*Article* **Low-Profile Frequency Reconfigurable Antenna for Heterogeneous Wireless Systems**

#### **Amjad Iqbal 1,, Amor Smida 2,3, Lway Faisal Abdulrazak 4, Omar A. Saraereh 5, Nazih Khaddaj Mallat 6, Issa Elfergani 7 and Sunghwan Kim 8,\***

Received: 5 August 2019; Accepted: 30 August 2019; Published: 31 August 2019

**Abstract:** A low-profile (0.21*λg*× 0.35*λg*× 0.02*λg*) and a simply-structured frequency-switchable antenna with eight frequency choices is presented in this paper. The radiating structure (monopole) is printed on a 1.6-mm thicker, commercially-available substrate of FR-4 ( *r* = 4.4, tan*δ* = 0.020). Specifically, it uses three PIN diodes in the designated places to shift the resonant bands of the antenna. The antenna operates at four different modes depending on the ON and OFF states of the PIN diodes. While in each mode, the antenna covers two unique frequencies (Mode 1 = 1.8 and 3.29 GHz, Mode 2 = 2.23 and 3.9 GHz, Mode 3 = 2.4 and 4.55 GHz, and Mode 4 = 2.78 and 5.54 GHz). The performance results show that the proposed antenna scheme explores significant gain (>1.5 dBi in all modes) and reasonable efficiency (>82% in all modes) for each mode. Using a high-frequency structure simulator (HFSS), the switchable antenna is designed and optimized. The fabricated model along with the PIN diode and biasing network is tested experimentally to validate the simulation results. The proposed antenna may also be combined in compact and heterogeneous radio frequency (RF) front-ends because of its small geometry and efficient utilization of the frequency spectrum.

**Keywords:** monopole antenna; S-parameters; frequency reconfigurable; 5G, 4/4.5G; LTE; ISM; WiFi; WiMAX; WLAN

#### **1. Introduction**

Recent advancements in wireless communication systems need transceivers to be efficiently utilized with heterogeneous systems such as wireless local area network (WLAN), Worldwide Interoperability for Microwave Access (WiMAX), Long-Term Evolution (LTE), Fourth-Generation (4G), LTE Advanced Pro (4.5G), Fifth-Generation (5G), and many more [1]. One of the important components of the transceiver is antenna. The antenna being an important component of the transceiver occupies a major portion of the system [2,3]. The size increases further by incorporating any tuning mechanism in it. Since, the antenna required for modern transceivers must be compact and frequency switchable to meet the standard of the transceivers. In this regards, many approaches have been used to reduce the size of the antenna and to make it tunable for many useful applications.

The antenna which is capable of switching its resonant band according to the user requirement, is known as a frequency-reconfigurable antenna. Various techniques have been adopted to change the surface current distribution of the antenna, which produces a highly-reconfigurable antennas. In [4,5], the lumped element between the monopole and the parasitic patch is used to switch the antenna from a single band to a dual band. A compact-sized frequency-reconfigurable antenna for mobile handsets is reported in [6]. The reported antenna in [6] covers Long-Term Evolution (LTE) LTE700, LTE2300, LTE2500, Global System for Mobile (GSM) GSM850, GSM900, GSM1800, GSM1900, and Universal Mobile Telecommunications System (UMTS) (1920–2170 MHz) bands for different switching conditions. A minimally-sized hexagonally-shaped reconfigurable slot antenna for WLAN is reported in [7]. The reported antenna covers dual bands (2.4 and 5.2 GHz) and a single band (2.4 GHz) for the PIN diode in the ON and OFF states, respectively. Two different types of frequency-reconfigurable antennas are reported in [8] for multiband applications. The reported antenna covers three bands: the Wireless Fidelity (WiFi) band, the Worldwide Interoperability for Microwave Access (WiMAX) band, and the WLAN band, for different biasing conditions of PIN diodes. Furthermore, a frequency-reconfigurable implantable antenna for the Medical Implant Communications Services (MICS: 402–405 MHz) and Industrial, Scientific, and Medical (ISM: 902–928 MHz) band is examined in [9]. Micro-electromechanical switches (MEMSs) are used in [10] to reconfigure the operating band of the antenna. However, the reported antenna cannot be efficiently used with any planner and compact components due to its large dimensions and 3D geometry. Following that, a polarization and frequency-reconfigurable antenna is reported in [11]. The optical switches are inserted between the main monopole and the parasitic patch in [12] to achieve different frequency responses with a wider bandwidth and high gain at the cost of a large antenna size. In [13], the PIN diode on the multi-layer antenna, control the patch and feeding lengths, generating two unique frequencies in the WLAN and 5G bands. In [14], two varactor diodes are connected with an F-shaped feed to reconfigure the operating band continuously in a wideband filtering patch antenna. Moreover, the hexa-band frequency-reconfigurable antenna for multi-standard wireless communication systems using PIN diodes is investigated in [15–17]. A fluidic channel-based frequency-reconfigurable monopole antenna is proposed in [18], which operates in three modes depending on the filled channels.

In the challenging world of communication, the size and performance of the reconfigurable antenna are important factors to be considered before using it in the RF front-end. So far, many antennas have been designed for reconfiguration of the frequency bands. However, their large dimensions and limited operational bands limit its applications in modern RF front-ends where limited space is available for the antennas. Therefore, the proposed antenna is designed with lower dimensions (0.21*λg*×0.35*λg*×0.02*λg*) and multiple frequency bands with high gain and efficiency for each operating band. The organization of the rest of the paper is as follows: Section 2 presents the antenna design equations and theoretical analysis and simulation results for each mode. Fabricated results and discussions, as well as a comparison with the state-of-the-art reconfigurable antennas are presented in Section 3.

