Weed Management Methods for Herbaceous Field Crops: A Review

Abstract

Weeds compete with crops for water and nutrients and can adversely affect crop growth and yield, so it is important to research effective weed control methods. This paper provides an overview of the impact of weeds on crop yield and describes the current state of research on weed management in field herbaceous crops. Physical weed control mainly refers to thermal technologies represented by flame weed control and laser weed control, which can efficiently and accurately remove weeds. Mechanical weed control requires a combination of sensor technologies, machine vision technology, and high-precision navigation to improve weed control accuracy. Biological weed control relies heavily on plant extracts and pathogens to create herbicides, but it is costly, and some can be toxic to mammals. Chemical weed control is a common method, resulting in environmental pollution and weed resistance. To reduce the use of chemical herbicides, scholars have proposed integrated weed management strategies, which combine biological control, control of the seed bank, and improve crop competitiveness. Integrated weed management strategies are considered to be the future direction of weed management. In conclusion, physical, mechanical, biological, and chemical weed control methods are commonly used in weed management. Each method has its applicable scenarios, and the implementation of integrated weed management strategies can lead to better weed control, improving crop yield and quality. The main objective of this review is to organize the research progress on weed management methods for herbaceous crops in the field and to provide a reference for the agricultural sector to develop weed control strategies. Specifically, this paper categorizes weed management methods into four groups, discusses and presents the advantages and disadvantages of the aforementioned weed control methods, and discusses future research directions.

1. Introduction

In the field of herbaceous field crops, there is a wide variety of crop species around the world. From rice in Asia to corn in Europe to potatoes in the Americas, a wide variety of herbaceous field crops have created a rich and diverse agricultural production system across the globe. Of all the species, only 15 to 20 have a significant impact on the world economy and are grown on about 1.6 billion hectares [1]. Herbaceous field crops can be categorized according to life cycle, climate, season, and use (Figure 1) [1]. Currently, it is widely recognized that weeds are the most important biological factor affecting crop growth and yield. Oerke concluded that weeds may cause a 34% reduction in crop yield [2]. Specifically, he assessed yield reductions in six herbaceous field crops, including a 23% reduction in wheat, 37% in rice, 40% in corn, 30% in potato, 37% in soybean, and 36% in cotton. Kraehmer and Baur [3] confirmed that the impact of weeds on the global economy exceeds USD 100 billion, and the cost of control is in the billions of dollars, according to Appleby et al. [4]. After the Second World War, chemical herbicides were mainly used in developed countries to increase productivity, causing serious damage to the environment [1]. The misuse of herbicides has also led to a significant increase in the number of herbicide-resistant weeds, making weeding even more difficult. In an attempt to solve this problem, farmers have started using higher doses and more frequent spraying. However, this approach has not addressed the root cause and has caused even more harm to the environment. Currently, there are four different approaches to weed control: physical, mechanical, biological, and chemical [1]. This paper analyzes the historical background and practical significance of these four approaches and describes the current state of development of weed management techniques for herbaceous field crops worldwide. Physical weed control mainly refers to thermal technologies represented by flame weed control and laser weed control, which can efficiently and accurately remove weeds [1,5]. Flame weeding and laser weeding use heat to destroy plant tissue and thus kill weeds, but flame weeding produces large amounts of carbon dioxide and poses a fire risk, and the high-energy beams of laser weeding can cause injury to people and animals. Traditional mechanical weeding may accidentally injure crops. Currently, the direction of global research is to combine traditional mechanical weeding technology with sensor technology, machine vision technology, and high-precision navigation technology to realize high-precision mechanical weeding [6]. Biological weed control relies heavily on plant extracts and pathogens to create herbicides, but it is costly, and some can be toxic to mammals. Chemical methods are commonly used for weed control, but there are issues with weed resistance and environmental pollution. In order to reduce the use of chemical herbicides, scholars have proposed integrated weed management (IWM) approaches that combine physical, mechanical, biological, and chemical methods and weed prevention measures (e.g., controlling the seed bank and improving crop competitiveness) [7,8,9,10]. Implementation of integrated weed management (IWM) can lead to better weed control and improved crop yield and quality [11]. IWM is widely used in cultivation in developed countries but is not common in developing countries. The growing interest in IWM in the global scientific community is illustrated in Figure 2, which reports the number of journal papers in the Web of Science database with the keywords integrated, weed, and management. As can be seen from the figure, the number of papers in this field has grown exponentially since 1943. IWM can effectively reduce the impact of weeds on crops and will be a focus of future research [12]. In conclusion, physical, mechanical, biological, and chemical weed control methods each have their own applicable scenarios, while integrated weed management provides better weed control and improves crop yield and quality [1].

To illustrate the contribution of this review, it is compared with three recent literature reviews on similar topics. The contributions of these three articles are shown in

Table 1. The contribution of these three literature reviews.

Article Number	Contribution of the Article	References
Article 1	Explore the application of artificial intelligence in weed management, focusing on weed recognition and deep learning.	[13]
Article 2	Describe traditional and non-traditional weed control strategies from a sustainability perspective, highlighting the value of applying precision weed control strategies.	[14]
Article 3	Present a four-step process for developing an integrated weed management strategy.	[1]

. Compared to article 1, this paper not only discusses the application of computer recognition technology in weed control from a macro perspective but also implements it in detail in mechanical weed control mechanisms. The “Section 8” explains in detail the four directions for the future development of the combination of artificial intelligence technology and weed control technology. Compared with articles 2 and 3, this review is more innovative and comprehensive. For example, in the section on physical weed control, the latest laser weed control solutions are added, and in the section on mechanical weed control, the application of ultrasonic sensors, LiDAR sensors, and spectral recognition technologies is added, along with a detailed description of the working principles of the above technologies. The main problems faced by current research and the development of weed control machinery are analyzed further to provide the reader with an understanding of the dilemmas encountered in current research. In the section on biological weed control, it is more comprehensive than the above-mentioned literature. All the sources of bioherbicides are described in detail, including allelochemicals, essential oils, pathogens, etc. The limitations and solutions of the bioherbicides not mentioned in article 3 are also introduced. In the section on chemical weed control, the origins and types of herbicides are reviewed in detail. The causes of resistance are explained, and the specific hazards of herbicides on different organisms and groundwater are carefully analyzed. Corresponding solutions are provided, introducing the latest concepts of precise chemical weed control. In the seventh part of the review, some traditional agro-technological methods, such as crop rotation, which were not mentioned in the above three articles, are introduced. The main objective of this review is to sort out the latest research advances in weed control techniques for herbaceous plants in the field and to provide guidance to the agricultural sector in developing weed control methods. For the sake of simplicity and readability, this paper classifies weed control methods into four categories and describes an integrated weed control strategy that combines the four methods mentioned above. Finally, the advantages and disadvantages of the above-mentioned weed control methods, as well as future research directions, are discussed and presented.

