**2. Negative impacts of the synthetic herbicides on human health and ecosystems**

Although synthetic herbicides are considered the best and effective methods to be used as weed control, the risk is very high if it is used indiscriminately. The types, quantity and frequency of applications of the synthetic herbicides can bring about various harmful effects to the environment and its ecosystems and in fact a threat to human health. The uses of synthetic pesticides including herbicides have directly and indirectly brought about several adverse effects to the soil surface, groundwater and organisms as well as in the atmosphere. The above changes have negative ramifications for the community and disrupting the ecological balance, hence risking human life.

**191**

growing locations.

*Roles of Terpenoids in Essential Oils and Its Potential as Natural Weed Killers: Recent…*

There is no segment of the population that is completely protectedc exposure and high risk groups are not only of the people of the developing countries but all the countries over the world. The hazards of synthetic pesticides are summarized by their impact through food commodities, surface water, groundwater and soil contamination and the effects on soil fertility (beneficial soil microorganisms) and non-target vegetation [9]. According to the report of Williams et al. [10] also highlighted the complaint of contacts diseases, acute ulcer, heart pain, skin rashes respiratory condition and eye problem of the people in the survey area. In another study conducted by Niemann et al. [11] on glyphosate; a common non-selective systemic herbicide promoted by the manufacturers as a safer herbicide, reported tracing of its residues in both humans and animal urine. It was then suggested that the use of glyphosate may have to be re-evaluated to reduce human exposure to the

Using synthetic herbicides even at recommended rates can lead to negative impacts on the ecosystems, especially the harmful effects that come from their residues. On the other hand, the efficiency of these compounds will be slowly decreased due to the increase in the resistance of the weed plants as a result of the continuous use of these compounds to control the same weed species. Hence, using these synthetic herbicides continuously becomes a double-edged sword. As it is well known, the insecticides are the most toxic to the environment, followed by fungicides and herbicides. But there are some herbicides that can be highly toxic and much more serious than the insecticides. Such problems that emanated from the utilization of synthetic herbicides cannot be overlooked regardless of the benefits accruing of its application [9, 12]. According to Williams et al. [10],the herbicide applied in agricultural lands can be leached and washed by rain or precipitation

which leading to extending their risks to the wider areas which applied.

**3. Development of weed resistance to synthetic herbicides**

Weed resistance to synthetic herbicides is considered to be the most difficult problem facing weed chemical control. The resistance comes when some of the weed species to be resistant to the herbicides naturally. Weed resistance can come in

The herbicide effects on the soil and the environment were detected and found to contaminate the groundwater and the source of fresh drinking water even at recommended levels, causing an acute decrease in the biomass such as microorganisms, plant tissues and soil organic matter [13]. On the other hand, most of the synthetic herbicides not only affect the target plants, but it can also cause low growth rates, reproductive problems, changes in appearance or behavior or even death of non-target plant species, making the use of herbicides more harmful [14]. Furthermore, a wide range of non-targeted plant species are more sensitive to herbicides, especially during the reproductive stages of their life cycle. This is further compounded by the effect of external factors such as wind and rainfall, which can adversely affect this most important stage of the plant's life cycle. In this regard, Boutin et al. [15] mentioned the delays in flowering and reduction in seed production observed in a large number of non-target plant species, including climbing and hedgerow plants, through the spraying of herbicides within the vicinity of their

*DOI: http://dx.doi.org/10.5772/intechopen.91322*

dangers of synthetic herbicides.

