Maize Genomics and Genetics 2024, Vol.15, No.2, 49-59 http://cropscipublisher.com/index.php/mgg 51 Another success story is the development of Quality Protein Maize (QPM), which contains higher levels of essential amino acids lysine and tryptophan. This improvement addresses the nutritional deficiencies in populations relying on maize as a staple food. QPM varieties were developed using conventional breeding techniques and have been widely adopted in developing countries to combat malnutrition (Tandzi et al., 2017). Conventional breeding has also played a crucial role in developing maize varieties resistant to diseases and pests. For instance, resistance to maize streak virus, a major disease in sub-Saharan Africa, has been successfully incorporated into maize varieties through traditional breeding methods (Figure 1) (Masuka et al., 2017) . Figure 1 Maize streak virus (MSV) score plotted against year of entry into regional trials of (a) early open-pollinated varieties (OPVs) and (b) intermediate-late OPVs from regional trials between 1999 and 2011 (Adopted from Masuka et al., 2017) Image caption: These two graphs illustrate the relationship between MSV scores and the year of entry into regional trials. Graph a shows a slight increase in MSV scores from 1998 to 2012, but the change is not significant (y = 2.3139 + 0.0074x, ns), with data points widely dispersed. Graph b shows a significant decrease in MSV scores (y = 3.3998 - 0.0552x, p<0.001), indicating that MSV scores significantly decrease over the years, with data points more concentrated along the downward trend. This suggests that over time, MSV scores have a statistically significant decreasing trend (Adopted from Masuka et al., 2017) 2.4 Limitations and challenges Despite its successes, conventional breeding faces several limitations and challenges. One significant challenge is the time required to develop new varieties. The process of creating inbred lines and evaluating hybrids can take several years, delaying the introduction of improved varieties. Additionally, conventional breeding relies heavily on the natural genetic variation present in maize populations, which may limit the range of achievable improvements (Morris et al., 2003).
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