**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**).


*Polymeric Systems for the Delivery of Herbicides to Improve Weed Control Efficiency DOI: http://dx.doi.org/10.5772/intechopen.104629*

#### **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**).

Some weeds produce seeds morphologically mimicking crop seeds thus escaping from physical separation. The maturity of wild mustard *Argemone mexicana* coincides


#### **Table 2.**

*Seed production potential of weeds.*

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].

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**).

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


**Table 3.** *Longevity of weed seeds.*


*Polymeric Systems for the Delivery of Herbicides to Improve Weed Control Efficiency DOI: http://dx.doi.org/10.5772/intechopen.104629*

#### **Table 4.**

*Nutrient composition of weeds.*

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**.

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].


#### **Table 5.**

*Yield reduction due to weed infestation.*

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**.

*Polymeric Systems for the Delivery of Herbicides to Improve Weed Control Efficiency DOI: http://dx.doi.org/10.5772/intechopen.104629*


#### **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 diseasecausing 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,

## *Polymeric Systems for the Delivery of Herbicides to Improve Weed Control Efficiency DOI: http://dx.doi.org/10.5772/intechopen.104629*

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 makeup 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**.

Low-dose herbicides such as pyrazosulfuron-ethyl, sulfosulfuron, metsulfuronmethyl, Quizalofop-ethyl, bispyribac sodium, etc. are available in the market, which



*Polymeric Systems for the Delivery of Herbicides to Improve Weed Control Efficiency DOI: http://dx.doi.org/10.5772/intechopen.104629*


#### **Table 7.**

*HRAC classification of herbicides.*

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


#### **Table 8.**

*Fate of herbicides in agro-ecosystem*

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].

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.

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

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

*Polymeric Systems for the Delivery of Herbicides to Improve Weed Control Efficiency DOI: http://dx.doi.org/10.5772/intechopen.104629*


#### **Table 9.**

*Persistence of herbicides in soil.*

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 [86–89]. 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].

**Figure 1.** *Advantages of herbicide encapsulation.*

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].