#### **2. Antenna Design Methodology**

The geometry (top view, bottom view, and side view) of the proposed frequency-switchable monopole antenna is illustrated in Figure 1. The proposed frequency-switchable monopole antenna was printed on a 1.6-mm thicker, commercially-available substrate of FR-4 ( *r* = 4.4, and tan*δ* = 0.020). The main radiating part of the antenna consisted of a hook-shaped monopole with three parasitic patches connected to it through three PIN diodes. A gap of 1 mm was kept between the parasitic patches to install the PIN diodes. The overall geometry of the proposed antenna was 0.21*λg*× 0.35*λg*× 0.02*λg*. The effective length of the monopole was calculated using transmission model theory [19]. The effective length of the monopole was calculated using Equation (1).

$$L\_{fr} = \frac{c}{4f\_r\sqrt{\varepsilon\_{eff}}}\tag{1}$$

and:

$$
\epsilon\_{eff} \approx \frac{\epsilon\_I + 1}{2} + \frac{\epsilon\_I - 1}{2} (1 + 12(\frac{w}{h}))^{-0.5} \tag{2}
$$

where *c* is the speed of light in a vacuum, *λg* is the guided wavelength,  *eff* is the effective dielectric constant, *w* is the width of the substrate, and *h* is the thickness of the substrate.

**Figure 1.** Geometry of the proposed antenna: (**a**) top view (*W*1 = 14.5 mm, *L*2 = 12 mm, *W*2 = 8 mm, *L*3 = 12 mm, and *L*1 = 25.5 mm); (**b**) bottom view (*W* = 17 mm, *L* = 28 mm, and *Lg* = 2 mm); (**c**) side view, (**d**) equivalent circuit model of the PIN diode in the ON, OFF state and the biasing circuit, and (**e**) fabricated prototype.

#### *2.1. Frequency Reconfigurability*

The frequency reconfigurability in the antenna is obtained by adjusting the ON and OFF states of the PIN diodes used between the corresponding parasitic patches. The proposed antenna operates at four different modes. The states of the PIN diodes at each mode are summarized in Table 1. The proposed antenna operates at two unique bands for each operating mode. The antenna operates at Mode 1 when the PIN diode (*D*1, *D*2, and *D*3) are in the ON state. The proposed antenna scheme operates at 1.8 GHz (GSM band) and 3.3 GHz (5G sub-6 GHz band) for operating mode 1, as illustrated in Figure 2. The same antenna operates at 2.25 GHz (3G advanced/LTE band) and 3.9 GHz (5G sub-6 GHz band), when it is operated in Mode 2 (*D*1 = ON, *D*2 = ON, and *D*3 = OFF). The antenna covers 2.4 GHz (WiFi/WLAN/ISM/Bluetooth band) and 4.5 GHz (5G sub-6 GHz band) upon operating with Mode 3

(*D*1 = ON, *D*2 = OFF, and *D*3 = OFF). The same antenna operates at 2.78 GHz (Airport surveillance radar band) and 5.54 GHz (WLAN band) when operated in Mode 4 (*D*1 = OFF, *D*2 = OFF, and *D*3 = OFF).

**Table 1.** States of the PIN diodes for various operating modes.

**Figure 2.** Reflection coefficient of the antenna for different operating modes.

#### *2.2. Switching Technique*

In this arrangement, the PIN diode acts as a variable resistor in the RF frequency range. The PIN diode Model HPND-4005 (640 *μ*m × 220 *μ*m) was used for frequency reconfiguration of the antenna. The equivalent circuit model of the PIN diode in the ON and OFF state is shown in Figure 1d [20]. The equivalent circuit model of the PIN diode is the series connection of the inductor (L = 0.15 nH) and low resistance resistor (R = 4.7 Ω) for the ON state of the PIN diode. The equivalent circuit model for OFF state of the diode was modeled as a parallel combination of the capacitor (C = 0.017 pF) and high-resistance resistor (R = 7 kΩ) in series connection with an inductor (L = 0.15 nH). For simplicity, the reconfigurability of the antenna was studied only in terms of the resistance, based on the concept that the PIN diode acts as an open circuit for a high resistor value and as a closed circuit for a lower value of the resistor.

#### **3. Results and Discussions**

The fabricated prototype of the antenna is shown in Figure 1e. Our simulation results are compared with the results obtained from the fabricated prototype. PIN diode Model HPND-4005 having dimensions of 640 *μ*m × 220 *μ*m was used for frequency reconfigurability in the antenna model. The proposed diode offered fewer insertion losses (0.4 dB) due to its small geometry and a small capacitance value (C = 0.017 pF). Conductive epoxy was used to mount the diodes on the antenna surface. Figure 1d shows the biasing circuitry of the antenna. It is illustrated from the biasing circuit that the PIN diode was excited through a current-limiting resistor (R = 47 Ω) and radio frequency (RF) chock (L = 68 nH).

**Figure 3.** Simulated (Sim) and measured (Meas) reflection coefficient of the antenna: (**a**) Mode 1; (**b**) Mode 2; (**c**) Mode 3; and (**d**) Mode 4.