**2.2 Ecosystems**

**2.1 Human health**

*Roles of Terpenoids in Essential Oils and Its Potential as Natural Weed Killers: Recent… DOI: http://dx.doi.org/10.5772/intechopen.91322*

## **2.1 Human health**

*Essential Oils - Bioactive Compounds, New Perspectives and Applications*

control measures. On the contrary, weed management is a systematic approach where the whole land use planning is carried out in advance to minimize the very invasion of weeds in aggressive forms and give crop plants a strong competitive advantage over the weeds. Weed control methods can be grouped into the culture, physical, biological and chemical [1]. Human efforts of controlling weeds began with the use of cultural practices such as tillage, planting, crop rotation, fertilizer application, irrigation, etc., that are adapted to create favorable conditions for the crop. If properly used, the practices can help in suppressing weeds. On the other hand, culture methods alone, cannot control weeds; it can only help to reduce the weed population. Culture methods therefore, can be effectively used in combination with other methods. Every method of weed control has its own advantages and disadvantages. No single method is successful under all weed situations. Most often, a combination of these methods gives effective and economic control than a single method. These methods of controlling weeds were later developed in the form of mechanical weed control such as hand pulling, hand hoeing, and planting in rows to facilitate machinery use, but again these methods did not attain the

Later, a new mechanism of weed control was developed through the use of chemical inputs. Chemical weed control began on a small scale. Since the 19th century, a combination of salt and ash powder was used to control weed plants which grow on either side of the railway. The use of synthetic herbicides, however, begun in the 1940s with the development of some organic herbicides, specifically the 2,4-D. This herbicide is considered as a growth regulator used in high doses to control broadleaf weeds [3]. Then, chemical weed control was widely used as a form of weed control and achieved a dominant role in the crop management, more efficient, economical and low costs as compared to other methods and contributed strongly to the increase in the agricultural yields and reduce losses due to weeds [4, 5]. As a result of using chemical weed control, the traditional method of weed control such as cultivation and hand weeding has been greatly been decreased [6]. A new method to control weeds was created by producing different types of synthetic herbicides according to the mode of action of these compounds against weed plants. By 1990s, the number of compounds that have been used in herbicides in many different formulas reached to more than 180 compounds [7]. According to a report of [8], the total value of the global's agrochemical market was between 31 and 35 billion US\$ and of the products, herbicides accounted for 48% followed by fungicides at 22%. Nowadays, chemical weed control becomes as an integral part of the complex world of technical inputs required for modern agricultural production and have been accepted as a standard tool of the trade by farmers throughout the world despite the negative effects of synthetic herbicides on the ecosystem [1].

**2. Negative impacts of the synthetic herbicides on human health** 

Although synthetic herbicides are considered the best and effective methods to be used as weed control, the risk is very high if it is used indiscriminately. The types, quantity and frequency of applications of the synthetic herbicides can bring about various harmful effects to the environment and its ecosystems and in fact a threat to human health. The uses of synthetic pesticides including herbicides have directly and indirectly brought about several adverse effects to the soil surface, groundwater and organisms as well as in the atmosphere. The above changes have negative ramifications for the community and disrupting the ecological balance,

**190**

desired benefits [2].

**and ecosystems**

hence risking human life.

There is no segment of the population that is completely protectedc exposure and high risk groups are not only of the people of the developing countries but all the countries over the world. The hazards of synthetic pesticides are summarized by their impact through food commodities, surface water, groundwater and soil contamination and the effects on soil fertility (beneficial soil microorganisms) and non-target vegetation [9]. According to the report of Williams et al. [10] also highlighted the complaint of contacts diseases, acute ulcer, heart pain, skin rashes respiratory condition and eye problem of the people in the survey area. In another study conducted by Niemann et al. [11] on glyphosate; a common non-selective systemic herbicide promoted by the manufacturers as a safer herbicide, reported tracing of its residues in both humans and animal urine. It was then suggested that the use of glyphosate may have to be re-evaluated to reduce human exposure to the dangers of synthetic herbicides.

#### **2.2 Ecosystems**

Using synthetic herbicides even at recommended rates can lead to negative impacts on the ecosystems, especially the harmful effects that come from their residues. On the other hand, the efficiency of these compounds will be slowly decreased due to the increase in the resistance of the weed plants as a result of the continuous use of these compounds to control the same weed species. Hence, using these synthetic herbicides continuously becomes a double-edged sword. As it is well known, the insecticides are the most toxic to the environment, followed by fungicides and herbicides. But there are some herbicides that can be highly toxic and much more serious than the insecticides. Such problems that emanated from the utilization of synthetic herbicides cannot be overlooked regardless of the benefits accruing of its application [9, 12]. According to Williams et al. [10],the herbicide applied in agricultural lands can be leached and washed by rain or precipitation which leading to extending their risks to the wider areas which applied.

