeds and their characteristics

The term “weed” refers to “a plant out of its place or a plant growing where it is not desired at that time” [7]. The definition implies that Echinochloa sp is a weed in rice fields, similarly, pigeon pea is also considered a weed in greengram fields. Rice var. Jaya is a weed in IR 8 rice fields. Weeds are notorious and unwanted plants, which affect crop production. Weeds are categorized into annuals, biennials, and perennials based on their ontogeny [8]. Annual weeds complete their life cycle in a season with abundant seed production, while biennials survive for two seasons, completing the vegetative phase in the first season and reproductive phase in the succeeding season. Perennial weeds live for more than two years and propagate through both seeds and underground storage parts such as tubers, rhizomes, stolon, etc. Seed propagation is the sole mechanism for dispersal in annual and biennial weeds, whereas perennial weeds are largely propagated through vegetative propagules (Table 1).

Weed	Scientific name	Family	Ontogeny	Mode of propagation
Country mallow	Abutilon indicum	Malvaceae	Annual	Seeds
Indian copper leaf	Acalypha indica	Euphorbiaceae	Annual	Seeds
Bristly starbur	Acanthospermum hispidum	Asteraceae	Perennial	Achenes
Khaki weed	Alternanthera echinata	Amaranthaceae	Biennial	Seeds
Alligator weed	Alternanthera philoxeroides	Amaranthaceae	Perennial	Seeds
Slender amaranth	Amaranthus viridis	Amaranthaceae	Annual	Seeds
Blistering ammannia	Ammannia baccifera	Lythaceae	Annual	Seeds
Mexican prickly poppy	Argemone mexicana	Papavaraceae	Annual	Seeds
Cape ash	Bergia capensis	Elatinaceae	Annual	Seeds
Purple chloris	Chloris barbata	Poaceae	Perennial	Seeds
Field bindweed	Convolvulus arvensis	Convolvulaceae	Perennial	Seeds and roots
Spreading dayflower	Cyanotis axillaris	Commonlinaceae	Annual	Seeds and bits of stem
Bermuda grass	Cynodon dactylon	Poaceae	Perennial	Seeds and stolon
Purple nutsedge	Cyperus rotundus	Cyperaceae	Perennial	Seeds and tubers
Umbrella sedge	Cyperus difformis	Cyperaceae	Annual	Seeds
Flat sedge	Cyperus iria	Cyperaceae	Annual	Seeds
Creeping wood sorrel	Oxalis corniculata	Oxaidaecae	Perennial	Seeds and tuberous roots
Johnson grass	Sorghum halpense	Poaceae	Perennial	Seeds and rhizome

Table 1.

Characteristic description of some important weeds.

Weeds adapt well to a diverse ecosystem, which makes weeds more competitive than crops. Weeds produce abundant seeds in a single season and enrich the weed seed bank. A field with a seed bank of 5500 seeds m⁻² will increase the seed count of 1,98,500 Nos. m⁻² in two years, when there is no adoption of weed control measures [9]. Weed seeds are lighter in weight and smaller. Weed Phalaris minor weighs a test weight of 1.5–2.1 g [10] compared to the test weight of wheat (40 g). Some weeds produce seeds without fertilization, i.e., apomixis (e.g., Taraxacum spp.) [9]. Further, weed seeds germinate earlier and establish rapidly before the establishment of crops. Certain weed species exhibit rapid seedling growth and attain earlier maturity. Carrot grass (Parthenium sp) enters the flowering phase four weeks after emergence [11]. Weeds produce flowers and set seeds well in advance before the harvest of a crop. Weeds exhibit environmental plasticity to withstand vagaries of climatic conditions (drought, heat, cold) and edaphic situations through better adaptive and survival mechanisms. Parthenium weed exhibits faster growth at elevated levels of carbon dioxide and temperature. Weeds are mostly self-sown plants, which do not require optimum climatic and soil conditions for establishment. Moreover, weeds are opportunistic plants, which colonize everywhere if it is not controlled properly (Table 2).

Weed species	Seed production per plant [12, 13, 14]
Redroot pigweed	1,17,400
Common lambsquaters	72,450
Common purslane	52,300
Shepherd’s purse	38,500
Carrot weed	30,000
Common ragweed	3380
Jungle rice	460–740
Wild oat	250

Table 2.

Seed production potential of weeds.

Some weeds produce seeds morphologically mimicking crop seeds thus escaping from physical separation. The maturity of wild mustard Argemone mexicana coincides with the harvest of mustard crop and produces seeds resembling mustard seeds [15]. Certain annual weeds produce more than one flush in a single season, which increases the weed seed bank in the soil. Carrot grass completes four to five generations in a year under ideal environmental conditions [16]. Weeds produce a huge number of seeds; however, not all seeds germinate at a time. Weed seeds have the ability to resist decaying and exhibit various modes of dormancy. Velvetleaf (Abutilon theophrasti) and Fieldbind weed (Convolvulus arvensis) showed dormancy due to hard seed coat [17, 18]. Weed seeds have more longevity and remain viable for many years due to the phenomenon of dormancy. Field sowthistle (Sonchus sp.) produces viable seeds even when plants are cut at the flowering stage (Table 3) [19].

Weed species	Seed viability	References
Parthenium sp.	8–10 years	[16]
Convolvulus arvensis	20 years or more	[20]
Chenopodium album	1700 year	[21]
Nelumbo nucifera	More than 3000 years	[22]
Stellaria media	More than 20 years	[23]

Table 3.

Longevity of weed seeds.

Weeds compete with crops efficiently for foraging nutrients from the soil with better-structured mechanisms. Weeds extract and accumulate more nutrients than crops, which make crops starve for nutrients. Crop nutrient contents, especially nitrogen, are closely correlated with crop yield potential, while an intense competition of weeds for nitrogen reduces the crop yield significantly. Weeds exhaust a huge amount of nutrients in soil in each season, thereby making soil progressively deficient in soil nutrients, thus affecting the crop growth and yield (Table 4).

Weed species	Nutrient content (%) [24, 25, 26]
	N	P	K
Amaranthus viridis	3.16	0.06	4.51
Chenopodium album	2.59	0.37	4.34
Achyranthus aspera	2.21	1.63	1.32
Cyperus rotundus	2.17	0.26	2.73
Ipomea carnea	1.90	0.75	2.50
Cynodon dactylon	1.72	0.25	1.75
Parthenium hysterophorus	2.54	0.44	1.23

Table 4.

Nutrient composition of weeds.