#### *3.1. Reflection Coefficient*

The reflection coefficient of the antenna was measured using a network analyzer for four operating modes of the antenna. The simulated and measured results for all operating modes of the antenna are compared in Figure 3. Figure 3a shows the reflection coefficient of the antenna when operated in Mode 1. The proposed antenna in Mode 1 operated at 1.8 and 3.3 GHz. The simulated 10-dB bandwidth of the antenna was 120 MHz (1.76–1.88 GHz) and 390 MHz (3.12–3.51 GHz) at the lower and higher resonant bands, respectively, for operating Mode 1. The measured 10-dB bandwidth for Mode 1 was recorded as 400 MHz (1.6–2 GHz) and 330 MHz (3.08–3.41 GHz) for the lower and upper operating bands, respectively. The same antenna operated at 2.25 and 3.9 GHz with different switching conditions (Mode 2). The simulated 10-dB bandwidth of 200 MHz (2.13–2.33 GHz) and 320 MHz (3.84–4.16 GHz) was quantified for lower and higher operating bands, respectively. The measured 10-dB bandwidth for Mode 2 was investigated as 300 MHz (2.05–2.35 GHz) and 420 MHz (3.78–4.2 GHz) for the lower and upper operating bands, respectively. In Mode 3, the antenna had resonant frequencies of 2.4 and 4.5 GHz with a simulated 10-dB bandwidth of 250 MHz (2.29–2.54 GHz) and 270 MHz (4.43–4.7 GHz). The measured 10-dB bandwidth for Mode 3 was recorded as 190 MHz (2.29–2.48 GHz) and 280 MHz (4.32–4.6 GHz) for the lower and upper operating bands, respectively. In Mode 4, the antenna operated at 2.78 and 5.54 GHz with a simulated 10-dB bandwidth of 310 MHz (2.63–2.94 GHz) and 320 MHz (5.37–5.69 GHz). The measured 10-dB bandwidth for Mode 4 was investigated as 350 MHz (2.6–2.95 GHz) and 360 MHz (5.35–5.71 GHz) for the lower and upper operating bands, respectively. The designed antenna can be used for different applications (GSM, 5G sub-6-GHz band, 3G Advanced/LTE, WiFi, WLAN, ISM, Bluetooth, and airport surveillance radars band) with different operating modes (Mode 1, Mode 2, Mode 3, and Mode 4).

#### *3.2. Surface Current Distribution*

The surface current distribution of each mode for their respective operating bands (Mode 1: 1.8 and 3.3 GHz, Mode 2: 2.25 and 3.9 GHz, Mode 3: 2.4 and 4.5 GHz, Mode 4: 2.78 and 5.54 GHz) is plotted in Figure 4. The surface current was distributed uniquely on the antenna surface for different operating bands. It is evident from the plots that the effective length was inversely proportional to the operating frequencies.

**Figure 4.** Surface current distribution of the antenna for different operating modes.

#### *3.3. Gain and Efficiency*

The simulated and measured antenna gains and efficiencies are presented in Figure 5. In Mode 1, the antenna's simulated (measured) gain was >1.5 dBi (>1.45 dBi) and >1.76 dBi (>1.72 dBi) for the lower and upper operating bands, respectively. The simulated (measured) antenna's efficiency in operating Mode 1 was >83% (>83.2%) and >82% (>82.1%) for the lower and upper bands, respectively. The simulated (measured) gain of the antenna in operating Mode 2 was >3.16 dBi (>3.14 dBi) and >3.22 dBi (>3.18 dBi) for the respective lower and upper bands. In Mode 2, the antenna's simulated (measured) efficiency was >88% (>87.5%) and >88% (>88.1%) for the lower and upper operating bands, respectively. The simulated (measured) antenna's efficiency in operating Mode 3 was >2.89 dBi (>2.85 dBi) and >2.75 dBi (>2.76 dBi) for the lower and upper bands, respectively. In Mode 3, the antenna's simulated (measured) efficiency was >88% (>87.5%) and >87% (>86.6%) for the lower and upper operating bands, respectively. The simulated (measured) antenna's efficiency in operating Mode 4 was >3.5 dBi (>3.48 dBi) and >3.79 dBi (>3.69 dBi) for the lower and upper bands, respectively. In Mode 4, the antenna's simulated (measured) efficiency was >88.3% (>88%) and >87% (>86.2%) for the lower and upper operating bands, respectively.

**Figure 5.** Gain and efficiency of the antenna (solid line = simulated, and dashed line = measured): (**a**) Mode 1; (**b**) Mode 2; (**c**) Mode 3; and (**d**) Mode 4.

#### *3.4. Radiation Pattern*

The antenna's radiation characteristics were analyzed using 2D and 3D radiation patterns for each resonant frequency of the four operational modes. The 2D radiation pattern of the antenna on both principal planes (*φ* = 0◦ and *φ* = 90◦) was measured in an anechoic chamber. The simulated and measured 2D radiation pattern is illustrated in Figure 6. The proposed antenna had an omni-directional radiation pattern for a lower operating band at Phi = 0◦ in all four operating modes. At Phi = 90◦, the antenna had the same radiation pattern as a figure of eight for all operating modes. For the upper operating band at Phi = 0◦, the radiation pattern was omni-directional for Mode 1; however, the radiation pattern was distorted in Mode 2, Mode 3, and Mode 4. The 3D pattern of the antenna is illustrated in Figure 7.

**Figure 6.** *Cont*.

**Figure 6.** Simulated and measured radiation pattern at resonant frequencies (at Phi = 0◦ and at Phi = 90◦) of the antenna: (**a**) Mode 1; (**b**) Mode 2; (**c**) Mode 3; and (**d**) Mode 4.

