Global Genetic Gain and Changes for Seed Yield, Agronomic, and Compositional Attributes in Soybean (Glycine max [L.] Merr.) from the 1920s to the 2010s: A Meta-Analysis

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Abstract
The analysis of genetic gain and changes in economic traits among modern and older cultivars is important to estimate breeding progress, evaluate the effectiveness of selection methods, identify optimised and neglected attributes, detect trade-offs, and inform strategies in crop improvement programs. This meta-analysis analysed global genetic gain and changes in seed yield, yield-related traits, and seed compositional attributes in soybean (Glycine max [L.] Merr.). A meta-database comprising 272 genetic gain data points recorded from 29 studies and 2555 mean performances captured from 34 research papers was compiled from research reporting cultivars released globally between the 1920s and 2010s. The mean values extracted from these studies were subjected to the R-Software analysis of variance (ANOVA) to reveal the effects of year of cultivar release and production region on the studied agronomic and compositional attributes. Boxplots were constructed to visualise data distribution and variability, and Pearson correlation and principal component analysis (PCA) were performed to examine inter-relationships among the measured traits with year of cultivar release and production region. Significant differences (p < 0.001) in mean performance were found among decades of cultivar release for seed yield (SY), number of seeds per pod (NSPod), days to flowering (DTF), days to maturity (DTM), and total biomass (TB). Cultivars released in the 2000s attained the highest mean SY of 3737.62 kg ha−1, compared to 2156.08 kg ha−1 observed for varieties released in the 1960s. Annual mean increases for SY ranged from 1.75 to 47 kg ha−1 year−1, driven primarily by improvements in number of seeds per plant (NSP), NSPod, plant height (PH), DTF, TB, and hundred seed weight (HSW). Conversely, seed protein content (SPC) declined across most breeding programs, while seed oil content (SOC) showed only modest improvement, and no consistent genetic gains were documented for micronutrient traits, including iron (Fe), zinc (Zn), calcium (Ca), or phytic acid (PA) content. Phytic acid, a potent chelator of essential minerals and a known inhibitor of protein digestibility in monogastric species, represents a critical but largely untracked compositional target in global soybean breeding. The absence of PA as a systematically monitored trait constitutes a significant gap in the literature, with direct implications for the nutritional quality of improved cultivars. Future breeding programs should adopt an integrated approach that simultaneously targets high SY, enhanced micronutrient bioavailability, and reduced antinutritional factor concentrations to advance both agricultural productivity and global food and nutrition security.

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