Weeds such as Digitaria sanguinalis (696), Echinochloa colona (674), Cynodon dactylon (813), Tephrosia purpurea (1108), and Tridax procumbens (1402) have higher transpiration coefficient than crops such as sorghum (394) and maize (352) [27]. Vegetative propagules of weeds (roots, stolons, rhizomes etc.) penetrate deeper soil strata and grow vigorously with larger food reserves supporting weeds to survive under stress conditions. Seeds of fieldbind weed present at a soil depth of 6 cm have the ability to germinate normally [28]. Similarly, weeds of carpetweed (Trianthema portulacastrum) have the potential to germinate from a soil depth of 9 cm [29]. Roots of sowthistle located in the soil depth of 50 cm produce shoots to reach above-ground [19]. Similarly, perennial weeds have regenerative ability while many weeds possess adaptive mechanism (disagreeable odor, bitter taste, spines, etc.), which repel animals from grazing (evasiveness). Animals, birds, winds, water, etc. disseminate weed seeds [30]. Field sowthistle disperses weeds to a distance of 100 m through wind [31]. Yellow mistletoe (Loranthus europaeus) is mostly dispersed through birds such as Mistle Thrush (Turdus viscivorus) and Eurasian jay (Garrulus glandarius) [32]. Most of the weeds exhibit C₄-type photosynthesis conferring the advantage to mitigate the impact of moisture stress during crop growth and utilize low levels of CO₂ in the crop microclimate for photosynthesis.

3. Impacts of weed infestation on crop production

Weeds are the major biotic threat, which affect yield and crop quality by exerting direct (allelopathy) and indirect (competition) influence on crops. Moreover, weeds serve as a reservoir of various crop insects and diseases. It also reduces the working efficiency of labor and agricultural machinery and increases the cost of cultivation.

The degree of competition of weeds on crops depends on weed flora infested, level and duration of weed infestation, competing ability of crops, and climatic factors that influence crop and weed growth. The yield reduction of crops due to weed infestation is directly correlated with the degree of weed competition. The increase of one kilogram of weed biomass reduces the crop biomass by one kilogram [33]. Weeds affect crop growth directly by releasing allelochemicals. Weed infestations cause 100% yield loss in crops if the weed remains uncontrolled. Weeds are responsible for 33% (one-third) of yield losses in crops among the agricultural pests in India [1]. The yield reduction due to weed infestation in various crops is presented in Table 5.

Crops	Per cent yield loss	Reference
Direct Seeded rice	21.4	[34]
Transplanted rice	13.8	[34]
Wheat	10–60	[35]
Maize	25.3	[34]
Sorghum	25.1	[34]
Pearlmillet	27.6	[34]
Fingermilet	50	[35]
Greengram	30.8	[34]
Chickpea	10–50	[35]
Redgram	16–65	[35]
Lentil	30–35	[35]
Horsegram	30	[35]
Soybean	31.4	[34]
Groundnut	35.8	[34]
Niger	20–30	[35]
Cotton	40–60	[35]
Jute	30–70	[35]
Sugarcane	25–50	[35]
Sugarbeet	70	[36]
Tomato	92–95	[37]
Okra	40–80	[38]
Cabbage	45–80	[39]
Cauliflower	61	[40]
Carrot	90	[41]
Radish	86	[42]
Peas	25–30	[40]

Table 5.

Yield reduction due to weed infestation.

Similarly, the estimated yield loss of grain crops in Australia was 2.52 billion USD due to weed infestation [43]. India suffers an economic loss of USD 11 billion annually due to weeds. In addition, higher monetary losses due to weeds were documented in rice (USD 4420 million) followed by wheat and soybean (USD 3376 and 1559 million, respectively). Annual yield loss of 3 million tons in China due to weed infestation in grain crops [44].

Weed infestation reduces crop and grain quality [45]. Certain weed species set seeds coinciding with crop maturity and few weeds produce seeds, which resemble crop seeds. Therefore, weed seeds have a chance to form admixture with crop seeds during the harvest thud affecting grain quality. Mustard seeds get contaminated with seeds of Mexican prickly poppy (Argemone mexicana) during the harvest. Weed infestation affects the quality of leafy and other vegetable crops. Commercially available wheat grain for household purposes was found to be contaminated with seeds of Phalaris minor @ 2–3 g/kg of grain [46]. Similarly, leaves of Loranthus (Dendrophthoe falcate) were plucked unwittingly impairing tea quality.

Weeds act as collateral hosts for various crop insects, diseases, and nematodes. Weeds act as a reservoir for various pests providing food and habitat that in turn affect crops. Weeds acting as hosts for pests and diseases are listed in Table 6.

Weeds	Crop insects/disease	Crop	Reference
Brachiaria mutica, Digitaria marginata, Dinebra retroflexa, Echinochloa crusgalli, Leersia hexandra	Blast disease (Pyricularia grisea)	Rice	[47]
Mikania cordata, Bidens biternata, Emilia sonchifolia, Polygonum chinense and Lantana camara	Tea mosquito bug (Helopeltis theivora)	Tea	[48]
Anagallis arvensis, Convolvulus arvensis and Chenopodium album	Alternaria blight	Mustard	[49]
Elytrigia spp, Agropyron spp, Festuca spp, Dactylis spp, Phleum spp and Lolium spp	Stem rust	Wheat	[50]

Table 6.

Weeds act as shelter for insect pest and diseases.

Weeds interfere with the movement of laborers while carrying out various farm operations viz. weeding, fertilizer application, spraying of chemicals, etc. Weeds also cause physical discomfort such as itching, allergy symptoms in human beings, and reduce efficiency during field operations. Parthenium weed causes human-related ailments such as asthma, skin rashes, eczema, swelling and itching of mouth and nose, etc. [51]. Fields infested with weeds such as Argemone mexicana and Amaranthus spinosus possess thorns and spines, respectively, which in turn restrict the movement of farm laborers causing hindrance to carrying out field operations.

Weed-free environment is prerequisite for attaining the maximum possible yield. Therefore, weed management practices raise the cost of cultivation and reduce the profit for farmers. The average cost of weed control is ₹4000 ha⁻¹ for winter season crops, while it is ₹6000 ha⁻¹ for crops that are grown during the rainy season [52]. Similarly, grain growers in Australia spent $113 per hectare for weed control [43].

4. Weed management strategies

Weed infestations are dynamic in nature. The adoption of high-input agricultural practices, use of high-yielding dwarf varieties and hybrids, and adoption of monoculture cause weed shift and composition of weeds. Moreover, invasion of alien weeds and consequences of climate change also determine the weed composition and weed dominance in field conditions. Therefore, ideal weed management strategies are crucial for establishing favorable environment for crops.