This review is divided into nine parts. Part 2 describes the research methodology of this paper, including the number of references and the criteria used to screen the literature. Part 3 describes physical weed control, using flame and laser weed control as examples. Part 4 describes precision mechanical weed control, which combines mechanical weed control with sensor technology, machine vision technology, and high-precision navigation technology. Part 5 introduces biological herbicides from various sources. Part 6 focuses on chemical weed control. Part 7 presents an integrated weed management strategy that combines the four weed control methods mentioned above. Part 8 discusses and suggests future directions for weed control technology. Part 9 reviews and summarizes the full text.

2. Material and Methods

This paper adopts a qualitative research methodology. The authors searched the “Web of Science” and “ScienceDirect” databases using the following five keywords: “integrated weed management”, “biological weed control”, “mechanical weed control”, “chemical weed control”, and “physical weed control”. The criteria for screening were that the articles were published between 2019 and 2023, and the type of literature was limited to review articles. Finally, the keywords and abstracts of the articles in the search results were read, and the literature with little relevance to the research topic of this paper was excluded. The final number of studies that met the conditions was 138. The number of references for each weed control method is shown in

Table 2. Number of references for each weed control method.

Weed Management Methods	Number of References
Physical weed control	20
Mechanical weed control	21
Biological weed control	63
Chemical weed control	34

.

3. Physical Weed Control

Physical weed control mainly refers to thermal technologies represented by flame weed control and laser weed control, which can efficiently and accurately remove weeds. Thermal technologies can be divided into three categories according to their mode of operation: direct heating methods such as flame, laser, and steam; indirect heating methods such as electric shock, microwave, and ultraviolet (UV); and liquid nitrogen or solid carbon dioxide freezing methods [15]. Indirect heating techniques such as microwave and ultraviolet (UV) and freezing methods [16] are still in the experimental stage and cannot yet be used on a large scale in the field. Flame and laser weeding are the most commonly used thermal methods. Thermal technologies are described below around these two methods.

3.1. Flame Weeding

Flame weeding, a direct thermal technique using propane gas burners or renewable sources like hydrogen [17], is often employed in organic agriculture to achieve combustion temperatures up to 1900 °C. When a flame touches a leaf, there is a swift increase in the plant cell tissue’s temperature to roughly 50 °C, resulting in the denaturation and clustering of membrane proteins [17]. Disruption of the cell membrane leads to loss of cellular function. As a result, affected tissues experience an expansion of intracellular water until dehydration occurs [18]. Weeds scorched by fire may perish in 2 to 3 days, or their capacity to combat the crop will significantly diminish. It is vital to differentiate between burning and flaming, given that plant tissues heat up quickly rather than ignite, leading to cell membrane rupture [18]. Flame weed control’s success is intimately linked to the variety of weeds and the size of seedlings, where dicotyledonous plants exhibit greater sensitivity than monocotyledonous ones. The ability of weeds to regenerate and different ignition methods (different ignition methods have different temperatures and burn times) also have an effect on the effectiveness of flame weed control [17,19]. For crops that can tolerate high temperatures (e.g., cotton, corn, sugarcane, etc.), directional flamethrowers are recommended for effective control of inter-plant weeds, while traditional mechanical techniques can effectively control inter-row weeds [20]. Heat-sensitive crops utilize shielding or parallel burner systems with angles varying from 22.5° to 45° to the horizontal, aiming to control the proliferation of weeds within the rows [19,21]. Countless efforts have gone into evaluating the amount of propane gas needed for flame weeding and the cost required to complete the ignition operation. Undoubtedly, flame weeding proves to be a more cost-effective alternative to herbicides, lessening manual weeding requirements. However, the process of flame weeding requires a significant quantity of gas [22] and generates CO₂, which may not be viable in the long term.

3.2. Laser Weeding

Laser technology is powered by electricity, which can be obtained through wind, water, solar, and other environmentally friendly means. Laser weed control uses the light that is absorbed most by the weed’s leaves to irradiate the weed. Lasers allow a large amount of energy to be highly concentrated in a laser beam that can be precisely aimed at a target. The efficient and accurate utilization of laser energy in weed control can efficiently and swiftly eradicate weeds while preserving the integrity of crops. In addition, laser weeding hinders weed regeneration and does not produce any pollution, which is a clear advantage over mechanical weeding. Laser weed control is a precision weed control method that combines weed identification technology, weed control actuators, and field navigation technology. Currently, there are two main methods for identifying weeds in the field, namely machine vision technology and spectral technology. Machine vision technology captures the external properties of plants, identifies weeds, and locates them by capturing images of the field environment and comparing crop and weed characteristics such as color, shape, and texture features. Spectral techniques capture the internal properties of plants, crops, and weeds belonging to different species and have different internal structures, and spectral techniques are able to differentiate between crops and weeds based on their spectral reflectance characteristics. After the weed recognition system was processed and identified, the location of the target weed was obtained. The robotic arm can then manipulate the laser module to aim the laser at the location of the weed. The weed is effectively and accurately killed by controlling the on and off of the laser. Field navigation systems can be mainly categorized into GPS navigation, machine vision navigation, and laser navigation.

Different types of lasers utilize a variety of materials and emit light with different wavelengths. The diode laser employs a semiconductor as its active medium, which is energized by an electrical current. Diode lasers display a broad spectrum of emission wavelengths, encompassing ultraviolet to infrared radiation, with their customary wavelengths spanning from 940 to 980 nm, accompanied by laser power varying from mW to several kW. A fiber laser is a solid-state laser that makes use of a doped glass fiber as its active medium. Fiber lasers can deliver up to several kilowatts of power [23]. The CO₂ laser uses a combination of nitrogen (N2), helium (He), and carbon dioxide (CO₂) as its active medium, which is energized by an electric gas discharge. It is customary for CO₂ lasers to emit light that falls within the far infrared spectrum. The three wavelength bands at 9.3 μm, 10.2 μm, and 10.6 μm are mostly used [24].