The herbicide effects on the soil and the environment were detected and found to contaminate the groundwater and the source of fresh drinking water even at recommended levels, causing an acute decrease in the biomass such as microorganisms, plant tissues and soil organic matter [13]. On the other hand, most of the synthetic herbicides not only affect the target plants, but it can also cause low growth rates, reproductive problems, changes in appearance or behavior or even death of non-target plant species, making the use of herbicides more harmful [14]. Furthermore, a wide range of non-targeted plant species are more sensitive to herbicides, especially during the reproductive stages of their life cycle. This is further compounded by the effect of external factors such as wind and rainfall, which can adversely affect this most important stage of the plant's life cycle. In this regard, Boutin et al. [15] mentioned the delays in flowering and reduction in seed production observed in a large number of non-target plant species, including climbing and hedgerow plants, through the spraying of herbicides within the vicinity of their growing locations.

## **3. Development of weed resistance to synthetic herbicides**

Weed resistance to synthetic herbicides is considered to be the most difficult problem facing weed chemical control. The resistance comes when some of the weed species to be resistant to the herbicides naturally. Weed resistance can come in different forms. One of the mechanisms is through the production of a new generation of weeds of different morphological and physiological characteristics with shorter life cycles that can survive from the application of the systemic herbicides. Thus, the number of these weeds surviving from the continuous application of herbicides which will gradually increase. While the other weed species in the same area will be controlled by applying herbicides [16]. Another way in the development of weed resistance comes from the application of contact herbicides, which are responsible for inhibiting the process of photosynthesis, such as attrition. The application of such herbicides, especially during unfavorable weather conditions can contribute to the absorption of a small amount of the active ingredient but ineffective to the targeted weeds. Over time, the continuous accumulation of contact herbicides developed a new generation of weed that is immune to the synthetic herbicides [17]. In September 2010, Dow AgroSciences' scientists stated that weeds which have become resistant to the glyphosate herbicide and the ensuing production of genetically modified plants are one of the solutions to resolve issues pertaining to weed competition [7]. Nowadays, there are more than 604 species of weed plants considered as resistance to synthetic herbicides, most of these weed species are resistant to herbicides which are responsible for inhibiting acetolactate synthase (ALS enzyme), photosystem II and acetyl-CoA carboxylase [18, 19]. **Figure 1**, showed the number of resistant plant species for several herbicides according to their modes of action [18].

In this regard, Sekutowski [20] and Weber and Golebiowska [21] reported that there are 10 species of weeds that pose the biggest threat to crops by causing yield losses. The weeds include the most important herbicide-resistant species which are characterized by multiple resistances such as rigid rye-grass (*Lolium rigidum* Gaud.), wild oat (*Avena fatua* L.) and redroot pigweed (*Amaranthus retroflexus*). As a result of weed resistance, many types of synthetic herbicide are re-evaluated as useless. Hence, a lot of conventional herbicides have been identified as resistance and ineffective against common weeds. For example, there are 116 useless conventional herbicides identified in Europe, where the highest number of herbicides is found in France (30 types) followed by Spain (26 types), United Kingdom (24 types), Belgium (18 types) and Germany (18 types) [22].

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pests and crop diseases [32, 33].