Weed management methods that commonly adopted in agriculture are prevention, cultural methods, mechanical methods, chemical weed management, and biological method. Weed management on a farm become successful when adoption of various weed management techniques as an integrated approach.

Cultural method encompasses crop management practices ranging from field preparation to crop harvest. Cultural method provides a favorable crop environment for crops to establish well to compete with weeds. Cultural method minimizes the yield reduction and maintains the purity of harvested grain. Similarly, cultural methods prevent the enrichment of weed seed bank. Cultural methods are cost-effective, feasible to adopt, and ecologically sound in nature; however, these are labor-intensive methods.

Mechanical and physical methods involve physical removal of weeds before sowing or planting crops or during the crop period. The method intends to either kill weeds or make them less favorable for weed seed germination and establishment. It includes tillage operations, manual weeding, hoeing, sickling, digging, dredging, chaining, mowing, cutting, stale seedbed, flooding burning, flaming, and mulching. This method is highly effective in controlling perennial weeds and reducing annual weed infestation in cropped lands. It saves time and labor for weeding. However, weeds found closely to crop are not removed through physical methods. Mechanical method warrants optimum soil moisture for weeding operations.

The use of chemicals was the third era of agronomical practices, which created a major impact in agriculture by substituting labor and mechanical energy [53]. The word “herbicide” is derived from Latin “herba” and “caedere” meaning “plant” and “to kill,” respectively. It implies that herbicides are used to kill the plant. Chemical weed control is the only strategy in areas of labor scarcity and, where mechanical and manual weeding is not feasible [54]. Herbicides are greatly differed in chemical structure, mobility in plants, mechanism of action, polarity, solubility, selectivity, etc. The pre-emergence herbicides control weeds that are emerged from soil. Selective herbicides with reference to crops are useful to eliminate mimicry weeds. Herbicides are effective in controlling problematic and perennial weeds. Chemical weed control is the cost-effective and reliable option compared to other weed management strategies. However, chemical weed control has certain limitations viz. herbicide drift, groundwater contamination, residual effect on succeeding crop, and risk of developing herbicide-resistant biotypes.

Biological weed management involves the use of living organisms such as disease-causing organisms, insects, animals, fish, and competitive plants to suppress the growth of weeds. Biological control does not eradicate weeds completely but it will minimize weed population. Biological control measures are effective against introduced weeds [55, 56]. The remarkable examples of the success of biological weed control were the eradication of Prickly pear (Opuntia spp.) in Australia and Lantana in Hawaii [57, 58].

Among different weed management strategies, chemical weed management is quite efficient, convenient, and economical to control weeds. There are different herbicides that are commercially available in the market to manage weeds. However, there are many factors that govern when, where, and how a particular herbicide is used for managing weeds.

5. Herbicides in weed management

Herbicides are a crucial component in chemical weed management. Due to labor shortage and hike in labor wages, farmers are forced to use herbicides in their fields. Herbicides are extensively used at a large scale to control weeds both in cropped and non-cropped areas. The application of herbicide has made remarkable transformation in agricultural production. Herbicides replace the manual and mechanical weed control in modern-day agriculture [59]. Hay [60] described the progress of herbicide evolution in agriculture.

Chemicals such as oil wastes, rock salts, copper salts, crushed arsenical ores, and sulphuric acid were used initially in the 1920s for eradicating weeds infested railway tracks, roads, and timber yards [61]. Pokorny synthesized 2,4-D herbicide in 1941 and found that 2,4-D was selectively toxic to broadleaved weeds. This work was the foundation for the development of herbicides. Herbicides occupy a major share (47.5%) in the pesticide market followed by insecticides and fungicides [6]. There are a variety of herbicides, which are commercially available for use viz. selective, nonselective, systemic, and contact herbicides. Herbicides are greatly varied in site of action and show selectivity for the control of weeds without affecting crops [62]. Plant factors include exposure of meristems to spray droplets of herbicides, leaf traits and root morphology affect the selectivity of herbicides. Plant characteristics of genetic make-up also influence the selectivity of herbicides. Herbicides kill target species alone without affecting nontarget species. Herbicide Resistance Action Committee (HRAC) [63] grouped herbicides based on mode of action are listed in Table 7.