Weed control experiments have utilized three varieties of lasers: the CO₂ laser [25], the diode laser [26], and the fiber laser [27,28]. Plants absorb a large amount of light energy from CO₂ lasers and fiber lasers, producing lethal injuries [29]. The plant’s surface absorbs the energy of the CO₂ laser, leading to the highest density of energy deposition. The water in the plant absorbs the energy emitted by the 2 μm fiber laser, which results in the heating of a large area of plant tissue [30]. Employing a thulium-doped fiber laser emitting at a wavelength of 2 μm yields superior benefits in weed control, as the radiation infiltrates the epidermis rather than being solely absorbed on the plant’s surface [30]. Various factors, including the type of weed, the location of the laser spot, the size of the weed, the area of the laser spot, and the amount of laser energy (J) used [25,31,32,33], all have an impact on the effectiveness of weed control. Compared to bigger plants, smaller weeds have a higher sensitivity to laser beams and consume less energy. Optimal herbicidal effectiveness is attained when the target plant’s meristematic tissues are revealed during the cotyledon phase or both the permanent leaf phases [31,32,33]. As the weed grows, its meristematic tissues become more developed. The ability of the weed to regrow after laser radiation becomes stronger (Figure 3). Laser weed control is highly accurate, eliminating weeds in the vicinity of the crop without causing any damage to their leaves and roots, as long as the crop does not shade the weed’s meristematic tissue. A reduction in the diameter of the laser beam will result in a greater concentration of laser energy, which will increase the efficiency of laser weed control (Figure 4). However, the diameter is too small and may not irradiate enough cells to kill the plant [32]. And, the smaller the diameter, the harder it is to accurately strike the meristematic tissue in rugged field conditions. Despite the advantages of laser weeding, there are numerous drawbacks. The heat generated by lasers can ignite dry materials in fields, such as straw, leaves, paper, and other combustible materials, which can cause fires. In addition, laser beams can have harmful effects on humans and animals.

The disadvantage of flame weed control is mainly that the combustion process produces large amounts of greenhouse gases, which exacerbates the greenhouse effect and is not environmentally friendly in the long run. The advantage of laser weeding is that the laser can be powered by electricity, which can be obtained through wind, solar, and other environmentally friendly means. Laser weeding can precisely eliminate weeds without accidentally injuring crops. The disadvantage is that it is only effective on small weeds and cannot eradicate large weeds. In addition to this, the heat generated by the laser may ignite dry materials in the field and start a fire. Laser beams may also have adverse effects on humans and animals.

4. Mechanical Weed Control

Conventional mechanical weed control has the potential to injure crops (Figure 5). If the crop is injured, crop growth is stunted and may lead to pathogen invasion. This can lead to secondary infections and reduced yields [35]. For example, in the case of mechanical injury, sugar beet roots may be susceptible to Fusarium or Phytophthora root rot [36]. Therefore, it is important to improve the accuracy of mechanical weed control to avoid crop damage.

Removing inter-row weeds with mechanical tools is not difficult, but removing intra-row weeds is a challenging task [6,37,38,39,40]. Removal of intra-row weeds is particularly important because the closer weeds grow to the crop, the greater the negative impact on resource competition [41,42]. The significance of precise mechanical weeding is shown in Figure 6. In order to improve the accuracy of mechanical weed control, it is necessary to combine traditional mechanical weed control techniques with sensor technology (Listing 1), machine vision technology, and high-precision navigation technology. Ultrasonic and LiDAR sensors can be integrated with conventional mechanical weeding tools. Ultrasonic and LiDAR sensors are also known as “distance sensors”; ultrasonic sensors are able to measure the distance between the sensor and the plant by analyzing the sound waves, while LiDAR sensors measure the distance by analyzing the phase difference between the emitted and reflected laser beams. LiDAR sensors are more accurate than ultrasonic sensors because they measure at a higher frequency [43], while ultrasonic sensors are more cost-effective and have high accuracy in the range of 100 mm to 10 m [44]. Detection of plants using LiDAR relies on the reflection of near-infrared light by chlorophyll, so the green hue of the vegetation results in elevated reflection values when using LiDAR sensors. LiDAR systems are effective in identifying and classifying weeds in large fields [45]. Similarly, the use of ultrasonic sensors can identify weeded areas in agricultural fields [46]. Machine vision technology can also be used for precision mechanical weed control. It can identify all the weeds in and around the crop from captured images of the field, calculate the location of the weeds, and finally drive actuators for precise weeding [47]. FarmWise’s weeding robots work autonomously, and their robots analyze images of plants using deep learning techniques to identify and remove weeds. Denmark’s F. Poulsen Engineering’s Robovator robot uses a machine vision system for identifying weeds and a tine rake equipped with blades. The rake teeth expand when they encounter the crop and close when they encounter weeds so that weeds can be removed from the rows, and the distance between the mowing work area and the crop can be adjusted according to the farmer’s needs and the crop’s stage of growth. In the future, machine vision technology will be developed and improved in the field of mechanical weeding to adapt to a variety of cropping situations, such as very narrow rows with row widths of less than 20 cm. To improve weeding accuracy, mechanical weeding robots also need a navigation system. GNSS uses signals from space satellites to transmit positioning and timing data to GNSS receivers. Currently, there are four navigation systems in the world: GPS (U.S.), GLONASS (Russia), Galileo (European Union), and Beidou (China). Navigation technology is widely used in several research fields, including precision agriculture, to provide relevant authorities with geographic information data about soil and yield [48]. However, Hiremath [49] pointed out that the accuracy of GNSS may not be sufficient for certain tasks, and navigation may not be successful when the signal is interrupted. In order to improve the accuracy of robotic operations, it is necessary to use a more precise navigation technique, usually RTK-GPS, which uses base stations near the farm to rectify and augment the signal and can provide sub-centimeter accuracy [50].

Listing 1. Fundamental criteria for the effectiveness of sensor-assisted mechanical weeding devices [6].