**Figure 7.** 3D pattern of the antenna for all operating bands.

#### *3.5. Specific Absorption Rate Analysis*

The proposed antenna covered different frequency bands such as GSM, 5G, 3G Advanced/LTE, WiFi, WLAN, ISM, and Bluetooth. The specific absorption rate (SAR) analysis was important for the proposed antenna, because most of the portable devices use these frequency bands. The SAR analysis was performed by placing the antenna on the three layers (skin, fat, and muscle) of a flat human model. The gap between the antenna and skin was kept as 4 mm. According to the International Commission of Non-Ionizing Radiation Protection (ICNIRP), the SAR value should not exceed 1.6 W/kg for one-gram and 2 W/kg for 10-gram standards [21,22]. Peak SAR values (10 grams) of 9.37, 8.99, 7.93, 5.98, 7.4, 7.11, 5.33, and 4.7 W/kg were noted for 1.8, 2.25, 2.4, 2.78, 3.3, 3.9, 4.5, and 5.54 GHz, respectively, for an input power of 1 W, as shown in Figure 8. The designed antenna exceeded the SAR limits (2 W/kg) for all operating bands using an input power of 1 W. However, many portable devices use power in the mW range [23]. Based on the above calculated SAR value, our antenna is safe if the input power is less than 213, 222, 252, 334, 270, 281, 375, and 425 mW for 1.8, 2.25, 2.4, 2.78, 3.3, 3.9, 4.5, and 5.54 GHz respectively. Hence, it is not recommended to install it directly into any portable device. An SAR reduction technique must be used before employing it in portable devices if the input power is more than the limit provided above.

An overall summary of the proposed antenna is presented in Table 2. Table 3 compares the performance of the proposed antenna with the state-of-the-art antennas. It can be noticed that the proposed antenna had smaller dimensions than the antennas presented in the literature. Furthermore, the proposed antenna used a fewer number of switches, which significantly reduce the system

complexity and enhance the power efficiency. Additionally, the antenna had reasonably good gain and efficiency in all operating bands.

**Figure 8.** Simulated specific absorption rate (SAR) of the antenna: (**a**) Mode 1; (**b**) Mode 2; (**c**) Mode 3; and (**d**) Mode 4.

**(d)**

**(c)**

**Table 2.** Summary of the simulated antenna under different operating modes.

**Table 3.** *Cont*.

*λg* = guided wavelength at the lower resonant frequency, NG = not given

#### **4. Conclusions**

A low-profile (0.21*λg*× 0.35*λg*× 0.02*λg*) and a simply-structured frequency-switchable antenna with eight frequency choices was presented in this paper. The antenna operated in four different modes depending on the ON and OFF states of the PIN diodes. In each mode, the antenna covered two unique frequencies (Mode 1 = 1.8 and 3.29 GHz, Mode 2 = 2.23 and 3.9 GHz, Mode 3 = 2.4 and 4.55 GHz, and Mode 4 = 2.78 and 5.54 GHz). The proposed antenna achieved high gain and reasonable efficiency in each mode. A fabricated model was tested experimentally to validate the simulation results. The proposed antenna can have a significant impact on compact and heterogeneous RF front-ends, because of its small geometry and efficient utilization of the frequency spectrum.

**Author Contributions:** Conceptualization, A.I.; methodology, A.I.; software, A.I.; validation, A.I., I.E. and A.S.; formal analysis, A.I.; investigation, A.I.; resources, L.F.A., O.A.S., S.K. and N.K.M.; data curation, A.I., N.K.M., I.E.; writing—original draft preparation, A.I.; writing—review and editing, A.I., A.S., L.F.A., I.E., O.A.S., S.K. and N.K.M.; visualization, A.I., S.K.; supervision, A.S., L.F.A., I.E., O.A.S., S.K. and N.K.M.; project administration, A.I., S.K.; funding acquisition, S.K.

**Funding:** Following are results of a study of the "Leaders in INdustry-university Cooperation +" Project, supported by the Ministry of Education and National Research Foundation of Korea. The authors extend their appreciation to the Deanship of Scientific Research at Majmaah University for funding this work under Project No. (RGP-2019-32).

**Conflicts of Interest:** The authors declare no conflicts of interest.

#### **References**

c 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).


**4. Summary**

It has been shown that metallic bipolar plates are prone to corrosion attack in the fuel cell environment. In addition, the use of precious noble metals is not economical in meeting the future cost projection of fuel cells to successfully compete with combustion engine vehicles. Though noble metals offer excellent conductivity and good corrosion resistance, they are phased out as viable bipolar plate materials over difficulty in forming them into thin strips. Composite-based bipolar plates are known for brittleness which in the long run may easily fail in operation of the fuel cell.

Metallic bipolar plates were proven to surpass the mechanical strength of graphite composite plates as well as giving acceptable electrical conductivity with minimal production cost on commercial scale. As indicated earlier, metallic plates are prone to corrosion in the fuel cell environment. Considerable research work has been conducted to enhance the material's corrosion resistance and interfacial contact resistance. It is concluded that metallic bipolar plates hold a promising potential as more research and development studies progress on its surface modification through different functional coatings.


**1. Introduction**

Agriculture is an approach of deploying natural resources to grow the desired plants. Since the induction of green revolution in the 1950s, the food production has been substantially increased that helped to meet food demands for the everincreasing world population [1]. Improved irrigation practices, tillage implements, fertilizers, and farm operations were some of the key outputs of green revolution. Nevertheless these practices have paved the way of agricultural sustainability yet

there are some concerns associated with these practices as, improved irrigation have given rise to salinity of soils, intensive tillage causes deterioration of soil structure, loss of soil organic carbon and destruction of natural habitats of different flora, higher yielding crop cultivars depleted soil nutrients. With all of the outputs of green revolution, introduction of pests is also acknowledged [2]. Disturbance in agricultural production due to invasion of other living organisms for their own existence is a natural phenomenon which cannot be stopped. These living organisms that survive on others are called as pests which include insects, plant pathogens, nematodes, rodents, and weeds.