Mode of action	Herbicide family	Herbicide
Inhibition of Acetyl CoA Carboxylase (ACCase)	Cyclohexanediones (DIMs)	Alloxydim, Butroxydim, Clethodim, Sethoxydim, Tepraloxydim, Tralkoxydim
	Aryloxyphenoxy-propionates (FOPs)	Clodinafop-propargyl, Cyhalofop-butyl, Diclofop-methyl, Fenoxaprop-ethyl, Fluazifop-butyl, Isoxapyrifop, Metamifop, Quizalofop-ethyl
	Phenylpyrazoline	Pinoxaden
Inhibition of Acetolactate Synthase (ALS)	Pyrimidinyl benzoates	Bispyribac-sodium, Pyriminobac-methyl, Pyrithiobac-sodium
	Sulfonanilides	Pyrimisulfan, Triafamone
	Triazolopyrimidine—Type 1	Cloransulam-methyl, Diclosulam, Florasulam, Flumetsulam, Metosulam
	Triazolopyrimidine—Type 2	Penoxsulam, Pyroxsulam
	Sulfonylureas	Amidosulfuron, Azimsulfuron, Bensulfuron-methyl, Chlorimuron-ethyl, Chlorsulfuron, Ethoxysulfuron, Flazasulfuron, Flucetosulfuron, Halosulfuron-methyl, Imazosulfuron, Metsulfuron-methyl, Orthosulfamuron, Pyrazosulfuron-ethyl, Rimsulfuron, Sulfosulfuron, Triasulfuron, Tribenuron-methyl, Trifloxysulfuron-Na, Triflusulfuron-methyl
	Imidazolinones	Imazamox, Imazapic, Imazapyr, Imazethapyr
	Triazolinones	Flucarbazone-Na, Propoxycarbazone-Na, Thiencarbazone-methyl
Inhibition of Photosynthesis at PS II—Serine 264 Binders	Triazines	Atrazine, Cyanazine, Cyprazine, Desmetryne, Dimethametryn, Prometon, Prometryne, Simetryne, Simazine, Terbumeton, Terbuthylazine, Terbutryne, Trietazine
	Triazolinone	Amicarbazone
	Triazinones	Ethiozin, Hexazinone, Isomethiozin, Metamitron, Metribuzin
	Uracils	Bromacil, Isocil, Lenacil, Terbacil
	Phenylcarbamates	Chlorprocarb, Desmedipham, Phenisopham, Phenmedipham
	Pyridazinone	Chloridazon, Brompyrazon
	Ureas	Benzthiazuron, Chloroxuron, Difenoxuron, Diuron, Fenuron, Fluometuron, Isoproturon, Linuron, Monuron, Neburon, Tebuthiuron
	Amides	Pentanochlor, Propanil
Inhbition ofPhotosynthesis at PS II—Histidine 215 Binders	Nitriles	Bromofenoxim, Bromoxynil, Ioxynil
	Phenyl-pyridazines	Pyridate
	Benzothiadiazinone	Bentazon
PS I Electron Diversion	Pyridiniums	Cyperquat, Diquat, Paraquat
Inhibition of Protoporphyrinogen Oxidase (PPO)	Diphenyl ethers	Acifluorfen, Chlornitrofen, Fluoronitrofen, Nitrofen, Oxyfluorfen
	Phenylpyrazoles	Pyraflufen-ethyl
	N-Phenyl-oxadiazolones	Oxadiargyl, Oxadiazon
	N-Phenyl-imides	Fluthiacet-methyl, Pentoxazone, Trifludimoxazin, Tiafenacil
Inhibition of Phytoene Desaturase	Phenyl ethers	Beflubutamid, Diflufenican, Picolinafen
	N-Phenyl heterocycles	Flurochloridone, Norflurazon
	Diphenyl heterocycles	Fluridone, Flurtamone
Inhibition of Hydroxyphenyl Pyruvate Dioxygenase (HPPD)	Triketones	Tembotrione, Tefuryltrione, Bicyclopyrone, Fenquinotrione
	Pyrazoles	Pyrasulfotole, Topramezone, Pyrazolynate, Pyrazoxyfen
	Isoxazoles	Isoxaflutole
Inhibition of Homogentisate Solanesyltransferase	Phenoxypyridazine	Cyclopyrimorate
Inhibition of Deoxy-D-Xyulose Phosphate Synthase	Isoxazolidinone	Clomazone, Bixlozone
Inhibition of Enolpyruvyl Shikimate Phosphate Synthase (ESPS)	Glycine	Glyphosate
Inhibition of Glutamine Synthetase	Phosphinic acids	Glufosinate-ammonium, Bialaphos or bilanafos
Inhibition of Dihydropteroate Synthase	Carbamate	Asulam
Inhibition of Microtubule Assembly	Dinitroanilines	Fluchloralin, Isopropalin, Nitralin, Oryzalin, Pendimethalin, Trifluralin
	Pyridines	Dithiopyr, Thiazopyr
	Phosphoroamidates	Butamifos, DMPA
	Benzoic acid	DCPA
	Benzamides	Propyzamide
	Carbamates	Carbetamide, Chlorpropham, Propham
Inhibition of Cellulose Synthesis	Triazolocarboxamide	Flupoxam
	Benzamides	Isoxaben
	Alkylazines	Triaziflam, Indaziflam
	Nitriles	Dichlobenil, Chlorthiamid
Uncouplers	Dinitrophenols	Dinosam, Dinoseb, DNOC
Inhibition of Very Long-Chain Fatty Acid Synthesis	Azolyl-carboxamides	Cafenstrole, Fentrazamide, Ipfencarbazone
	α-Thioacetamides	Anilofos, Piperophos
	Isoxazolines	Pyroxasulfone, Fenoxasulfone
	Oxiranes	Indanofan, Tridiphane
	α-Chloroacetamides	Acetochlor, Alachlor, Butachlor, Dimethachlor, Metazachlor, Metolachlor, Pretilachlor, Propachlor,
	α-Oxyacetamides	Mefenacet, Flufenacet
	Thiocarbamates	Butylate, Cycloate, EPTC, Molinate, Thiobencarb, Tiocarbazil, Tri-allate
	Benzofurans	Benfuresate, Ethofumesate
Auxin Mimics	Pyridine-carboxylates	Picloram, Clopyralid, Aminopyralid
	Pyridyloxy-carboxylates	Triclopyr, Fluroxypyr
	Phenoxy-carboxylates	2,4,5-T, 2,4-D, 2,4-DB, Fenoprop, MCPA, MCPB
	Benzoates	Dicamba, Chloramben, TBA
	Quinoline-carboxylates	Quinclorac, Quinmerac
	Pyrimidine-carboxylates	Aminocyclopyrachlor
	Phenyl carboxylates	Chlorfenac, Chlorfenprop
Auxin Transport Inhibitor	Aryl-carboxylates	Naptalam, Diflufenzopyr-sodium
Inhibition of Fatty Acid Thioesterase	Benzyl ether	Cinmethylin, Methiozolin
Inhibition of Solanesyl Diphosphate Synthase	Diphenyl ether	Aclonifen
Inhibition of Lycopene Cyclase	Triazole	Amitrole

Table 7.

HRAC classification of herbicides.

Low-dose herbicides such as pyrazosulfuron-ethyl, sulfosulfuron, metsulfuron-methyl, Quizalofop-ethyl, bispyribac sodium, etc. are available in the market, which control weeds efficiently at lower concentration. However, active molecules are lost through various degradation processes in agro-ecosystem and reduce the weed control efficiency. The offsite movement of herbicides also poses serious environmental hazards in certain circumstances.

6. Issues associated with chemical weed control

Each herbicide molecule is unique in its herbicidal activity. The nature of herbicide, soil, and climatic conditions influence the behavior and weed control efficiency of herbicides. Herbicides are subjected to various forms of degradation on reaching soils, which in turn reduce the weed control activity of herbicides. Herbicidal activity and persistence of herbicides in soil are determined by various factors viz. soil sorption coefficient, leaching potential, and volatilization behavior of herbicide molecule. Soil with high content of clay or organic matter facilitates more adsorption of herbicide, while dry soils have more unoccupied binding sites, promoting the binding of herbicide molecules thus affecting the herbicidal activity. Soil microbial population also influences the fate of applied herbicide in agro-ecosystem. The pre and postemergence herbicides experience different modes of loss in soils. In spite of loss of herbicidal activities, maintenance of herbicides above the threshold level is crucial to achieve the desired effect on weeds. The fate of herbicides in solid is summarized in Table 8 [64, 76].