To prevent harm to crop plants, enhance the accuracy of implement guidance, and accurately distinguish between rows of crops or individual plants at various growth phases. Adapt to diverse field features like landscape inclines, assorted soil varieties, intense weed presence, gaps in crop rows (absent crops), and different crop looks (e.g., the elevation of crops and the contrast between red and. green plants). Financial advantages are essential to offset the increased costs of acquisition. Facilitate novice operators in executing sophisticated weed management tasks (e.g., harvesting near a row of crops). Maintain a uniform depth of operation for the tools within the soil. Extend the duration of operation from daytime to nighttime. Streamline the workload through automated guidance of the implement (s). Broadening the range of operation through enhanced travel velocity. The reliability of every component in the system to withstand diverse environmental factors, such as dust, water, temperature fluctuations, machine oscillations, and both lateral and horizontal forces encountered during the treatment application, is crucial.

There are still many issues that need to be addressed in the development of mechanical weeding robots that enable precise weeding. Firstly, weeding robots need a lightweight design. Mechanical weeding robots will need to perform tasks in the field for long periods of time if weed densities are to be kept low, and if the robot is too heavy, the disadvantages of soil compaction will outweigh the advantages of using mechanical weeding. Secondly, sloping terrain is also a problem. Uneven terrain affects the accuracy of mechanical weeding and can lead to accidental crop damage. Finally, sensor-based systems can operate efficiently in specific situations, but complex agricultural environments present challenges for accurate sensor judgment. Examples include varying weather [52], different soil properties in different regions, a wide variety of cropping systems, leaf litter in the field, wind and sand, waterlogging, unregulated lighting conditions, randomness of weed distribution in the field [53,54], differences in the appearance, color, and size of different weed species [55], and differences in the appearance of the same weed at different stages of growth. Therefore, it is important to keep experimenting to obtain the best combination of sensors in order to control weeds successfully in complex environments while preventing crop damage.

5. Biological Weed Control

The European Weed Research Society defines biological weed control as the intentional employment of indigenous or non-native species, like phytophagous arthropods, nematodes, and plant pathogens, to manage specific weed groups. The Weed Science Society of America defined weed biological control as the employment of an agent, a mix of agents, or biological mechanisms to achieve weed control, categorizing various microbial and microbial species as biological control agents. Cordeau and his team [56] categorized biocontrol agents into three groups: macro-organisms (like predators, parasitoid insects, and nematodes), microorganisms (including bacteria, fungi, and viruses), chemical agents (such as pheromones), and natural substances (sourced from plants or animals).

Beginning in the 1980s, the focus of scientists, business entities, and interested parties has increasingly been on biological regulation, playing a crucial role in advancing organic farming practices with an emphasis on sustainability. Schwarzländer and others [57] referenced findings from the fifth edition of Biological Control of Weeds, indicating that Australia, North America, South Africa, Hawaii, and New Zealand (in a decreasing sequence) lead the research in biocontrol and bioherbicide application, with the trio of entomopathogenic orders (Coleoptera, Lepidoptera, and Diptera) comprising roughly 80% of all employed biocontrol agent species, and 66% of targeted weeds are successfully manageable. Currently, the market share of bioherbicides (including biofungicides, biobactericides, bioinsecticides, and bionicides) is less than 10% [58], but the interest in bioherbicides is increasing day by day.

Table 3. Development of bioherbicides from different sources with their target weeds [64].

Source	Phytotoxic Effects	Target Weeds	References
Achillea santolina L.	Hinders growth and modifies metabolic activities	Medicago polymorpha L.	[65]
Brassica napus L.	Suppress germination and root length	Phalaris minor Retz., Convolvulus arvensis L., Sorghum halepense (L.) Pers.	[66]
Carum carvi L.	Damage to leaves and biochemical alterations in plant tissues	Echinochloa crus-galli (L.) P. Beauv.	[67]
Chrysanthemum coronarium L.	Inhibition of sprouting and development	Sinapis arvensis L., Phalaris canariensis L.	[68]
Cynara cardunculus L.	Inhibit the sprouting and development, leading to necrosis or chlorosis	Trifolium incarnatum L., Silybum marianum (L.) Gaertn., P. minor	[69]
Cymbopogon nardus (L.) Rendle.	Suppress the sprouting and growth of plants while lowering the levels of chlorophyll and proteins	Digitaria horizontalis Willd., Cenchrus echinatus L.	[70]
Mimosa pigra L.	Deceleration in root development	Lactuca sativa L., Ruellia tuberosa L.	[71]
Parthenium hysterophorus L.	The process of seed sprouting and growth	Oryza sativa f. Spontanea Roshev., Echinochloa colona (L.) Link., Euphorbia hirta L., Ageratum conyzoides L.	[72]
Pinus densiflora Siebold and Zucc.	Suppressed development of shoots and roots	Lolium multiflorum Lam., Digitaria sanguinalis (L.) Scop.	[73]
Pinus nigra J.F. Arnold	Suppressed development of shoots and roots	P. canariensis, Trifolium campestre Schreb., S. arvensis	[74]
Sinapis alba L. Fungi	Reduced dry biomass	Amaranthus powellii S. Watson, Setaria viridis (L.) P. Beauv.	[75]
Ascochyta agropyrina	Reduced root growth	Chenopodium album L., Cirsium arvense (L.) Scop., Mercurialis annua L., Sonchus oleraceus L., Setariavirdis (L.) P. Beauv.	[76]
Diaporthe gulyae	Necrosis	Papaver rhoes L., Ecballium elaterium (L.) A. Rich., Urtica dioica L., Hedysarum coronarium L.	[77]
Fusarium fujikuroi	Chlorosis, necrosis, and reduced height and root length	Cucumis sativus L., Sorghum bicolor (L.) Moench.	[78]
Phoma herbarum	Maximum toxicity	P. hysterophorus, Lantana camara L., Hyptis suaveolens (L.) Piot., Sida acuta Burm.f.	[79]
Lasiodiplodiapseudotheobromae	Suppression of germ activity	Solanum lycopersicum L., Amaranthus hybridus L., E. crus-galli	[80]
Myrothecium roridum	Gangrene	Eichhornia crassipes (Mart.) Solms	[81]
Bacteria	Reduced weed density	Solidago canadensis L.	[82]
Pseudomonas aeruginosa	Hindered the sprouting, growth, and germination of seeds	A. hybridus, S. lycopersicum, E. crus-galli, Pennisetum purpureum Schumach.	[78,83]
Pseudomonas fluorescens	Inhibit the sprouting and development of roots	Bromus tectorum L., Aegilops cylindrical Host, Taeniatherum caput-medusae (L.) Nevski	[84,85]

lists several different sources of bioherbicides and their target weed species. Biological herbicides are not only environmentally friendly but also have unique modes of action and molecular target sites that differ from those of synthetic herbicides [59]. Compared with chemical herbicides, bioherbicides have a shorter half-life and lower field weed control efficiency, which limits their commercial value [60,61]. Biological herbicides can partially replace chemical herbicides [62] and have been recognized as an important component of weed management in recent years [63]. Since sustainable weed control is not reliant on just one method, it is recommended to employ both bioherbicides and other weed control techniques.