Hence, the prevailing situation demands some weed control measures other than

*Application Potentials of Plant Growth Promoting Rhizobacteria and Fungi as an Alternative…*

Broadly speaking the control of weeds using microbes in green areas is a green approach that may reduce costs, decrease dependence on synthetic chemicals, and lower the negative impressions of chemicals on the environment. Microorganisms in the form of bioherbicides can be more selective than synthetic chemicals (herbicides) and target only the desired species [19]. Bioherbicides also lessen the chances of induction of resistance in the target weed species, due to the involvement of a number of mechanisms [20]. Therefore, keeping in view the abovementioned (even more) limitations of conventionally outdated methods necessitates the adoption of newer methods based on biological agents that are environmentally safer, friendly, economic, and feasible. We tried to highlight the need for adoption of innovative methods of weed control with higher efficacy. We then focused on harmful aspects of the judicious use of herbicides that in turn causes threats to environmental quality, food security, and human health followed by future research aims for

About one third of the total costs in field crop production is taken away by the weed management. There exist a variety of weed control strategies that can be applied depending upon various cropping systems [21]. Traditional farming practices generally rely on the application of herbicides and manual weeding. Generally,

weed control measures include physical, chemical, and biological methods.

Physical approaches of weed control include mechanical (tillage), manual methods, crop rotations, and crop fertilization and are separately discussed with

An increase in the density of weed species has been observed where monocropping was adopted. However due to the diverse nature of crop rotations, the density of such weeds can be tackled for profitable crop production [22]. Using a cover crop in rotation with the main crop is an attractive solution to cope with weed infestations [23, 24]. The integration of cover crops with no-till system has shown significant reduction (78%) in weed density in the USA [25]. The weeds with similar life cycles that match with the crop pose serious threats to crop production. These cover crops when used properly in rotation with the main crop compete with weeds for available nutrients, light, space, and water sources, hence reducing their emergence and numbers [26]. However the ability of cover crops to control weeds is largely governed by the growth habit and performance of the cover crop in a desired area [27]. That makes the use of this method to be only a small scale.

Increasing the competitive ability of crops against weeds is an important aspect to avoid field losses due to weeds and has been seen as a strategy for integrated weed management systems [28]. It can be achieved through manipulating fertilizer

chemicals, and in this context, biological control is gaining much importance around the world. Biological control is a general term used to define the introduction of organisms mostly bacteria and fungi in order to solve one or more problems in the farmer's field [17, 18]. Biological control using bacteria (bacterial herbicides), fungi (mycoherbicides), and viruses has recently gained much attention. Different kinds of fungi showing herbicidal activity are potential candidates of *Phoma* and *Sclerotina* genera. Among the bacteria some members of *Pseudomonas* and

*Xanthomonas* depict these attributes.

*DOI: http://dx.doi.org/10.5772/intechopen.86339*

improvement.

**2. Weed control options**

**2.1 Physical weed control**

possible limitations.

**249**

Among the agricultural crop pests, weeds are the most potent crop pests reducing crop yields by almost 34% followed by animal pests (18%) and plant pathogens (16%) worldwide [3]. Weeds are unwelcomed plants that interfere with the management of agricultural production systems, compete with the main crops for available nutrient resources and space and reduce growth, yield, and quality of agricultural produce up to a certain extent [4]. Generally, they produce a larger number of seeds, which may remain dormant in the soil seedbank for several decades, having greater plasticity and equipped with specialized seed dispersal mechanisms. Further, they exhibit the ability to invade newly disturbed areas and compete with crops for scarcely available moisture, nutrients, and light [5]. Apart from yield and production losses, they may also provide niches and harbor insects, plant pathogens, and other pests, hence increasing their incidence of attack to the main crop [6]. Weeds are the firstborn problem in agriculture since about 10,000 BC [7] representing the main hindrance in profitable agricultural production under natural resource management. The presence of weeds in natural ecosystems causes various direct and indirect losses, including interference with successful crop production, damage to biodiversity, loss of possibly fruitful land, loss of grazing areas and livestock production, obstruction of navigational and irrigation channels, and reduction of available water in water bodies. Most of the weeds belong to families Poaceae and Asteraceae. A majority of the weeds are terrestrial plants, a few are aquatic weeds and some are parasitic weeds [8]. Globally, reduction losses of wheat yield due to weed infestation are 23% [2]. The economic losses incurred due to this wheat yield reduction amount to Rs. 146 billion [9].

In the light of the abovementioned properties and harmful effects of weeds, it becomes important to control them. Appropriate weed control strategy in arable soils employs both the direct and indirect methods. Direct methods include those with the prime objective of weed control such as mechanical, manual, chemical and biological weed control and indirect being the cultural and preventive practices reducing germination, growth and vigor of weeds [10]. Many practices are available to control and manage weeds in agricultural crops. In ancient times when synthetic herbicides were not introduced, people tried polyculture, crop rotation, and other management practices that have shown sustainability with low inputs [11]. Until recently, weeds were being controlled by manual, mechanical, and chemical methods [12]. However there were drawbacks associated with each of these methods that severely limited their practical use, for example, herbicides cast detrimental effects on environment, humans, and animals [13]. They also cause contamination of water bodies and pollute natural resources like air, soil, and plants, thus destroying nontarget entities such as wildlife [14]. Also due to repeated herbicide applications, there is an increasing trend in herbicide-resistant weed species [15]. Mechanical weeding on the other side requires several repetitions and is only feasible for crops sown in rows; therefore weeds grown near to crop plants and within rows are escaped of control [10]. Similarly, hand weeding needs a huge number of labor and hence cannot be applied on a large scale [10]. Therefore, repeated manual weeding and nonavailability of labor make this method unfeasible and uneconomic [16].