Process of degradation	Description	Factors affecting degradation	Examples
Transport of active molecules (physical process)
Volatilization	Lost via evaporation from soil surface	Vapor pressure, temperature and wind velocity	Dinitroanilines, Thiocarbamates [64]
Adsorption	Interactions with soil	Organic matter, clay content, Soil moisture	Bipyridinium [65], Pendimethalin [66]
Leaching	Offsite transport of herbicide molecules into soil	Herbicide solubility, soil texture and rainfall	Bromacil, diuron [67], thifensulfuron-methyl [68], sulfentrazone [69]
Physical drift	Transport of spray droplets by wind	Wind velocity and droplet size	—
Degradation of active molecules
Photo-decomposition	Degraded by sunlight	Chemical structure, duration and intensity of exposure to sunlight	Dinitramine, Nitralin, Fluchloralin [70], Paraquat [71]
Chemical degradation	Breakdown of active molecules into metabolite through different chemical process (hydrolysis, oxidation-reduction reaction, etc.)	Chemical nature	Sulfonylurea [72]
Microbial degradation	Degradation of active molecules through soil microbes.	Soil pH, moisture content, organic matter and temperature	Sulfonylurea [72], Oxyfluorfen [73], 2,4-D [74], Glyphosate [75]

Table 8.

Fate of herbicides in agro-ecosystem

Direct application of herbicide in soil as pre-emergence or pre-plant incorporation poses a serious threat to the environment compared to other methods of herbicide applications. Leaching of herbicides especially ureas, sulfonylurea, and uracil herbicides contaminates groundwater. Herbicides with higher solubility, mobility, and sorption to soil particles are categorized with higher potential herbicides for groundwater contamination. Herbicides that persist in the soil impede the germination of succeeding crops through phytotoxicity effect. Persistence of herbicide in soil is listed in Table 9. Further, nontarget plant species are also affected due to spray drift and inappropriate application of herbicides.

Persistence in soil [77]
Less than 1 month	1–3 months	3–6 months	More than 6 months
2,4-D, MCPA, Glyphosate	Butachlor, Alachlor, Halosulfuron, Pyrazosulfuron-ethyl, Metribuzin, Bispyribac-sodium, Fluzifop-butyl, Metsulfuron-methyl, Oxyfluorfen	Pendimethalin, Fluchloralin, Isoproturon, Imazethapyr, Oxadiazon, Linuron	Atrazine, Simazine, Paraquat, Diquat, Chlorsulfuron, Diuron, Bromacil, Imazapyr, Sulfentrazone, Trifluralin, Picloram

Table 9.

Persistence of herbicides in soil.

Herbicide poses serious health hazards such as cancer, neurological disorders, and respiratory and reproductive related problems on the prolonged exposure to herbicide [78, 79, 80, 81, 82].

Herbicide-resistant weeds are superweeds, which evolve resistance against the use of single or multiple herbicides. The factors for the development of herbicide resistance among weeds are due to the repeated application of same herbicide or herbicides with a similar mode of action [83]. There are 266 weed species, which developed resistance against herbicides. Further, infestation of herbicide-resistant weeds has been reported in 71 countries [84]. The control of superweeds requires alternate strategies other than herbicides, which incur additional cost for managing resistant weeds. Herbicide-resistant weeds also pose weed shift in specific regions.

Application of selective herbicides increases risk of infestation of nonselective weeds. Herbicides do not exert consistent weed control since interaction of herbicides with the environment is dynamic in nature. Herbicides also affect non-target weed species in certain regions posing the threat to biodiversity. Therefore, chemical weed control has several issues on the herbicide use efficiency besides posing threat to nontarget sites.

7. Herbicide encapsulation: an innovative approach

Conventional herbicide formulations are recommended at a higher dosage over the minimum threshold level to complement the herbicide losses encountered in agroecosystem to achieve higher weed control [85]. Further, a significant quantity of applied herbicides undergoes various degradation paths causing environmental pollution. Herbicide encapsulation is the smart delivery approach, which addresses and resolves the constraints of conventional chemical weed management. Encapsulation involves the entrapment of herbicides in polymeric systems to safeguard the active molecules from the environmental vulnerability and achieve controlled release of herbicides in the target environment. The active ingredients are encapsulated in the shell materials for improving weed control efficiency through prolonged release of active ingredients in the soil. Encapsulation promotes the stability of active ingredients and reduces the herbicide requirement significantly by minimizing the loss of herbicides into the environment [86, 87, 88, 89]. Herbicide encapsulation is a versatile technology performed at nano and micro-scale by incorporating active ingredients into the suitable carrier [90]. The assembly of active ingredients and carrier material resulted in sustained release of active ingredients for a longer period at the desired rate. Similarly, encapsulated formulation reduces herbicide dosage coupled with slow-release results in reducing the residue buildup in soil and eliminating phytotoxicity [91]. Sulfentrazone, a pre-emergence herbicide was encapsulated using calcium alginate and calcium chloride as cross-linker [92]. The resultant formulation offered controlled release of sulfentrazone and minimized the leaching potential of herbicide. Similarly, encapsulation of atrazine with starch polymer impeded volatilization [93]. Nano-encapsulated atrazine in poly epsilon-caprolactone carrier system exhibited targeted weed control at ten times lower dose of the recommended level of herbicide [94]. In addition, it reduced the soil mobility of atrazine in soils. Meanwhile, smart delivery of herbicide shows higher efficacy of weed control and exhausts the weed seed bank resulting in less emergence of weeds (Figure 1) [95].

Figure 1.

Advantages of herbicide encapsulation.

8. Polymers for herbicide encapsulation

Generally, carriers are polymeric materials that are employed for the encapsulation of herbicides to develop a smart delivery system. There are numerous carrier materials (natural, synthetic, and semisynthetic polymers) available for herbicide encapsulation. However, synthetic polymer has less significance than natural and semisynthetic polymer since it is not degradable in nature and remains as a contaminant in the soil. In contrast, natural carrier materials are advantageous since they are eco-friendly, biocompatible, cost-effective, easily available, and biodegradable in nature [96, 97]. Alginates, chitosan, starch, pectin, lignin, Arabic gum, cyclodextrin, cellulose, and gelatin are the biopolymers employed for herbicide encapsulation [90, 98, 99, 100, 101]. Commonly used synthetic polymers are polycaprolactone, polyurethane, polyvinyl alcohol, and polystyrene sulfonate [99, 102, 103]. Semisynthetic polymers are natural polymers with side-chain modification through the replacement of hydrogen from hydroxyl group of glucose repeating units with ethyl, methyl, carboxymethyl, and carboxyethyl moieties.

8.1 Natural polymers

Alginate is an anionic linear polysaccharide polymer that naturally exists in the cell walls of brown seaweed viz. Ascophyllum nodosum, Laminaria hyperborea, and Macrocystis pyrifera [104, 105]. Alginate has been explored for the controlled release of active compounds via ionotropic gelation method [92]. Leaching potential of sulfentrazone herbicide was reduced by developing sustained release of herbicide by exploiting alginate polymeric system. Tebuthiuron was encapsulated using alginate as carrier material to impede leachability of herbicides in agroecosystem [106]. Similarly, starch is a homopolysaccharide that is made up of two distinct molecules of amylose and amylopectin [90].

