Maize Genomics and Genetics 2024, Vol.15, No.2, 60-69 http://cropscipublisher.com/index.php/mgg 63 designed to tolerate glyphosate, a broad-spectrum herbicide. This innovation allowed farmers to control a wide range of weeds with a single herbicide application, leading to more efficient and cost-effective farming practices (Perry et al., 2016). 3.2 Major commercial herbicide-tolerant maize varieties Several major commercial herbicide-tolerant maize varieties have been developed and widely adopted. One notable example is the glyphosate-tolerant maize, which has been extensively used in the United States. This variety has shown a reduction in herbicide use by 1.2% compared to non-adopters, highlighting its efficiency in weed management (Perry et al., 2016). Another significant development is the creation of maize varieties tolerant to both glyphosate and glufosinate. The SCB-29 maize event, for instance, expresses both CP4-EPSPS and bar genes, providing robust tolerance to these herbicides and demonstrating stable integration and expression of the transgenes over multiple generations (Yu et al., 2023). 3.3 Case studies of successful genetic engineering for herbicide tolerance Several case studies illustrate the success of genetic engineering in developing herbicide-tolerant maize. One such example is the development of maize tolerant to dicamba, a herbicide used for broadleaf weed control. Transgenic maize plants expressing the dicamba monooxygenase (DMO) gene linked with a chloroplast transit peptide (CTP) have shown enhanced tolerance to dicamba, significantly exceeding the recommended application rates without crop injury (Cao et al., 2011). Another case study involves the engineering of maize with tolerance to aryloxyphenoxypropionate (FOP) and synthetic auxin herbicides like 2,4-D. This was achieved by developing an enzyme with robust and specific activity for these herbicide families, providing farmers with additional tools for effective weed management (Fu et al., 2021) (Figure 2). These advancements underscore the potential of genetic engineering to enhance herbicide tolerance in maize, offering significant benefits in terms of weed control, crop yield, and environmental sustainability. However, the continuous evolution of herbicide-resistant weeds remains a challenge, necessitating ongoing research and development to sustain the effectiveness of these genetically engineered crops (Bonny, 2016). 4 Agronomic and Environmental Impacts 4.1 Agronomic benefits of herbicide-tolerant maize Herbicide-tolerant maize has significantly improved weed management and crop yield. The adoption of genetically modified (GM) crops, including herbicide-tolerant maize, has led to a reduction in chemical pesticide use by 37% and an increase in crop yields by 22% on average (Klümper and Qaim, 2014). Specifically, transgenic maize plants expressing genes for herbicide tolerance, such as those tolerant to dicamba, have shown enhanced weed control and increased crop yield potential (Cao et al., 2011). Additionally, the development of maize tolerant to both glyphosate and glufosinate has provided a robust weed management system, delaying the development of herbicide resistance in weeds and maintaining high crop yields (Yu et al., 2023). 4.2 Environmental concerns and considerations While herbicide-tolerant maize offers agronomic benefits, there are concerns regarding its impact on biodiversity. The widespread adoption of GM crops has led to homogenization of agricultural practices, which can affect non-target biota and soil food web properties (Rizzo et al., 2022). Studies have shown that the herbicide systems associated with GM crops can alter the fungal-to-bacterial biomass ratios in soil, potentially impacting soil health and biodiversity (Rizzo et al., 2022). Additionally, the potential for indirect harmful effects on farmland biodiversity through losses in food resources and shelter has been highlighted, although these effects have not been consistently demonstrated (Devos et al., 2008). The introduction of herbicide-tolerant maize has also contributed to reduced tillage practices, which are beneficial for soil conservation. The Farm Scale Evaluations in the UK demonstrated that GM herbicide-tolerant crops generally required fewer herbicide applications and less active ingredient compared to conventional crops, which can reduce soil disturbance and erosion (Champion et al., 2003). Moreover, the use of broad-spectrum herbicides
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