Plant extracts are suitable raw materials for the development of bioherbicides, and some of them have been used in the production of bioherbicides. These extracts are very effective in controlling weeds. Many plant extract compounds have significant inhibitory effects on weed growth while being harmless to crops [86]. The cause could be attributed to the varying sensitivities of the enzymes targeted or the diverse receptors in weeds that identify and react to these substances [68]. Some plants are capable of producing allelochemicals such as fatty acids, phenols, flavonoids, terpenoids, and steroids [87], which can impede the growth and reproduction of nearby plants, including weeds. Phytotoxic aqueous extracts from sorghum bicolor are examples of bioherbicides that effectively control weed growth and are not harmful to crops [88]. Employing sorghum extracts led to a significant 40% decrease in E. crus-galli’s population, culminating in an 18% rise in rice production [89]. Another typical example is that substances extracted from the leaves, stems, flowers, and roots of Brassica nigra (L.) hinder the germination and growth of weeds [90,91,92]. The mechanism of action is that high levels of glucosinolates (bitter sulfur-containing compounds found mainly in Brassica juncea) are converted to isothiocyanates, thiocyanates, and nitriles by enzymatic hydrolysis, thereby inhibiting a wide range of weeds. Isothiocyanate hinders the sprouting of weeds Matricaria inodora L., E. crus-galli, Sonchus asper L., A. hybridus, and Alopecurus myosuroides Huds. It interacts with cell-destroying, sulfhydryl-containing enzymes to inhibit a wide range of fungi, pathogenic bacteria, and food spoilage bacteria [91]. In addition, extracts of Pisum sativum L. also showed significant phytotoxicity, strongly inhibiting the germination and growth of Polygonum persicaria L., A. hybridus, Galinsoga parviflora Cav., C. album, Medicago sativa L., with up to 83% inhibition in petri dish experiments and up to 89% under field conditions [93].

Beyond allelochemicals, essential oils play a crucial role as raw materials in creating bioherbicides. This originates from various plant segments, including leaves, bark, flowers, fruits, seeds, roots, and the entire plant [94]. Essential oils are natural volatile compounds that contain terpenoids, especially monoterpenes and sesquiterpenes, which are potent phytotoxic agents against a wide range of weeds, making them a suitable choice for the manufacture of novel bioherbicides [95]. Essential oils are phytotoxic and can cause weed chlorosis, leaf scorch, reduced growth, mitotic inhibition, membrane depolarization, reduced levels of chlorophyll, cell breathing, and oxidative harm [96]. Verdeguer et al. stated [97] that essential oils extracted from Cistus ladanifer L. were effective against A. hybridus, Portulaca oleracea L., C. album, Conyza Canadensis, and Parietaria judaica L., inhibiting their germination and growth.

Natural by-products have also been noted to inhibit the growth of weeds, and these substances can also be used as raw materials for bioherbicides. Boydston and colleagues [98] produced a by-product called corn gluten meal (CGM) during the wet milling of maize, which was highly effective in weed control. C. album, S. nigrum, Agrostis palustris Huds., P. oleracea, and R. crispus were suppressed after CGM was applied to the surface of greenhouse soil [99]. Mustard seed meal (MSM), a by-product of mustard oil pressing technology, also possesses herbicidal properties. MSM contains glucosinolates, which are effective in inhibiting plant growth [100].

Bacteria can also be used as bioherbicides, and among the currently known strains, Curtobacterium sp. (MA01) [101], Pseudomonas fluorescens (D7 strain) [84,102], P. fluorescens (WH6 strain) [103,104], P. fluorescens (BRG100 strain) [105,106], and P. viridiflava (CDRT_C14 strain) [107] have suitable herbicidal effects and can be used as bioherbicides. The main difficulty in the development of bacterial bioherbicides is the identification and search for suitable strains that can directly interact with and control the target weeds [108]. The weed control efficacy of bacterial bioherbicides is strongly influenced by environmental conditions, and studies have shown that a dew period of 25 °C is required to maintain more than 60% mortality [109]. In addition, many bacterial bioherbicides may take longer to suppress weeds than chemical herbicides [84]. Some bacterial bioherbicides also adversely affect other flora and fauna around the crop [84,103]. Therefore, bacterial bioherbicides also suffer from a long duration of action and adverse effects on the surrounding environment. Further research is needed to solve the above problems if bacterial bioherbicides are to be widely used.

Several fungal-based bioherbicides have been developed and commercially used in Australia, Canada, China, South Africa, the Netherlands, and the United States [56,110,111,112,113,114,115,116,117,118]. The literature has shown that fungi of the genus Colletotrichum are among the most commonly used species in fungal bioherbicide formulations [117,119,120,121,122,123,124,125]. Current studies using fungi of this genus have produced several herbicides, including BioMal^® (derived from C. gloeosporioides f. sp. malvae) [120,121,126], Collego™/LockDown™ (derived from C. gloeosporioides f. sp. aeschynomene) [113,120], Lubao1 and Lubao 2 (from C. gloeosporioides) [114], Velgo^® (from C. coccodes) [119], and C. truncatum (not yet commercially developed) [122]. Although fungi are effective in eliminating weeds, they are not widely used because they are usually more expensive than chemical herbicides, and their success in weed control is not as suitable as that of chemical herbicides [127,128]. These limitations are shared by several other developed fungal bioherbicides such as Casst™ (from Alternaria cassiae) [111], DeVine^® (from Phytophthora palmivora) [129], Dr. Biosedge^® (from Puccinia canaliculata) [130], Sarritor™ (from Sclerotinia minor) [131,132], Smolder^® (from Alternaria destruens) [127,132], and Woad Warrior^® (from Puccinia thalaspeos) [133]. Therefore, future research should be directed toward reducing the production cost of fungal bioherbicides while searching for fungi with better herbicidal efficacy.