Starch is extensively found in cereal grains, roots, tubers, and fruits, which is also employed as a carrier for smart release of herbicide. Herbicides such as 2,4-D and 2,4,5-T were encapsulated with corn, wheat, potato, and cassava starches [107]. Encapsulation with wheat and potato starches exhibited slower release of herbicide because of higher amylose content and molecular weight of starch in wheat and potato starch. Sulfentrazone herbicide was encapsulated using starch via solvent evaporation method for season-long weed control by reducing leaching potential of herbicide [108]. Atrazine was encapsulated by utilizing starch as carrier and resultant formulation minimize the volatilization loss over the conventional formulation [93].

Chitosan is a nontoxic, biodegradable, and biocompatible polymer obtained through the deacetylation of chitin, which is usually found in the cell walls of fungi and bacteria. Chitosan is a cationic linear polysaccharide, which is highly efficient carrier system for agrochemicals [89, 109]. Paraquat-loaded chitosan/tripolyphosphate nanoparticles reduced the soil sorption of paraquat, thus improving the stability of herbicide [99]. Cellulose and its derivatives are explored as a carrier system for the smart delivery of active compounds. The formulation was developed by mixing chitosan and glyphosate at different molar ratios in water for the mart delivery of glyphosate, where chitosan polymer plays a dual role as eco-friendly adjuvant and polymeric carrier of glyphosate facilitating prolonged release of herbicide [110].

Cellulose is a polysaccharide that is biodegradable in nature and available in abundance at a lower cost. Alginate/cellulose-based delivery system containing imazethapyr offered the extended-release of active material [100].

Pectin is a polysaccharide and an anionic biopolymer, which is abundantly present in higher plants' cell walls. Pectin is a biodegradable, nontoxic, and easily available natural polymer. Pectin is composed of D-galacturonic acid units, which are linked by α-(1-4) glycosidic linkage [111]. Nowadays, pectin is also explored as a carrier system for the controlled release of the active ingredient due to its characteristics viz. more stability at acidic and high-temperature conditions, gelation property, non-toxicity, biocompatibility, and easily available at a cheaper cost [112]. Six percent pectin and two percent calcium chloride were found as optimum concentration for smart release of herbicides via ion gelation technique [113]. Metsulfuron-methyl loaded in pectin nanoparticles were found to be effective with higher herbicidal activity at a lower application rate as compared to the commercial herbicide [87].

Lignin is another important polymer that is obtained as a byproduct in pulp and paper industries. Lignin exhibits UV shielding property and antimicrobial activity, which attracted lignin to explore as a polymer for the delivery of herbicide. Moreover, lignin is relatively available in abundance at a lower cost [114, 115]. Dicamba herbicide was encapsulated in lignosulfonate carrier system for sustained release [116]. Lignin-polyethylene glycol-based chloridazon and metribuzin were synthesized for minimizing leachability of herbicides in light-textured soils [117].

Cyclodextrin is a cyclic oligosaccharide consisting of glucose units, derived through enzymatic conversion of starch. β-Cyclodextrin is a highly preferred molecule for encapsulation of active molecules since it is easily available at a lower cost [118]. Cyclodextrin has a unique structure, which enables it to form inclusion complex with hydrophobic active molecules. Terbuthylazine herbicide molecule was encapsulated using cyclodextrin, which showed improved solubility and bioavailability of herbicide molecule [119].

Guar gum is a neutral polysaccharide made up of the main chain of D-mannopyranose residues linked together by β-(1,4) glycosidic bonds and a secondary chain of D-galactopyranose residues linked together by α-(1,6) glycosidic bonds. Solubility of guar gum in cold water rises in proportion to the galactose/mannose molar ratio [120]. Herbicide formulation of guar gum-g-cl-polyacrylate/bentonite clay hydrogel composite was employed for pre-emergence application, while guar gum-g-cl-poly N-isopropylacrylamide nano hydrogel was used for the post-emergence application [121]. The encapsulation efficiencies of imazethapyr into guar gum-g-cl-polyacrylate/bentonite clay hydrogel composite ranged from 75.99 to 98.96% and guar gum-g-cl-poly N-isopropylacrylamide nano hydrogel ranged from 67.98 to 80.90%. The time to release 50 percent of the loaded imazethapyr (t_1/2) was between 0.06 and 4.8 days in CGNHG, while it was from 4.4 to 12.6 days in GG-HG system, Encapsulation of bioherbicides were also attempted using natural carrier materials such as Arabic gum, Persian gum/gelatin and gelatin [101].

8.2 Synthetic polymers

Polycaprolactone is biodegradable and hydrophobic polyester belonging to the aliphatic family. Polycaprolactone is utilized as a smart delivery vehicle for various active ingredients since it is biocompatible, cost-effective and possesses unique mechanical properties [122, 123]. Encapsulation of pretilachlor in polycaprolactone polymer enhanced the stability and herbicidal activity of herbicide [124]. Encapsulated atrazine and paraquat herbicides in poly-ε-carpolactone carrier system minimized the environmental impacts associated with the use of herbicides [125]. Similarly, poly-ε-caprolactone based atrazine nanocapsules reduced the soil mobility of herbicide and showed higher weed control efficiency at a lower application rate [94, 126, 127].

Polyurea is a product derived from the reaction of isocyanates and amines. Polyurea is used as shell material for herbicide encapsulation since it has high thermal stability and is available at a lower cost. Polyurea was utilized as a polymer for encapsulation of oxyfluorfen to reduce the phytotoxic effect on non-target plants [128]. Polyurea-based pretilachlor microcapsule formulation was synthesized through polymerization, which was found to be efficient in controlling weeds [129]. Polyurea-based pendimethalin encapsulated formulation reduced the usage of organic solvents during the manufacture of emulsifiable concentrate formulation eliminating the environmental pollution due to its application [130]. Pendimethalin was encapsulated using shell material made up of polyurethane urea to improve weed control efficiency [131].

Polyvinyl alcohol is a water-soluble polymer being widely explored for herbicide encapsulation. Glyphosate, a non-selective herbicide was encapsulated using polyvinyl alcohol and polystyrene sulfonate to minimize the herbicide loss in the environment [103]. Further, polyurethane polymeric systems are also utilized for the controlled delivery of active ingredients. Polyurethane is a synthetic polymer composed of urethane units, which is biocompatible and biodegradable in nature [132]. Trifluralin loaded in polyurethane network through interfacial polymerization protected the active ingredient from volatilization and photodegradation [133].