*Application Potentials of Plant Growth Promoting Rhizobacteria and Fungi as an Alternative… DOI: http://dx.doi.org/10.5772/intechopen.86339*

Hence, the prevailing situation demands some weed control measures other than chemicals, and in this context, biological control is gaining much importance around the world. Biological control is a general term used to define the introduction of organisms mostly bacteria and fungi in order to solve one or more problems in the farmer's field [17, 18]. Biological control using bacteria (bacterial herbicides), fungi (mycoherbicides), and viruses has recently gained much attention. Different kinds of fungi showing herbicidal activity are potential candidates of *Phoma* and *Sclerotina* genera. Among the bacteria some members of *Pseudomonas* and *Xanthomonas* depict these attributes.

Broadly speaking the control of weeds using microbes in green areas is a green approach that may reduce costs, decrease dependence on synthetic chemicals, and lower the negative impressions of chemicals on the environment. Microorganisms in the form of bioherbicides can be more selective than synthetic chemicals (herbicides) and target only the desired species [19]. Bioherbicides also lessen the chances of induction of resistance in the target weed species, due to the involvement of a number of mechanisms [20]. Therefore, keeping in view the abovementioned (even more) limitations of conventionally outdated methods necessitates the adoption of newer methods based on biological agents that are environmentally safer, friendly, economic, and feasible. We tried to highlight the need for adoption of innovative methods of weed control with higher efficacy. We then focused on harmful aspects of the judicious use of herbicides that in turn causes threats to environmental quality, food security, and human health followed by future research aims for improvement.

#### **2. Weed control options**

there are some concerns associated with these practices as, improved irrigation have given rise to salinity of soils, intensive tillage causes deterioration of soil structure, loss of soil organic carbon and destruction of natural habitats of different flora, higher yielding crop cultivars depleted soil nutrients. With all of the outputs of green revolution, introduction of pests is also acknowledged [2]. Disturbance in agricultural production due to invasion of other living organisms for their own existence is a natural phenomenon which cannot be stopped. These living organisms that survive on others are called as pests which include insects, plant

Among the agricultural crop pests, weeds are the most potent crop pests reducing crop yields by almost 34% followed by animal pests (18%) and plant pathogens (16%) worldwide [3]. Weeds are unwelcomed plants that interfere with the management of agricultural production systems, compete with the main crops for available nutrient resources and space and reduce growth, yield, and quality of agricultural produce up to a certain extent [4]. Generally, they produce a larger number of seeds, which may remain dormant in the soil seedbank for several decades, having greater plasticity and equipped with specialized seed dispersal mechanisms. Further, they exhibit the ability to invade newly disturbed areas and compete with crops for scarcely available moisture, nutrients, and light [5]. Apart from yield and production losses, they may also provide niches and harbor insects, plant pathogens, and other pests, hence increasing their incidence of attack to the main crop [6]. Weeds are the firstborn problem in agriculture since about 10,000 BC [7] representing the main hindrance in profitable agricultural production under natural resource management. The presence of weeds in natural ecosystems causes various direct and indirect losses, including interference with successful crop production, damage to biodiversity, loss of possibly fruitful land, loss of grazing areas and livestock production, obstruction of navigational and irrigation channels, and reduction of available water in water bodies. Most of the weeds belong to families Poaceae and Asteraceae. A majority of the weeds are terrestrial plants, a few are aquatic weeds and some are parasitic weeds [8]. Globally, reduction losses of wheat yield due to weed infestation are 23% [2]. The economic losses incurred due to this wheat yield reduction amount to Rs. 146 billion [9].

In the light of the abovementioned properties and harmful effects of weeds, it becomes important to control them. Appropriate weed control strategy in arable soils employs both the direct and indirect methods. Direct methods include those with the prime objective of weed control such as mechanical, manual, chemical and biological weed control and indirect being the cultural and preventive practices reducing germination, growth and vigor of weeds [10]. Many practices are available to control and manage weeds in agricultural crops. In ancient times when synthetic herbicides were not introduced, people tried polyculture, crop rotation, and other management practices that have shown sustainability with low inputs [11]. Until recently, weeds were being controlled by manual, mechanical, and chemical methods [12]. However there were drawbacks associated with each of these methods that severely limited their practical use, for example, herbicides cast detrimental effects on environment, humans, and animals [13]. They also cause contamination of water bodies and pollute natural resources like air, soil, and plants, thus destroying nontarget entities such as wildlife [14]. Also due to repeated herbicide applications, there is an increasing trend in herbicide-resistant weed species [15]. Mechanical weeding on the other side requires several repetitions and is only feasible for crops sown in rows; therefore weeds grown near to crop plants and within rows are escaped of control [10]. Similarly, hand weeding needs a huge number of labor and hence cannot be applied on a large scale [10]. Therefore, repeated manual weeding and nonavailability of labor make this method unfeasible

pathogens, nematodes, rodents, and weeds.

*Sustainable Crop Production*

and uneconomic [16].

**248**

About one third of the total costs in field crop production is taken away by the weed management. There exist a variety of weed control strategies that can be applied depending upon various cropping systems [21]. Traditional farming practices generally rely on the application of herbicides and manual weeding. Generally, weed control measures include physical, chemical, and biological methods.

#### **2.1 Physical weed control**

Physical approaches of weed control include mechanical (tillage), manual methods, crop rotations, and crop fertilization and are separately discussed with possible limitations.

