Field Crop 2024, Vol.7, No.3, 158-170 http://cropscipublisher.com/index.php/fc 160 While genetic improvements have historically been credited with significant yield gains, recent studies suggest that the contribution of genetic technologies to yield potential is smaller than previously thought. Instead, climate and agronomic management practices play a more substantial role in recent yield gains (Rizzo et al., 2022). For instance, a study conducted in Nebraska found that 48% of the yield gain was associated with climate trends, 39% with agronomic improvements, and only 13% with genetic yield potential improvements (Rizzo et al., 2022). This underscores the importance of focusing on agronomic practices to enhance maize composition. In summary, the protein and starch contents of maize are significantly influenced by agronomic practices such as nitrogen fertilization, irrigation, and environmental conditions during grain filling. Understanding these influences can help optimize maize production for both nutritional and industrial purposes. 3 Agronomic Practices Affecting Maize Protein Content 3.1 Fertilization strategies Fertilization strategies play a crucial role in determining the protein content of maize. Nitrogen (N) fertilization, in particular, has been shown to significantly impact grain protein concentration. A meta-analysis of 21 studies revealed that increasing levels of nitrogen fertilizer consistently enhanced grain protein concentration, with a protein change of +14% for low, ≤70 kg N ha⁻¹; +21% for medium, >70–150 kg N ha⁻¹; and +24% for high, >150 kg N ha⁻¹ (Correndo et al., 2021). Additionally, integrated agronomic practices that include optimal fertilizer management have been found to improve nitrogen use efficiency (NUE) and grain nitrogen content, thereby promoting higher protein levels in maize (Vanlauwe et al., 2011; Liu et al., 2018; Zhou et al., 2019). The balanced application of other nutrients, such as phosphorus (P) and potassium (K), also affects the protein content in maize. Phosphorus fertilizer can promote root growth and energy metabolism, thereby indirectly enhancing protein synthesis efficiency (Grant and Flaten, 2019). Potassium fertilizer helps improve maize's stress resistance, water use efficiency, and photosynthetic efficiency, which in turn increases grain protein content (Pettigrew, 2008). 3.2 Irrigation techniques Irrigation techniques also influence maize protein content. Proper irrigation management can improve the biological value of protein in maize. For instance, irrigation has been shown to enhance the amino acid balance, which is crucial for the nutritional value of maize protein (Mason and D'croz-Mason, 2002). However, the timing and frequency of irrigation are critical. Studies have indicated that nighttime irrigation and low-frequency irrigation can lead to higher contamination with mycotoxins, which may indirectly affect protein quality (Herrera et al., 2023). Moreover, different irrigation levels combined with nitrogen fertilization can alter starch properties and phytic acid content, which are related to protein content in maize (Kaplan et al., 2019). Advanced irrigation technologies such as drip irrigation and sprinkler irrigation can more precisely control water supply, reducing water wastage and preventing overly wet or dry soil conditions. Studies have found that drip irrigation can significantly improve maize's water use efficiency and protein content (Oerke, 2006). Combining water-saving irrigation techniques with scientific fertilization schemes can further enhance the protein content and overall nutritional quality of maize. 3.3 Crop rotation and soil management Crop rotation and soil management practices are essential for maintaining soil fertility and enhancing maize protein content. Integrated Soil Fertility Management (ISFM), which includes the combined application of organic inputs and fertilizers, has been shown to maximize the agronomic efficiency of applied nutrients, thereby improving grain protein content (Vanlauwe et al., 2011). Additionally, practices such as subsoiling tillage and optimal planting density have been found to increase soil mineral nitrogen content and root length, which are crucial for nitrogen uptake and protein synthesis in maize (Liu et al., 2017; Zhou et al., 2019). Crop rotation can improve soil structure, increase soil nutrients, and reduce the frequency of pest and disease occurrences. For example, rotating with leguminous crops can enhance the nitrogen content in the soil, thereby
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