Different scholars do not share the same understanding of “whether bioherbicides pose health hazards to humans and animals”. Bailey [133] states that bioherbicides are natural products that can be used to control weeds. Although bioherbicides are composed of compounds derived from nature, it should not be assumed that they do not cause any harm. Vegetation emits harmful toxins that could negatively impact the well-being of non-plant life in their surroundings, along with specific bacteria, viruses, and fungi, posing risks to both human and animal health [134]. Therefore, these toxins must be strictly controlled to prevent harm to crops or favorable wildlife [59]. Conversely, some contend that bioherbicides possess brief half-lives and are not enduring in the environment over extended durations, implying a low probability of polluting soil and water or harming unintended organisms. As an illustration, bioherbicides originating from allelochemicals exert a negligible effect on both ecosystems and human health. Certain allelochemicals can dissolve in water, thereby obviating the necessity for surfactants when applied [88,135]. Compared to chemical herbicides, bioherbicides have a more environmentally friendly composition, lower toxicity, and shorter presence in the environment.

Although bioherbicides are more environmentally friendly, there are some disadvantages. Initially, bioherbicides are known for their brief environmental half-life, a key factor in reducing environmental pollution. However, herbicides must exist for a certain period of time to achieve the desired weed control effect [136], and the short half-life of bioherbicides in the environment makes them generally effective in weed control. Second, due to the complexity of the extraction process, many allelochemicals are too costly to be widely used as herbicides. For example, cyclic tetrapeptide toxin has been shown to be a very effective bioherbicide, but its cost is too high to be widely used [137]. Finally, certain plant extracts that are toxic to target weeds also exhibit significant toxicity to mammals, such as AAL toxins [58], raising concerns about the safety of using them as herbicides for weed control.

Very little research has been performed on bioherbicide formulations and mechanisms of action. Going forward, there is a need to focus on improving the formulation of bioherbicides, as this is the main factor affecting their weed control effects. The use of multiple surfactants or nanoformulations could be considered to enhance the penetration and assimilation of bioherbicides in weeds. In addition, plant extracts that can inhibit the growth of weeds have complex and expensive extraction steps and, therefore, cannot be used in large quantities in the field. To solve this problem, the chemical industry could investigate simpler extraction methods. In the future, scientists working on bioherbicides will continue to explore and discover new varieties of plants containing a variety of toxic compounds, along with pathogens that are more effective at weed control, in order to develop more effective bioherbicides.

6. Chemical Weed Control

Modern farming practices depend extensively on chemical methods for weed management in farmlands, with herbicide-induced environmental contamination emerging as a significant worldwide issue [138]. The presence of weeds can diminish agricultural output and lead to considerable financial damage. Consequently, it is crucial for agriculturalists to focus intently on weed control on their farms. Following the discovery of 2,4-D in the 1940s, herbicides have become the leading global approach for managing weeds. Advancements in technology for controlling weeds lessened the necessity for hands-on removal and necessitated that cultivators acquire new machinery and agricultural methods. Currently, a diverse range of herbicides are available, each utilizing a distinct MOA to eliminate weeds. The development of various herbicides, each with unique action mechanisms, made them an essential tool for weed management, especially during a period when the manual weed removal job market was dwindling. Excessive dependence on this control method has resulted in the emergence of herbicide-resistant weeds, now a significant worldwide concern in agricultural practices.

The American Society of Weed Science defines herbicides as either organic or inorganic substances that hinder plant development. The use of herbicides in intensive cropping systems is essential for weed management as they provide fast and effective weed control, greatly increase crop yields, and reduce expenditure and labor [139]. A large number of herbicides have been produced for use on field herbaceous crops. Herbicides are categorized based on their chemical family, application period (preplant, pre-emergence, and postemergence), action mechanism, formulation, absorption site, and specificity [137]. Various elements, such as the crop’s genetic makeup, weed diversity, and unique pedo-climatic conditions, affect the selection of herbicides. The consistent and regular use of the same herbicide in the same crop and area resulted in the development of resistance in numerous weeds. Weed resistance (WR) refers to the ability of a weed population to survive the use of a normal herbicide dose and an increase in weed resistance to pesticides, which can be inherited [140]. Currently, globally, WR weeds appear 510 times (species × action site), encompassing 262 species (152 dicots and 110 monocots), as many as 23 of the 26 recorded herbicide action locations [141]. Regarding their action site/mechanism, herbicides fall into seven categories [142]: those reliant on light (hindering photosynthesis, pigment generation, cell membrane breakdown, and suppression), those that block fatty acid production, those that impede cell growth, those that regulate auxin-like growth, those that inhibit amino acid synthesis, those that impede respiration, and a mechanism of action that remains unidentified. Elements, including climatic conditions (like temperature, sunlight exposure, air and soil moisture), size of droplets, volume of spray, and the composition of herbicides, among others, influence the efficiency of chemical weed management [143]. In order to minimize the amount of herbicides used, experts have proposed different mathematical models to determine the amount of herbicide required to limit crop yield reduction below a predetermined limit, usually using a symmetric sigma curve [144]. The use of granular or microencapsulated herbicides in no-till or reduced tillage systems provides better weed control compared to liquid formulations, probably due to their greater mobility in different soil layers [139]. Furthermore, for managing weeds resistant to treatment, it is advised to alternate herbicides based on various action modes/sites or employ a combination of herbicides.