Polylactic acid is a biodegradable polymer derived from renewable sources such as corn, wheat, and rice. Polylactic acid is an aliphatic semicrystalline polyester which is hydrolyzable, eco-friendly, and biocompatible in nature [134, 135]. Microparticles of metazachlor herbicide were synthesized with low molecular weight polylactic acid for the controlled release of active molecules [136, 137]. Encapsulation of metolachlor herbicide was also attempted using a high molecular weight of polylactic acid for smart delivery of herbicide [138]. Similarly, Poly (lactic-co-glycolic acid) is a biopolymer composed of monomers of lactic and glycolic acids [139], exploited as carrier system for the smart delivery of atrazine herbicide to reduce the environmental impacts associated with application of herbicide [140].

8.3 Semi-synthetic polymers

Cellulose and its derivatives are exploited as a carrier system for the smart delivery of active compounds in agriculture. The two primary classes of cellulose derivatives are cellulose ethers and cellulose esters, which have varied levels of mechanical and physicochemical properties.

Ethyl cellulose is a derivative of cellulose in which the hydroxyl group of cellulose is substituted with the ethyl ester group [141]. Ethyl cellulose is a hydrophobic polymer utilized for improving the stability of the active ingredient to achieve higher use efficiency. 2,4-D herbicide was loaded in ethyl cellulose microspheres to achieve sustained release of herbicides [142]. Ethyl cellulose-loaded alachlor formulation reduced the soil mobility of herbicide which achieved prolonged weed control at a lower application rate [143]. Norfluazon based controlled release system using ethyl cellulose reduced the soil mobility of herbicide and protected the active ingredient from photodegradation [144, 145]. Solvent evaporation method was utilized to introduce atrazine, a broadleaf weed control herbicide, into ethyl cellulose-controlled release formulations [146], to sustain the release of herbicide.

Carboxymethyl cellulose is a cellulose derivative that is anionic in nature with high solubility in water. Carboxylmethyl cellulose readily forms gel in solutions of multivalent cations, such as aluminum or iron cations, to generate hydrogels. Controlled release formulations of acetochlor were synthesized with various modified forms of clay/carboxymethyl cellulose (Figures 2–4 and Table 10) [147].

Figure 2.

Solvent evaporation technique.

Figure 3.

Ion gelation method.

Figure 4.

Preformed polymerization technique.

Herbicide	Polymer	Encapsulation technique	Characteristics of formulation	Authors
Trifluralin	Polyurethane	Interfacial polymerization	Microencapsulation protected herbicide from photo degradation and volatilization	[133]
Pretilachlor	Poly ε-caprolactone	Interfacial deposition of preformed polymer	Encapsulation efficiency was 99.5 ± 1.3%. Enhanced herbicidal activity with less environmental toxicity	[124]
2,4-D	Ethyl cellulose	Emulsion solvent evaporation	Encapsulation efficiency of 7.7–27% with prolonged release of 2,4-D	[142]
Tebuthiuron	Sodium alginate	Ion gelation technique	Controlled release carrier system for tebuthiuron	[106]
Chloridazon and Metribuzin	Lignin and ethyl cellulose		Reduced leaching and photo degradation of herbicides	[117]
Terbuthylazine	β-cyclodextrin	Kneading method	Improved herbicide solubility	[119]
Atrazine	Poly ε-caprolactone with chitosan as coating agent	Modified interfacial deposition of preformed polymer	Improved adhesive property of herbicide on foliage of target weeds	[98]
Paraquat	Chitosan/tripolyphosphate	Ionic gelation	Encapsulation efficiency of polymeric system was 65% with stability of 60 days. Reduced Soil sorption of herbicide in soils	[99]
Tribenuron-methyl	Zein	Solvent evaporation	Encapsulation efficiency was 81± 3% with enhanced solubility, controlled release formulation improved weed control	[148]
Sulfentrazone	Sodium alginate	Ionotropic gelation	Minimized herbicide leaching into the soil	[92]
Metazachlor	Polylactic acid/polyethylene glycol	Solvent evaporation	Controlled release system for the delivery of herbicides for prolonged weed control	[137]
Pendimethalin	Starch	Solvent evaporation	Slow release system depends on soil moisture availability and non-toxic to earthworms	[88]
Metsulfuron-methyl	Pectin	Emulsification	Encapsulation efficiency of 63 ± 2% with increased herbicidal activity at lower dose	[87]
Tebuthiuron	Sodium alginate	Ionotropic gelation	Reduced herbicide loss due to leaching	[149]
Imazapic and Imazapyr	Alginate/chitosan and Chitosan/tripolyphosphate	Ionotropic gelation	Enhanced herbicidal activity and less toxic	[150]
Imazethapyr	Alginate and Alginate/cellulose	Ionotropic gelation	Extended release of Imazethapyr for 30 days of application	[100]
Atrazine	Poly ε-caprolactone	Interfacial deposition of preformed polymer	Improved post-emergence activity at lower dose (ten-fold lower than recommended levels) in controlling target weeds. Reduced soil mobility	[126, 127, 151]
Norflurazon	Ethyl cellulose	Solvent evaporation	Prolonged release and reduced soil mobility and offered protection from photo degradation	[144]
Atrazine	Poly ε-caprolactone	Interfacial deposition	Enhanced pre-emergence herbicidal activity at ten times of lower dose	[94]
Oxyfluorfen	Polyurea	Interfacial polymerization	Reduced phytotoxicity in rice	[128]
Metazachlor	Poly lactic acid	Solvent evaporation	Enhanced herbicidal activity on target plants	[138]
Atrazine	Poly (lactic-co-glycolic acid)	Modified precipitation method	Encapsulation efficiency of 50%	[140]
Norflurazon	Ethyl cellulose	Oil in water emulsion through solvent evaporation	Controlled release formulation (depends on active ingredient loaded, emulsifying and pore forming agent)	[145]
Alachlor	Ethyl cellulose	Oil in water solvent evaporation	Reduced herbicide loss due to leaching by 39% and minimized the risk of groundwater contamination; Encapsulated formulation showed better efficacy for 30 days of application	[143]
Metazachlor	Poly (lactic acid)	Oil in water solvent evaporation	Encapsulation efficiency of 30%. Release rate of herbicide depends on particle size and loading efficiency	[136]
Savory essential oil (Bioherbicides)	Arabic gum, Persian gum/gelatin and Persian gum	Complex coacervation	Better stability for 42 days; Increment in herbicidal activity with encapsulation	[101]
Glyphosate	Metal nanoparticles such as iron oxide and silver nanoparticle and water soluble polymer such as polyvinyl alcohol and poly Styrene Sulfonate	Spray drying method	Higher encapsulation and weed control efficiency	[103]
Sulfentrazone	Starch	Solvent evaporation	Reduced horizontal and vertical leachability potential of herbicide; Offered season long weed management in black gram	[108]
Metribuzin and Tribenuron	Poly(3-hydroxubutyrate)	High energy ball milling	Higher efficiency of weed control	[96]
Picloram	Chitosan and sodium ligno-sulfonate	Layer by layer technique	Altered the release of herbicide and improved photo stability of herbicides	[152]
Atrazine and alachlor	Starch	Solvent evaporation	Encapsulated formulation resulted in reduced mobility and volatilization losses	[93]
Dicamba	Copper chitosan nanoparticles	Green chemical reduction method	Reduced leaching losses	[153]
Pendimethalin	Polyurethane urea	Interfacial polymerization	Encapsulation efficiency was 53.2–89.1% with enhanced stability	[131]

Table 10.

