Does growing latitude influence soybean seed critical amino acid content? A meta-analysis

Soybean [Glycine max (L.) Merr.] seed critical amino acid (CAA) content is a key quality factor for buyers worldwide, yet its spatial variability remains unclear. This study evaluated the correlation between growing latitude and CAA content in soybean seed using published data. Pearson's r values were extracted, converted to Fisher's Z as an effect-size metric, and analyzed using a random-effects model. Lysine, cystine, methionine, and tryptophan showed no significant correlations, while threonine exhibited a significant negative correlation with absolute latitude (Z = −0.50, p = 0.04). Subgroup analysis revealed additional significant correlations for lysine in soybean meal (Z = −0.74, p = 0.04) and for threonine (chemical methods to measure CAAs: Z = −0.58, p = 0.03; studies conducted outside the United States: Z = −0.78, p = 0.02). The absence of consistent correlations across multiple soybean-growing regions worldwide suggests that latitude alone should not determine a buyer's purchasing decision regarding soybean CAA content.