The main problem with herbicides is that they pollute the environment. Herbicides can contaminate the soil. A variety of elements, including the makeup of the soil, its chemical characteristics, the actions of microbes, and weather patterns like humidity, heat, and solar exposure, influence the breakdown duration of herbicides [145]. At temperatures below 20 °C [146], clay loam soils may retain atrazine and trifluralin for several years. The presence of herbicides in the soil also affects the microbial community in the soil. The composition of soil bacteria changes in response to herbicides, and the type of herbicide also affects the composition of soil bacteria [147]. The introduction of glyphosate, glufosinate, and dicamba has been linked to a rise in antibiotic-resistance genes and mobile genetic elements within soil microbial populations [148]. In addition, it was found that herbicides facilitated the transfer of these genes between bacteria, leading to increased conjugation of plasmids resistant to multiple drugs [148]. The infiltration of herbicides into the soil also poses the risk of groundwater contamination [149,150]. Herbicides are the main pesticides found in groundwater, according to Gaw et al. (2008), who tested a total of 163 wells in 14 different areas of New Zealand. Research conducted by Okada and colleagues in 2020 showed that glyphosate was present in 77 percent of urban wetlands and 79 percent of urban streams, proving that herbicides can contaminate groundwater. Protecting our waters from herbicide contamination is an important factor in sustainable agriculture. The dispersal of herbicides like glyphosate through sprays can inflict significant damage on adjacent plants, impacting a broad spectrum of flora [151]. Various elements influence spray drift, such as the direction and velocity of the wind, the steadiness of the air, humidity levels, the method of application, and the size of the droplets [152]. Additionally, herbicides pose a lethal risk to beneficial insects in crops and adjacent ecosystems. The effects of herbicide application on ladybird beetles are varied, with mortality rates as high as 80% following 2,4-D application [153]. Mixing herbicides causes larvae to shrink in size and the targeted elimination of males. Experiments have shown that earthworms exposed to 2,4D at 10 mg/kg soil had a mortality rate of 30–40 percent, while exposure to 500 mg/kg soil for several hours resulted in up to 100 percent mortality [154]. Furthermore, researchers have observed alterations in the intestinal microbiota of honeybees after they are exposed to glyphosate formulations either orally or topically.

Precision chemical weed control is the spraying of herbicides only where weeds are found, which can effectively reduce the amount of herbicides used. Thibault Maillot et al. investigated the long-term effects of precision chemical weed control on weeds in the field (crop yield loss, biodiversity, etc.) [155]. The experiment showed that spraying herbicides only in specific locations where weeds grow reduced herbicide use by 34%, and average crop yields were as suitable as if the herbicides had been sprayed uniformly throughout the field, proving the feasibility of precision chemical weed control [155].

7. Integrated Weed Management Strategies

In order to reduce the use of herbicides, many scholars have proposed integrated weed management (IWM) strategies that integrate various weed control methods. IWM employs a variety of non-chemical methods such as biological control (e.g., insects or plant pathogens), controlling the seed bank, grazing, crop rotations, the use of mulch plants, and adjusting the timing of planting. The use of agro-technical methods for weed control does not require significant costs but significantly improves field ecology [156]. The following will focus on agro-technical methods not mentioned above: soil solarization, cover cropping, crop rotation, and improving the competitiveness of crops.

Solarizing soil is frequently regarded as an immediate and efficient method for weed management. In order to take advantage of the solar radiation and keep the soil warm, it is necessary to apply transparent polyethylene film to the plowed. The process of solarization allows the soil’s surface layers to reach temperatures exceeding 40 degrees Celsius. Additionally, the presence of 50–55 °C at 5 cm [157] can be fatal to a wide range of soil-borne pests, mainly fungi and nematodes, as well as weed seeds, as it hinders their ability to germinate. Although typically limited in greenhouse environments [158], soil solarization is universally recognized as a highly efficient technique for managing parasitic plants, particularly those of the Orobanche and Phelipanche genera, prevalent in field crops [159,160]. Solarization poses the greatest threat to annual weeds, whereas perennials that reproduce via rhizomes, tubers, and other means are not susceptible. Typically, they exhibit tolerance, possibly due to the soil’s limited heat penetration and their ability to swiftly regenerate from partially impaired underground organs [161]. Temperatures in Mediterranean, tropical, and subtropical climatic regions can rise substantially to 40 °C in summer [160], and soil solarization is well suited in these regions.

Employing cover cropping has proven effective in curtailing weed proliferation. Within conventional or organic agricultural practices, cover cropping is commonly used, and there is a frequent inverse relationship between the existence of cover crops and weed biomass. Berti and team [162] found that integrating cover cropping with zero tillage leads to better weed management than single methods due to the combined effect of plant remnants and allelochemicals in the soil, which together impede weed seed germination. Employing cover crops presents substantial benefits in boosting soil fertility, diminishing soil erosion, and curbing weed proliferation. The technique hinders weed management in both a physical and chemical sense: the first hinders weed competition for space, water, light, and nutrients, while the second emits phytotoxic substances that obstruct weed sprouting and initial growth. The success of cover crops in weed elimination is intricately linked to various elements, such as the type and management of cover crops (for instance, the date of planting), the weed community’s makeup, the environmental and soil circumstances, the quantity of plant remnants, and the decomposition speed [163,164].

Crop rotation is one of the oldest and most effective agricultural technological tools for weed control. It is a sustainable and environmentally friendly weed control strategy [156]. In the tillage system, crop types involved in crop rotation have an effect on weed density and weed species in the field [156]. The use of the crop rotation system can effectively reduce weed density [165]. In contrast to crop rotation, continuous cropping does not lead to changes in field ecology. It can increase the field densities of weeds that are suited to the current environment, accelerating the rate at which weeds develop resistance and thus affecting crop yields [165]. Crop rotation systems can bring about changes in herbicide types, sowing, and harvest dates, reducing weed densities by up to 65% [166].

Improving crop competitiveness is an essential aspect of integrated weed management and includes careful selection of competitive crop varieties, precise row spacing, precise row orientation, and the practice of sowing crops at higher densities [167]. For example, Qian-Nan Sheng and other scholars found that it is an ecologically sustainable weed control measure to regulate the community structure between oilseed rape and weeds by accelerating the growth of crops through the rational application of fertilizers and giving full play to the competitive advantage of oilseed rape to inhibit the occurrence of grasses [168]. Grazing management on certain rangelands allows pastures to remain competitive with weeds. The effectiveness depends on the duration, intensity, and density of grazing, as well as the type and number of grazing animals [169]. Improving crop competitiveness by selecting more competitive crop varieties [170] and optimizing environmental conditions [171] can be useful in controlling weeds.

The need to adopt IWM will further increase as the number of weeds resistant to herbicides will continue to increase due to the absence of new herbicide modes of action in the market.