Brief overview of herbicide encapsulation.

9. Release profile of encapsulated herbicides

Herbicide encapsulation protects the active compound from different losses viz. leaching, volatilization, adsorption, photodecomposition, etc. The loss of herbicides is controlled by altering the release rate of active ingredients from the polymeric systems. Therefore, herbicide encapsulation serves as a platform to design herbicide formulation with varying release patterns of active molecules. Encapsulated formulation modified the herbicide release profile. The encapsulation of herbicides minimizes the adverse consequence in soil environment due to use of herbicides. Similarly, encapsulation technique offers an extended period of weed control at a lower dosage.

The particle size of the formulation greatly influences the release rate of active ingredients into the environment [142, 154, 155, 156]. Polymer-solvent ratio, water diffusion rate, pH of the releasing medium, molecular weight of the polymer, nature of interaction between shell and core materials (active molecules), polymers, methodology, and preparation conditions also govern the release profile of active molecules [143, 153, 157, 158, 159].

Encapsulation of metribuzin and chloridazon in lignin-polyethylene glycol system coated with ethyl cellulose (20%) and dibutyl sebacate (2.25%) resulted in the controlled release formulation and time taken for the delivery of 50% corresponding herbicide were 16.94 and 65.39 h, respectively [117]. Release kinetics study of paraquat loaded in pectin/chitosan/tripolyphosphate nanoparticles revealed that polymeric system sustained release of paraquat compared to that of conventional formulation where a significant amount of paraquat was not released until 30 min of incubation [160]. Similarly, alginate/chitosan-based paraquat nano-formulation modified the release profile of paraquat, which achieved 100% herbicide release in eight hours of incubation. The release of paraquat was extended for two hours compared to that of free form of paraquat in water medium [161]. Conventional paraquat released 92% of active molecules after 350 min of incubation, while paraquat from chitosan/tripolyphosphate nanoparticles diffused only 72% during the same period [99]. The commercial formulation of imazethapyr released more than 76% of active herbicide molecules in less than one day, whereas the time taken for fifty percent release of the active molecule from alginate and alginate/cellulose beads were 11.30 and 43.73 days respectively [100]. Laboratory studies on the release profile of starch-encapsulated atrazine revealed that 70% of active ingredients were delivered in three days, while the remaining quantities of herbicides were released over 16 days of incubation. However, the maximum release was noticed after 15 days of application under field conditions as against the peak release of herbicide, which was observed in three days of incubation in in vitro study [162]. Moreover, multilayer encapsulation of active ingredients resulted in the reduction of burst release and extended the release period (Figure 5) [152].

Figure 5.

Overview of encapsulated herbicide formulation.

10. Longevity of weed control by encapsulated herbicides

Encapsulated herbicide formulation delivers active material to the target environment in a sustained pattern thus protecting active molecules from environmental vulnerability and eventually resulted in an efficient and extended period of weed control. Ethyl cellulose-based microencapsulated alachlor formulation showed greater herbicidal activity even after 30 days of application achieving long-term weed control [143]. Efficacy of free and encapsulated metribuzin in poly (3-hydroxybutyrate) was tested against Avena fatua as target species [96]. The results revealed that encapsulated metribuzin offered prolonged weed control of 70 days against Avena fatua, whereas 40% of germinated weeds were observed at 42 days with application of conventional metribuzin formulation. Similarly, metazachlor and pendimethalin were encapsulated separately using terpolymer (L-Lactide/Glycolide/PEG) where the weed control was effective against target weed species for 2–3 months [163]. Encapsulated formulation of pendimethalin herbicide delivered herbicide sustainably during the period of forty days to achieve season-long weed control [88].

11. Residual effect of encapsulated herbicide formulation

Encapsulation of herbicides exhibits the same herbicidal activity at a lower dose as compared to its conventional formulation. Controlled release formulation reduced the amount of active ingredient applied to the environment thus reducing the residue buildup in agro-ecosystem. Herbicide encapsulation conferred the controlled release of active material to maintain the threshold level of herbicides for an extended period to control weeds [164]. Encapsulated herbicide formulation was not active in soil to affect the succeeding crop [165]. Residual effect of poly (ε-caprolactone) based atrazine formulation was validated on soybean plants [94]. The results showed that nano-formulation enhanced the short-term without causing a long-term residual effect.

12. Conclusion

Weeds are the crucial yield-limiting factor in crop production that affect crop growth and yield dither directly or indirectly. There are different weed management options are available; however, chemical weed control strategy is quite effective among them. Herbicide is an important component in weed management and registered a major share in the pesticide market to improve crop productivity. Application of herbicide poses several environmental consequences since herbicides are subjected to different degradation processes in agro-ecosystem. Encapsulation of herbicides is an innovative strategy and offers a controlled release system to address the issues of chemical weed control. Polymers are unique and explored their specific characteristics for the encapsulation of active ingredients. There are numerous polymers available to design smart release formulation. Herbicide encapsulation improves stability of active ingredient and safeguards the active molecules from environmental vulnerability. Further, it enhances the bioactivity, which helps to achieve prolonged weed control with higher efficiency. Encapsulated formulation achieves the same weed control efficiency at lower dosage as compared to the conventional formulation. Research evidence showed that there was no significant residue carryover due to application of encapsulated formulation. Therefore, the development of encapsulated herbicide formulation has greater scope in crop protection. Moreover, encapsulated formulation will make a greater revolution in the chemical era to manage weeds at a lower rate of application. However, costlier instruments are required for designing and characterization of encapsulated formulation, and regulatory evaluation of nano-formulation are few limitations for development and commercialization of smart herbicides.

Conflict of interest

The authors declare no conflict of interest.