An increase in the density of weed species has been observed where monocropping was adopted. However due to the diverse nature of crop rotations, the density of such weeds can be tackled for profitable crop production [22]. Using a cover crop in rotation with the main crop is an attractive solution to cope with weed infestations [23, 24]. The integration of cover crops with no-till system has shown significant reduction (78%) in weed density in the USA [25]. The weeds with similar life cycles that match with the crop pose serious threats to crop production. These cover crops when used properly in rotation with the main crop compete with weeds for available nutrients, light, space, and water sources, hence reducing their emergence and numbers [26]. However the ability of cover crops to control weeds is largely governed by the growth habit and performance of the cover crop in a desired area [27]. That makes the use of this method to be only a small scale.

Increasing the competitive ability of crops against weeds is an important aspect to avoid field losses due to weeds and has been seen as a strategy for integrated weed management systems [28]. It can be achieved through manipulating fertilizer timing, rate of fertilizer, and placement methods effectively [29]. Nitrogenous fertilizers have been known to involve in the activation of dormant weed seeds, thus directly affecting specific weed densities. The most agricultural weeds have shown growth rates equal to that of wheat in response to the added nitrogen [30]. However, it is not well known that phosphorus levels of soil affect weed growth and crop as well, but it is a fact that the crop-weed competition is considerably affected by phosphorus fertilizations, for instance, Bansal [31] reported that weed-crop (fenugreek) competition was increased with higher P levels. Similarly, Santos et al. [32] reported that lettuce (*Lactuca sativa* L.) showed a higher competitive ability than the common purslane (*Portulaca oleracea* L.) but not smooth pigweed (*Amaranthus hybridus* L.) with higher P levels than lower levels. Therefore, due to this uncertainty, this method is not widely adopted and acceptable.

and pinoxaden. However, for the control of broad-leaved weeds, major chemical herbicides are carfentrazone, 2,4-D, and metsulfuron [44]. Herbicides account for 44% of all pesticides worldwide [45]. Nevertheless, chemical methods have controlled the weeds resultantly improving the yields of diverse crops from 10 to 50% [4]. However, the continuous application of such herbicides had led to intraspecific selection of weeds and caused the development of herbicide-resistant biotypes of weeds [46, 47]. Approximately, 300 herbicide-resistant weeds have been reported in 15 families of synthetic herbicides [45, 48, 49] (**Table 1**).

*Application Potentials of Plant Growth Promoting Rhizobacteria and Fungi as an Alternative…*

A major portion of applied herbicides falls on nontarget species and soil [50]. Some herbicides like triazines and sulphonylureas may persist in soil long enough to affect the growth of subsequent sensitive crops [38]. Herbicides have also caused toxicity and diseases to exposed animals [51]. Herbicides in soil however may not reduce the population of soil microflora and microfauna but may induce

The magnitude of issues caused by herbicides is much bigger than the outcomes of herbicides (**Figure 1**). Therefore it is a dire need of the hour to move toward some newer methods other than chemicals that can ensure environmental safety

**Herbicide-resistant weeds Common names Herbicide (s)** *Eichhornia crassipes* Water hyacinth 2,4-D,Glyphosate

*Chenopodium album* Common lambsquarters Triazine *Salsola kali* Russian thistle Sulfonylurea *Senecio vulgaris* Common groundsel Triazine (atrazine) *Sesbania exaltata* Hemp sesbania Glyphosate *Cyperus* Purple nutsedge Sulfonylureas *Avena fatua* Wild oat Glyphosate

and resource conservation and sustain crop production economically.

intraspecific and interspecific selections [38].

*DOI: http://dx.doi.org/10.5772/intechopen.86339*

*Some worst weeds that evolved resistance against chemical herbicides.*

*Disadvantages of herbicides to all life forms. Modified and redrawn from [1].*

**Table 1.**

**Figure 1.**

**251**

Manual weed control methods involve plucking, uprooting, and hoeing with and/or without hand-driven machines [33] and are in use since ancient times. Manual weed control is one of the most efficient methods and is applicable in areas where the labor is easily available. However, immediate availability of labor before the weeds have grown in crops [10], repeated hand weedings [16] and adoptation on only small scale farming are the major limitations of this method to be adopted. Mechanical methods use tillage implements such as cultivators, weeders, and different types of harrows which are being drawn by animals (in the past) or by engines (until recently) around world [34]. Tillage practices in the field affect weed management, weed seed bank in the soil, and soil disturbance patterns. Deep cultivation can be used to burry weeds that germinate in the upper soil layers such as *Phalaris minor* in wheat. However, timely sown wheat in integration with zero tillage has shown significant results in the reduction of *Phalaris minor* infestations, obtaining higher grain yields of wheat [35, 36]. Tillage for weed control is not suitable for all crops and is only limited to crops sown on rows with suitable row-torow spacing. Weeds that grow in close association with crop plants are not managed properly by this method, and those weeds which are grown within crop rows cause more losses than those sown in between crop rows [10, 37]. Moreover, some weeds may regenerate which are not completely uprooted, and root injury to main crop may occur [38]. However, the use of tillage implements for weed control are associated with adverse environmental impacts such as deterioration of soil structure, disturbed soil biological processes and soil erosion [39], leaching of nutrients which would otherwise be available to plants and eutrophication [40]. Therefore the efficiency of mechanical weed control measures is less than that of chemical weed control [22, 38]. Tillage practices done for weeding aggravate more soil compaction than other tillage operations due to a shorter cover of wheel tracks [38].