*Roles of Terpenoids in Essential Oils and Its Potential as Natural Weed Killers: Recent…*

Therefore, due to the similarity of the active ingredient in most of the herbicides that belong to the same group, there is no major new site-of-action herbicide has been introduced into the marketplace in the last two decades [23]. Conditions for the registration of synthetic herbicides also become strict and complicated. This is considered the greatest loss in the herbicides production sector. For example, the number of new herbicides active substances in Europe declined to 336 in 2009 from 945 in 1999 [23]. In fact, chemical herbicides development efforts worldwide whose market used to be monopolized by glyphosate-based herbicide have diminished since 1996 as a result of the glyphosate-resistant which is considered the most widely used pesticide in the world [24]. Therefore, the trend today is toward the use of natural alternative compounds, but still possess their herbicidal potential and safety versus the currently used synthetic and non-ecofriendly materials, wherein the plant defense tactic characterized by plant secondary metabolites, come to the forefront as promising solutions to replace the

The word allelopathy comes from the Greek words "Allelon" meaning "each other" and "Pathos" refers "to suffering". This allelopathic phenomenon whereby a plant response in defense of the neighboring plants, insects, microorganism and animals results in the production of natural chemicals called allelochemicals or phytochemicals [25, 26]. Allelopathy phenomenon is defined by Kato-Noguchi [27] as an important defense mechanism of the plant which results in the manufacturing of secondary metabolites. This term is also defined by the International Allelopathy Society in 1996th as a science that study any process involving secondary metabolites produced by plants, micro-organisms, viruses, and fungi that influence growth and development of agricultural and biological systems, excluding animals [28]. Usually, the allelochemical compounds released from donor plants caused negative effects on organisms found within the surrounding environment. However, some of the recent researchers reported the effects of the allelochemical produced by plants belonging to the plant family (Fabaceae) can be positive to the surrounding environment. As for an instance, the allelochemicals residues produced by these plant types such as legumes help to fix nitrogen from the air thus enriching the soil [29]. Most of the plants including weeds release allelochemical compounds as defense mechanisms. These compounds can be released from donor plant parts into the environment by leaching, volatilization, exudate from living plant tissue or by the decomposition of plant residues as shown in **Figure 2**. Hence, it was responsible for inhibiting the germination and growth of neighboring organisms [30]. One of the major advantages of allelopathy involves the release of plant biochemical compounds into the environment that inhibits germination or suppresses the growth of the surrounding vegetation. Another form of allelopathic potential can be tapped from trees that produce biochemical compounds for its survival and hence its reproduction. An example of such plants is eucalyptus; these types of plants can be used in agricultural production as cover crops to control weeds [31]. Lately, the allelopathic phenomenon has gained prominent attention and has been successfully employed in field crop production toward the improvement of crop productivity and for the protection of the environment through eco-friendly control of weeds,

The motivation for the use of the allelochemical compound in weed control is attributed to its phytotoxic effects similar to the phytotoxic effects of the synthetic herbicides in inhibiting seed germination and reducing seedling growth. The

*DOI: http://dx.doi.org/10.5772/intechopen.91322*

conventional herbicides.

**4. Allelopathic compounds**

*Number of resistant plant species for several herbicides according to their modes of action [18].*

*Roles of Terpenoids in Essential Oils and Its Potential as Natural Weed Killers: Recent… DOI: http://dx.doi.org/10.5772/intechopen.91322*

Therefore, due to the similarity of the active ingredient in most of the herbicides that belong to the same group, there is no major new site-of-action herbicide has been introduced into the marketplace in the last two decades [23]. Conditions for the registration of synthetic herbicides also become strict and complicated. This is considered the greatest loss in the herbicides production sector. For example, the number of new herbicides active substances in Europe declined to 336 in 2009 from 945 in 1999 [23]. In fact, chemical herbicides development efforts worldwide whose market used to be monopolized by glyphosate-based herbicide have diminished since 1996 as a result of the glyphosate-resistant which is considered the most widely used pesticide in the world [24]. Therefore, the trend today is toward the use of natural alternative compounds, but still possess their herbicidal potential and safety versus the currently used synthetic and non-ecofriendly materials, wherein the plant defense tactic characterized by plant secondary metabolites, come to the forefront as promising solutions to replace the conventional herbicides.
