Journal of Energy Bioscience 2025, Vol.16, No.3, 151-162 http://bioscipublisher.com/index.php/jeb 158 maize and wheat, if combined with technologies such as straw processing, precision fertilization and water-saving irrigation, there is room for improvement in yield, efficiency and environmental performance. 8 Case Studies 8.1 Breeding methods for energy maize in the USA In the United States, researchers have conducted many field trials and found a strong connection between maize grain yield and stover dry matter (SDM), with a correlation coefficient of 0.67. They also focused on improving the saccharification efficiency (how well the stover breaks down into sugars). There was a big difference in saccharification rates among over 100 maize varieties. Some had only 20%, while others reached 33%. This trait also had a relatively high genetic heritability (H² could be up to 0.71). Different maize varieties had different ratios of cellulose and lignin in their stover. Materials with high cellulose and low lignin release sugar more easily and have better saccharification. Some parent lines (NAM parental lines) and certain inbred lines performed well in both saccharification efficiency and cellulose content, making them easy to categorize. After harvesting the grain, the remaining stover is fermented to produce sugar for biofuel production. This approach makes agricultural production more economically valuable and environmentally friendly. Especially in the Midwest of the United States, where maize and soybean crop rotation systems are common, maize stover is particularly well-suited for second-generation biofuels. 8.2 EU bioenergy maize projects In some European countries, such as Spain, the Mediterranean, and along the Atlantic coast, local maize landraces are gaining attention. These varieties evolved in less-than-ideal environments, producing high stover yields and accumulating more cellulose. These landraces have now become important materials for biofuel breeding. Researchers evaluated more than 100 old European maize varieties and found that they performed very stably in different regions and years. Some varieties, such as "Amylomaize-Cudillero" and "Lusitano-Asturias", can achieve a dry weight of 10 tons/hectare of straw without nitrogen fertilization. These old varieties generally have a higher cellulose content and a lower lignin ratio, which makes them easier to enzymatically hydrolyze and is also conducive to the saccharification process. For example, the "Cudillero" variety has a high straw yield, with a cellulose content of 38% and a lignin content of less than 15%, making it very suitable for biorefining (Munaiz et al., 2021). 8.3 Progress in tropical and subtropical regions In tropical and subtropical regions, maize breeding has also made significant progress in the direction of biofuels. These areas often have dry weather, high temperatures, and poor soil, making it difficult to grow high-yield maize. Some tropical hybrids, especially quality protein maize (QPM), show good stover quality, with high cellulose, low lignin, and high saccharification efficiency (Prasanna et al., 2021) (Figure 2). Under drought or nitrogen-limited conditions, these materials maintain more than 90% of their stover yield, while regular varieties often lose more than 30%. Now, many tropical maize breeding programs use doubled haploid (DH) technology and marker-assisted selection (MAS). Researchers use MAS to introduce genes that help with cell wall breakdown and stover yield into high-quality tropical maize backgrounds. The selected varieties outperform the control varieties in dry matter accumulation and saccharification efficiency, with an average improvement of 15% to 20%. 9 Challenges and Future Directions 9.1 Genetic bottlenecks and insufficient germplasm resources Although we have found some maize materials suitable for biofuels through screening and biological analysis in recent years, such as high-cellulose, low-lignin quality protein maize (QPM), the genetic resources of maize are still not rich enough (Pratikshya et al., 2025). This resource limitation not only affects the improvement of
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