### **2.2 Chemical weed control**

The application of synthetic chemicals for crop protection began after the second world war when most of the selective herbicides for broad-leaved weeds were commercialized in 1946 [41]. However, with the advancement in crop protection measures usually at the start of the twentieth century, copper and sulfuric acid containing herbicides were developed [42]. Herbicides are chemical compounds which kill or control weeds and are largely synthesized by crop protection industries nowadays available for almost all cultivated crops. They were rapidly adopted by farming communities as they do not require much labor and hence are not costly; no risks of soil erosion and energy efficiency are further advantages of herbicides [43]. The most widely used chemicals in wheat to control grassy and non-grassy weeds are clodinafop, tralkoxydim, Atlantis (meso-/iodosulfuron), sulfosulfuron,

#### *Application Potentials of Plant Growth Promoting Rhizobacteria and Fungi as an Alternative… DOI: http://dx.doi.org/10.5772/intechopen.86339*

and pinoxaden. However, for the control of broad-leaved weeds, major chemical herbicides are carfentrazone, 2,4-D, and metsulfuron [44]. Herbicides account for 44% of all pesticides worldwide [45]. Nevertheless, chemical methods have controlled the weeds resultantly improving the yields of diverse crops from 10 to 50% [4]. However, the continuous application of such herbicides had led to intraspecific selection of weeds and caused the development of herbicide-resistant biotypes of weeds [46, 47]. Approximately, 300 herbicide-resistant weeds have been reported in 15 families of synthetic herbicides [45, 48, 49] (**Table 1**).

A major portion of applied herbicides falls on nontarget species and soil [50]. Some herbicides like triazines and sulphonylureas may persist in soil long enough to affect the growth of subsequent sensitive crops [38]. Herbicides have also caused toxicity and diseases to exposed animals [51]. Herbicides in soil however may not reduce the population of soil microflora and microfauna but may induce intraspecific and interspecific selections [38].

The magnitude of issues caused by herbicides is much bigger than the outcomes of herbicides (**Figure 1**). Therefore it is a dire need of the hour to move toward some newer methods other than chemicals that can ensure environmental safety and resource conservation and sustain crop production economically.

#### **Table 1.**

timing, rate of fertilizer, and placement methods effectively [29]. Nitrogenous fertilizers have been known to involve in the activation of dormant weed seeds, thus directly affecting specific weed densities. The most agricultural weeds have shown growth rates equal to that of wheat in response to the added nitrogen [30]. However, it is not well known that phosphorus levels of soil affect weed growth and crop as well, but it is a fact that the crop-weed competition is considerably affected by phosphorus fertilizations, for instance, Bansal [31] reported that weed-crop (fenugreek) competition was increased with higher P levels. Similarly, Santos et al. [32] reported that lettuce (*Lactuca sativa* L.) showed a higher competitive ability than the common purslane (*Portulaca oleracea* L.) but not smooth pigweed (*Amaranthus hybridus* L.) with higher P levels than lower levels. Therefore, due to

this uncertainty, this method is not widely adopted and acceptable.

tracks [38].

**250**

**2.2 Chemical weed control**

*Sustainable Crop Production*

Manual weed control methods involve plucking, uprooting, and hoeing with and/or without hand-driven machines [33] and are in use since ancient times. Manual weed control is one of the most efficient methods and is applicable in areas where the labor is easily available. However, immediate availability of labor before the weeds have grown in crops [10], repeated hand weedings [16] and adoptation on only small scale farming are the major limitations of this method to be adopted. Mechanical methods use tillage implements such as cultivators, weeders, and different types of harrows which are being drawn by animals (in the past) or by engines (until recently) around world [34]. Tillage practices in the field affect weed management, weed seed bank in the soil, and soil disturbance patterns. Deep cultivation can be used to burry weeds that germinate in the upper soil layers such as *Phalaris minor* in wheat. However, timely sown wheat in integration with zero tillage has shown significant results in the reduction of *Phalaris minor* infestations, obtaining higher grain yields of wheat [35, 36]. Tillage for weed control is not suitable for all crops and is only limited to crops sown on rows with suitable row-torow spacing. Weeds that grow in close association with crop plants are not managed properly by this method, and those weeds which are grown within crop rows cause more losses than those sown in between crop rows [10, 37]. Moreover, some weeds may regenerate which are not completely uprooted, and root injury to main crop may occur [38]. However, the use of tillage implements for weed control are associated with adverse environmental impacts such as deterioration of soil structure, disturbed soil biological processes and soil erosion [39], leaching of nutrients which would otherwise be available to plants and eutrophication [40]. Therefore the efficiency of mechanical weed control measures is less than that of chemical weed control [22, 38]. Tillage practices done for weeding aggravate more soil compaction than other tillage operations due to a shorter cover of wheel

The application of synthetic chemicals for crop protection began after the second world war when most of the selective herbicides for broad-leaved weeds were commercialized in 1946 [41]. However, with the advancement in crop protection measures usually at the start of the twentieth century, copper and sulfuric acid containing herbicides were developed [42]. Herbicides are chemical compounds which kill or control weeds and are largely synthesized by crop protection industries nowadays available for almost all cultivated crops. They were rapidly adopted by farming communities as they do not require much labor and hence are not costly; no risks of soil erosion and energy efficiency are further advantages of herbicides [43]. The most widely used chemicals in wheat to control grassy and non-grassy weeds are clodinafop, tralkoxydim, Atlantis (meso-/iodosulfuron), sulfosulfuron,

*Some worst weeds that evolved resistance against chemical herbicides.*

#### **Figure 1.**

*Disadvantages of herbicides to all life forms. Modified and redrawn from [1].*