The amount of suitable land for growing agricultural food crops is limited. As the growth of the global population continues to increase, the demand for feeding the world's population has increased in difficulty. The problem arises from a number of factors, for example, weeds, pests and a lack of land (O'Gara, 2010). Weeds restrict the growth of most agricultural crops (O'Gara, 2010). It reduces the quantity and quality of food crops (O'Gara, 2010). Invasive weeds compete with agricultural crops for space, light, moisture and nutrients (O'Gara, 2010). Herbicides were introduced to reduce the yield loss and to control the growth of weeds (Bhadoria, 2010). The use of herbicides in large doses, have negative impacts on the environment and human health (Bhadoria, 2010). Synthetic herbicide creates pollution and increases the number of herbicide resistant weeds (Bhadoria, 2010). Scientists have researched alternatives to lessen the use of harmful herbicides, the alternatives that are considered in replacing the use of herbicides are: allelopathic plants, biological control for plants, plant breeding and genetic modification for plants.
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2)Allelopathy is the effect of one plant on other surrounding plants, which occur due to the release of chemical compounds (allelochemicals) (Bhadoria, 2010). Allelopathic plants may affect other plants positively or negatively by promoting or inhibiting growth of plants respectively (Bhadoria, 2010). The inhibition and promotion of growth in plants is due to the different concentrations of allelochemicals that is released into the soil (Putnam, 1988). This property of plants has been proposed as a major solution in the reduction of the use of synthetic herbicides for weed control in an agricultural environment (Xuan et al, 2005). An allelopathic plant undergoes a mechanism of plant interference that is mediated by the release of secondary metabolites into the soil through various plant tissues and through the decomposition of parts of the allelopathic plant (Bhadoria, 2010). From studies of Duke & Weston (2003), allelopathic interference was found to be useful to a variety of weed and crop species in an agricultural setting of environment (Duke & Weston, 2003). With the information from the studies of allelopathy and the allelochemical mechanism, the weed management strategies will be enchanced and the use of synthetic herbicides could be replaced with the use of the non-synthetic chemicals from allelopathy (Duke & Weston, 2003). The agricultural crops that have been investigated to have allelopathic potential and weed-suppressive traits are: Rice, Wheat, Rye, Cucumber, Barley, etc (Duke & Weston, 2003).
To find ways to control the aggressive growth of invasive weeds, scientists have looked into the study of genetic modification in crops to reduce the use of harmful herbicides. There is a method known as Integrated Weed Management, which include a combination of cultural, mechanical, and chemical tools to control excessive weed growth (Cassman & Knezenc, 2003). Herbicide-tolerant crops have been developed from genetic modification as a weed control technology (Cassman & Knezenc, 2003). Herbicide-tolerant crops are produced from genetic modification by the insertion of a transgene from an organism into an agricultural crop (Cassman & Knezenc, 2003). Currently available herbicide-tolerant crops include corn, soybean, canola and cotton (Cassman & Knezenc, 2003). This transgenic alternative still requires the use of herbicides, however it does not require the use in large amounts, compared to the use of herbicides alone (Cassman & Knezenc, 2003). This method requires not only the use of herbicides and gene modification but also mechanical management - physically pulling out weeds (Cassman & Knezenc, 2003). Glyphosate and glyfosinate, a non-selective herbicide is the chemical that the genetically modified plants are tolerant to (Cassman & Knezenc, 2003). The use of glyphosate and glyfosinate have little soil residual activity and therefore will have less replanting restrictions compared to the use of normal herbicides, more crops can be planted in a lesser period of time (Cassman & Knezenc, 2003). The 2 chemical ingredients in the herbicides have lower toxicity to other organisms compared to many herbicides and the chemicals used decompose rapidly in soil, hence pose less danger in leaching and contaminating water (Cassman & Knezenc, 2003). A disadvantage to this alternative is a single selection pressure, which may cause weed to become herbicide resistant (Cassman & Knezenc, 2003). It is more likely for invasive weeds to mutate when the same herbicide is used over and over, there are already several weed species that are resistant to glyphosate, this includes the rigid ryegrass in Australia, ryegrass in California and horseweed in Delaware and Tennessee (Cassman & Knezenc, 2003). The resistance to the glyphosate is due to the repeated use of herbicide in the absence of other weed management, cultural and mechanical tools. (Cassman & Knezenc, 2003). There are other ways in which genetic engineering is used to manage excessive growth of weeds in agricultural environments, another involving engineering plants that will be able to compete with weeds better for limiting resources, which will resort to less use of herbicides and fertilizers (Lopez-Arrendondo & Herrera-Estrella, 2012).
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Biological control is the use of living organisms to manage a problem (McFadyen, 1998). Biological control is widely used in the control of weed invasion alongside allelopathy and genetic modification in plants (Templeton, 1988). There are three common approaches to biological control, they are: classical biological control - using exotic natural pathogens -, integrated pest management - generally used for insect and disease control- and biological pesticides (Templeton, 1988). Weeds in ecosystems are targets for biological control due to its rapid growth (McFadyen, 1998). One way that biological control is used, is through biological herbicides (Templeton, 1988). Biological herbicides are termed mycoherbicides, as they contain fungal plant pathogens that have specific weed hosts (Templeton, 1988). The fungal plant pathogens can be used to control the growth of weeds, as a classical biological control agent (Smith, TeBeest & Templeton, 1979). The use of mycoherbicides is a known sustainable way to control the excessive growth of herbicides (Templeton, 1988). The use of bio-control for weeds is more sustainable and less harmful than the use of synthetic herbicides however, bio-control for weeds is a specific and selective method (Ghosheh, 2005). The method requires a host-specific agent and therefore target crops need to be properly identified (Ghosheh, 2005). The use of biological weed control in the form of a bioherbicide aims to regulate the weed populations rather than completely eradicate it (Ghosheh, 2005).
There are a number of ways to control weed growth, the use of synthetic herbicides is only one. The application of synthetic herbicides on agricultural land has negative impacts on the environment and on the health of living organisms, leaching of herbicide chemicals to water poses some danger as such chemicals have some level of toxicity. There are substitutes to synthetic herbicide, which does not completely eradicate its use however it lessens it. The alternatives have some disadvantages, like creating a 'super-weed' that is resistant to solutions to weed management and others solutions may be less efficient. The most ideal way of weed management is integrated management - combinations of cultural, mechanical, biological and chemical tools.