8. Discussions

Physical weed control mainly refers to thermal technologies represented by flame weed control and laser weed control, which can efficiently and accurately remove weeds. The disadvantage of flame weed control is that the combustion process produces large amounts of greenhouse gases, which contributes to the greenhouse effect and is not environmentally friendly in the long run. The advantage of laser weeding is that the laser can be powered by electricity, which can be obtained through wind, solar, and other environmentally friendly means. Laser weeding is able to accurately remove weeds without accidentally injuring crops. The disadvantage is that laser weeding is only effective on small weeds and cannot eradicate large weeds. The heat generated by lasers can ignite dry materials in the field and cause fires. In addition, laser radiation may have adverse effects on humans and animals. Mechanical weed control robots also have some problems. First, current weeding robots are not light enough, and prolonged use can lead to soil compaction in the field; continued efforts in the direction of lightweight design are needed in the future. Second, uneven terrain in the field can affect the accuracy of mechanical weeding, leading to crop damage. Finally, sensor-based recognition systems have poor judgment accuracy in complex and changing natural environments. In order to improve the accuracy of sensor-based recognition in complex environments, future experiments need to be conducted continuously to obtain the best sensor-based system. The advantages of bioherbicides mainly lie in their ecological friendliness. The disadvantage is that bioherbicides have a short half-life and too little time to act on weeds, making them ineffective in weed control. Many bioherbicides have complex and costly extraction processes, making it difficult to popularize them in the field. In addition, some bioherbicides are toxic to mammals at the same time as they are toxic to target weeds, raising concerns about the safety of using them as herbicides for weed control. Future research in bioherbicides should focus on improving the formulation of the active ingredient of bioherbicides, and the use of multiple surfactants or nanoformulations could be considered to improve the ability of bioherbicides to penetrate weeds. The chemical industry needs to investigate easier methods of extracting plant extracts. In order to solve the problem of complex and costly steps in extracting raw materials for bioherbicides. In addition, scientists need to continue searching for new varieties of plants containing various toxic compounds and pathogens that are more effective at weed control in order to develop new bioherbicides. The advantages of chemical herbicides are that they are fast, effective, and cheap. The main disadvantage is that they pollute the environment and have harmful effects on soil, microorganisms, and groundwater. To reduce the use of chemical herbicides, future research should focus on precision chemical weed control, as well as integrated weed management strategies that incorporate multiple weed control methods.

In recent years, artificial intelligence, intelligent perception, and computer vision technology have been constantly integrated into agricultural production. Intelligent weeding robots have come into being, promoting the transformation and upgrading of weeding technology toward automation and intelligence. Intelligent weeding robots can effectively reduce labor costs, improve production efficiency, and reduce environmental pollution [172]. With the development of smart agriculture, unmanned farms are beginning to be practiced, and intelligent weeding robots will be essential equipment for integrated weed management. Lian Hu and other scholars [172] believe that the future of intelligent weeding technology will be researched around the following four aspects: (1) Intelligent sensing. Multi-sensor information fusion technology allows weed control robots to understand the farmland environment in real time and comprehensively in order to complete operations in a variety of complex farmland environments. Machine vision, global navigation satellite systems, radar, and other sensing technology fusion algorithms, as well as multidisciplinary cross-fertilization technology, provide a wealth of sensory information for weeding robots. This information is used to identify crop rows and weeds while simultaneously locating the crop plants. (2) Precision weeding. A high-precision weeding device is an important part of the intelligent weeding robot, directly affecting the weeding effect. The weeding device combines robotic technology and automatic control technology to achieve high-precision weeding. Precision weeding devices in the inter-plant and near-inter-plant areas will be important directions for future breakthroughs in farmland weeding technology. (3) High-efficiency operation. The application of artificial intelligence and other technologies will improve the quality and efficiency of weeding operations, making them develop toward wide operation, group intelligent operation, and multi-functional operation. (4) Intelligent decision making. With the integration of the Internet of Things, big data, and cloud computing, among other technologies, the robot weeding operation will be remotely controlled by the cloud platform. This can not only realize the functions of remote monitoring of robot operation and operation task scheduling but also make intelligent decisions.

9. Conclusions

There is a wide variety of herbaceous field crops, only 15 to 20 of which have a significant impact on the world economy. Weeds may cause a 34% reduction in crop yields. There are four methods of controlling weeds: physical, mechanical, biological, and chemical. Physical weed control mainly refers to thermal technologies represented by flame weed control and laser weed control, which can efficiently and accurately remove weeds. Flame weed control causes damage to plant tissues and rupture of cell membranes through heat, resulting in the death of the weed or loss of its ability to compete with the crop. The effectiveness of flame weed control is related to weed species, seedling size, and method of ignition. Flame weeding produces large amounts of carbon dioxide and is therefore considered environmentally unfriendly. Laser technology provides efficient and accurate weed control powered by renewable energy. Machine vision and spectroscopic techniques identify the location of the weeds, and the laser emits a beam of light to destroy the weeds. The effectiveness of laser weed control is affected by a number of factors. There are many advantages to laser weed control, but one needs to be aware of the risk of fire and the potential hazards of lasers to humans or animals. Conventional mechanical weed control may cause crop damage and lead to secondary infections and reduced yields. To improve the accuracy of weeding, a combination of sensor technology, machine vision technology, and high-precision navigation technology can be used. Mechanical weeding robots have problems with lightweight design, precise work on sloping terrain, and sensor accuracy in complex agricultural environments. Biological weed control is the use of organisms or bioproducts to control weeds. Several plant extracts have been used to produce bioherbicides that inhibit weed growth and are not harmful to crops. For example, sorghum extracts are effective in controlling weed growth, resulting in increased rice yields. Substances in the black brassica plant can hinder weed germination and growth. Plant essential oils and pathogens are also important components of bioherbicides that suppress a wide range of weeds. Bioherbicides can be harmful to humans, animals, and the environment but are also considered by some to be more environmentally friendly. Bioherbicides also have some disadvantages, such as average weed control and high cost, and some bioherbicides are toxic to mammals. Future research directions include improving formulations, using surfactants or nanoformulations to enhance penetration, and improving methods for extracting plant extracts. Reliance on chemical herbicides for weed control creates problems of weed resistance and pollution of the environment. Herbicides pollute the environment, affect soil and groundwater quality, alter microbial communities, and harm vegetation and beneficial insects. To reduce the use of herbicides, many scholars have proposed integrated weed management strategies and precision chemical weed control methods. Integrated weed management strategies incorporate a variety of methods, including biological control, controlling the seed bank, and improving crop competitiveness, which are increasing in importance every day.